Sensorineural hearing loss (SNHL) is a type of
hearing loss
Hearing loss is a partial or total inability to Hearing, hear. Hearing loss may be present at birth or acquired at any time afterwards. Hearing loss may occur in one or both ears. In children, hearing problems can affect the ability to Language ...
in which the root cause lies in the
inner ear
The inner ear (internal ear, auris interna) is the innermost part of the vertebrate ear. In vertebrates, the inner ear is mainly responsible for sound detection and balance. In mammals, it consists of the bony labyrinth, a hollow cavity in the ...
or sensory organ (
cochlea
The cochlea is the part of the inner ear involved in hearing. It is a spiral-shaped cavity in the bony labyrinth, in humans making 2.75 turns around its axis, the modiolus. A core component of the cochlea is the Organ of Corti, the sensory org ...
and associated structures) or the
vestibulocochlear nerve
The vestibulocochlear nerve or auditory vestibular nerve, also known as the eighth cranial nerve, cranial nerve VIII, or simply CN VIII, is a cranial nerve that transmits sound and equilibrium (balance) information from the inner ear to the bra ...
(
cranial nerve
Cranial nerves are the nerves that emerge directly from the brain (including the brainstem), of which there are conventionally considered twelve pairs. Cranial nerves relay information between the brain and parts of the body, primarily to and ...
VIII). SNHL accounts for about 90% of reported hearing loss . SNHL is usually permanent and can be mild, moderate, severe, profound, or total. Various other descriptors can be used depending on the shape of the
audiogram
An audiogram is a graph that shows the audible threshold for standardized frequencies as measured by an audiometer. The Y axis represents intensity measured in decibels and the X axis represents frequency measured in hertz. The threshold of hear ...
, such as high frequency, low frequency, U-shaped, notched, peaked, or flat.
''Sensory'' hearing loss often occurs as a consequence of damaged or deficient cochlear
hair cell
Hair cells are the sensory receptors of both the auditory system and the vestibular system in the ears of all vertebrates, and in the lateral line organ of fishes. Through mechanotransduction, hair cells detect movement in their environment. ...
s. Hair cells may be abnormal at birth or damaged during the lifetime of an individual. There are both external causes of damage, including
infection
An infection is the invasion of tissues by pathogens, their multiplication, and the reaction of host tissues to the infectious agent and the toxins they produce. An infectious disease, also known as a transmissible disease or communicable dise ...
, and
ototoxic
Ototoxicity is the property of being toxic to the ear (''oto-''), specifically the cochlea or auditory nerve and sometimes the vestibular system, for example, as a side effect of a drug. The effects of ototoxicity can be reversible and temporary, ...
drugs, as well as intrinsic causes, including
genetic mutations
In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA or viral replication, mit ...
. A common cause or exacerbating factor in SNHL is prolonged exposure to environmental noise, or
noise-induced hearing loss
Noise-induced hearing loss (NIHL) is a Hearing loss, hearing impairment resulting from exposure to loud sound. People may have a loss of perception of a narrow range of Frequency, frequencies or impaired perception of sound including hyperacusi ...
. Exposure to a single very loud noise such as a gun shot or bomb blast can cause noise-induced hearing loss. Using headphones at high volume over time, or being in loud environments regularly, such as a loud workplace, sporting events, concerts, and using noisy machines can also be a risk for noise-induced hearing loss.
''Neural'', or "retrocochlear", hearing loss occurs because of damage to the
cochlear nerve
The cochlear nerve (also auditory nerve or acoustic nerve) is one of two parts of the vestibulocochlear nerve, a cranial nerve present in amniotes, the other part being the vestibular nerve. The cochlear nerve carries auditory sensory information ...
(CVIII). This damage may affect the initiation of the nerve impulse in the cochlear nerve or the transmission of the nerve impulse along the nerve into the
brainstem
The brainstem (or brain stem) is the posterior stalk-like part of the brain that connects the cerebrum with the spinal cord. In the human brain the brainstem is composed of the midbrain, the pons, and the medulla oblongata. The midbrain is cont ...
.
Most cases of SNHL present with a gradual deterioration of hearing thresholds occurring over years to decades. In some, the loss may eventually affect large portions of the
frequency range
A frequency band is an interval in the frequency domain, delimited by a lower frequency and an upper frequency. The term may refer to a radio band or an interval of some other spectrum.
The frequency range of a system is the range over which i ...
. It may be accompanied by other symptoms such as ringing in the ears (
tinnitus
Tinnitus is the perception of sound when no corresponding external sound is present. Nearly everyone experiences a faint "normal tinnitus" in a completely quiet room; but it is of concern only if it is bothersome, interferes with normal hearin ...
) and dizziness or lightheadedness (
vertigo
Vertigo is a condition where a person has the sensation of movement or of surrounding objects moving when they are not. Often it feels like a spinning or swaying movement. This may be associated with nausea, vomiting, sweating, or difficulties w ...
). The most common kind of sensorineural hearing loss is age-related (
presbycusis
Presbycusis (also spelled presbyacusis, from Greek πρέσβυς ''presbys'' "old" + ἄκουσις ''akousis'' "hearing"), or age-related hearing loss, is the cumulative effect of aging on hearing. It is a progressive and irreversible bilateral ...
), followed by
noise-induced hearing loss
Noise-induced hearing loss (NIHL) is a Hearing loss, hearing impairment resulting from exposure to loud sound. People may have a loss of perception of a narrow range of Frequency, frequencies or impaired perception of sound including hyperacusi ...
(NIHL).
Frequent symptoms of SNHL are loss of acuity in distinguishing foreground voices against noisy backgrounds, difficulty understanding on the telephone, some kinds of sounds seeming excessively loud or shrill, difficulty understanding some parts of speech (
fricative
A fricative is a consonant produced by forcing air through a narrow channel made by placing two articulators close together. These may be the lower lip against the upper teeth, in the case of ; the back of the tongue against the soft palate in t ...
s and
sibilant
Sibilants are fricative consonants of higher amplitude and pitch, made by directing a stream of air with the tongue towards the teeth. Examples of sibilants are the consonants at the beginning of the English words ''sip'', ''zip'', ''ship'', and ...
s), loss of directionality of sound (especially with high frequency sounds), perception that people mumble when speaking, and difficulty understanding speech. Similar symptoms are also associated with other kinds of hearing loss; audiometry or other diagnostic tests are necessary to distinguish sensorineural hearing loss.
Identification of sensorineural hearing loss is usually made by performing a
pure tone audiometry
Pure-tone audiometry is the main hearing test used to identify hearing threshold levels of an individual, enabling determination of the degree, type and configuration of a hearing loss and thus providing a basis for diagnosis and management. Pur ...
(an audiogram) in which bone conduction thresholds are measured.
Tympanometry
Tympanometry is an acoustic evaluation of the condition of the middle ear eardrum (tympanic membrane) and the conduction bones by creating variations of air pressure in the ear canal.
Tympanometry is an objective test of middle-ear function. It is ...
and
speech audiometry may be helpful. Testing is performed by an
audiologist
Audiology (from Latin , "to hear"; and from Greek , ''-logia'') is a branch of science that studies hearing, balance, and related disorders. Audiologists treat those with hearing loss and proactively prevent related damage. By employing various ...
.
There is no proven or recommended treatment or cure for SNHL; management of hearing loss is usually by hearing strategies and hearing aids. In cases of profound or total deafness, a
cochlear implant
A cochlear implant (CI) is a surgically implanted neuroprosthesis that provides a person who has moderate-to-profound sensorineural hearing loss with sound perception. With the help of therapy, cochlear implants may allow for improved speech und ...
is a specialised hearing aid that may restore a functional level of hearing. SNHL is at least partially preventable by avoiding environmental noise, ototoxic chemicals and drugs, and head trauma, and treating or inoculating against certain triggering diseases and conditions like
meningitis
Meningitis is acute or chronic inflammation of the protective membranes covering the brain and spinal cord, collectively called the meninges. The most common symptoms are fever, headache, and neck stiffness. Other symptoms include confusion or ...
.
Signs and symptoms
Since the
inner ear
The inner ear (internal ear, auris interna) is the innermost part of the vertebrate ear. In vertebrates, the inner ear is mainly responsible for sound detection and balance. In mammals, it consists of the bony labyrinth, a hollow cavity in the ...
is not directly accessible to instruments, identification is by patient report of the symptoms and
audiometric testing. Of those who present to their doctor with sensorineural hearing loss, 90% report having diminished hearing, 57% report having a plugged feeling in ear, and 49% report having ringing in ear (
tinnitus
Tinnitus is the perception of sound when no corresponding external sound is present. Nearly everyone experiences a faint "normal tinnitus" in a completely quiet room; but it is of concern only if it is bothersome, interferes with normal hearin ...
). About half report vestibular (vertigo) problems.
For a detailed exposition of symptoms useful for screening, a self-assessment questionnaire was developed by the
American Academy of Otolaryngology
American(s) may refer to:
* American, something of, from, or related to the United States of America, commonly known as the "United States" or "America"
** Americans, citizens and nationals of the United States of America
** American ancestry, pe ...
, called the Hearing Handicap Inventory for Adults (HHIA). It is a 25-question survey of subjective symptoms.
Causes
Sensorineural hearing loss may be genetic or acquired (i.e. as a consequence of disease, noise, trauma, etc.). People may have a hearing loss from birth (
congenital
A birth defect, also known as a congenital disorder, is an abnormal condition that is present at birth regardless of its cause. Birth defects may result in disabilities that may be physical, intellectual, or developmental. The disabilities can ...
) or the hearing loss may come on later. Many cases are related to old age (age-related).
Genetic
Hearing loss can be inherited. More than 40 genes have been implicated in the cause of deafness. There are 300 syndromes with related hearing loss, and each syndrome may have causative genes.
Recessive
In genetics, dominance is the phenomenon of one variant (allele) of a gene on a chromosome masking or overriding the effect of a different variant of the same gene on the other copy of the chromosome. The first variant is termed dominant and t ...
,
dominant,
X-linked
Sex linked describes the sex-specific patterns of inheritance and presentation when a gene mutation (allele) is present on a sex chromosome (allosome) rather than a non-sex chromosome (autosome). In humans, these are termed X-linked recessive, ...
, or
mitochondrial
A mitochondrion (; ) is an organelle found in the cells of most Eukaryotes, such as animals, plants and fungi. Mitochondria have a double membrane structure and use aerobic respiration to generate adenosine triphosphate (ATP), which is use ...
genetic mutations can affect the structure or metabolism of the inner ear. Some may be
single point mutations, whereas others are due to
chromosomal abnormalities
A chromosomal abnormality, chromosomal anomaly, chromosomal aberration, chromosomal mutation, or chromosomal disorder, is a missing, extra, or irregular portion of chromosomal DNA. These can occur in the form of numerical abnormalities, where ther ...
. Some genetic causes give rise to a late onset hearing loss. Mitochondrial mutations can cause SNHL i.e. m.1555A>G, which makes the individual sensitive to the ototoxic effects of
aminoglycoside antibiotics
Aminoglycoside is a medicinal and bacteriologic category of traditional Gram-negative antibacterial medications that inhibit protein synthesis and contain as a portion of the molecule an amino-modified glycoside (sugar). The term can also refer ...
.
* The most common cause of recessive genetic congenital hearing impairment in developed countries is
DFNB1
Gap junction beta-2 protein (GJB2), also known as connexin 26 (Cx26) — is a protein that in humans is encoded by the ''GJB2'' gene.
Clinical significance
Defects in this gene lead to the most common form of congenital deafness in developed c ...
, also known as Connexin 26 deafness or
GJB2
Gap junction beta-2 protein (GJB2), also known as connexin 26 (Cx26) — is a protein that in humans is encoded by the ''GJB2'' gene.
Clinical significance
Defects in this gene lead to the most common form of congenital deafness in developed c ...
-related deafness.
* The most common syndromic forms of hearing impairment include (dominant)
Stickler syndrome
Stickler syndrome (hereditary progressive arthro-ophthalmodystrophy) is a group of rare genetic disorders affecting connective tissue, specifically collagen. Stickler syndrome is a subtype of collagenopathy, types II and XI. Stickler syndrome i ...
and
Waardenburg syndrome
Waardenburg syndrome is a group of rare genetic conditions characterised by at least some degree of congenital hearing loss and pigmentation deficiencies, which can include bright blue eyes (or Heterochromia iridum, one blue eye and one brown ey ...
, and (recessive)
Pendred syndrome
Pendred syndrome is a genetic disorder leading to congenital bilateral (both sides) sensorineural hearing loss and goitre with euthyroid or mild hypothyroidism (decreased thyroid gland function). There is no specific treatment, other than support ...
and
Usher syndrome
Usher syndrome, also known as Hallgren syndrome, Usher–Hallgren syndrome, retinitis pigmentosa–dysacusis syndrome or dystrophia retinae dysacusis syndrome, is a rare genetic disorder caused by a mutation in any one of at least 11 genes result ...
.
* Mitochondrial mutations causing deafness are rare:
MT-TL1
Mitochondrially encoded tRNA leucine 1 (UUA/G) also known as MT-TL1 is a transfer RNA which in humans is encoded by the mitochondrion, mitochondrial ''MT-TL1'' gene.
Structure
The ''MT-TL1'' gene is located on the Locus (genetics), p arm of the ...
mutations cause
MIDD
Diabetes and deafness (DAD) or maternally inherited diabetes and deafness (MIDD) or mitochondrial diabetes is a subtype of diabetes which is caused from a point mutation at position 3243 in human mitochondrial DNA, which consists of a circular ge ...
(Maternally inherited deafness and diabetes) and other conditions which may include deafness as part of the picture.
*
TMPRSS3
Transmembrane protease, serine 3 is an enzyme that in humans is encoded by the ''TMPRSS3'' gene.
Function
This gene encodes a member of the serine protease family. The encoded protein contains a serine protease domain, a transmembrane domain, ...
gene was identified by its association with both congenital and childhood onset autosomal recessive deafness. This gene is expressed in fetal cochleae and many other tissues, and is thought to be involved in the development and maintenance of the
inner ear
The inner ear (internal ear, auris interna) is the innermost part of the vertebrate ear. In vertebrates, the inner ear is mainly responsible for sound detection and balance. In mammals, it consists of the bony labyrinth, a hollow cavity in the ...
or the contents of the
perilymph
Perilymph is an extracellular fluid located within the inner ear. It is found within the scala tympani and scala vestibuli of the cochlea. The ionic composition of perilymph is comparable to that of plasma and cerebrospinal fluid. The major ca ...
and
endolymph
Endolymph is the fluid contained in the membranous labyrinth of the inner ear. The major cation in endolymph is potassium, with the values of sodium and potassium concentration in the endolymph being 0.91 mM and 154 mM, respectively. I ...
. It was also identified as a tumor associated gene that is overexpressed in
ovarian tumor
Ovarian tumors, or ovarian neoplasms, are tumors arising from the ovary. They can be benign or malignant (ovarian cancer). They consist of mainly solid tissue, while ovarian cysts contain fluid.
Histopathologic classification
Ovarian tumors are ...
s.
*
Charcot–Marie–Tooth disease
Charcot–Marie–Tooth disease (CMT) is a hereditary motor and sensory neuropathy of the peripheral nervous system characterized by progressive loss of muscle tissue and touch sensation across various parts of the body. This disease is the mos ...
an inherited neurological disorder with delayed onset that can affect the ears as well as other organs. The hearing loss in this condition is often ANSD (auditory neuropathy spectrum disorder) a neural cause of hearing loss.
*
Muckle–Wells syndrome
Muckle–Wells syndrome (MWS) is a rare autosomal dominant disease which causes sensorineural deafness and recurrent hives, and can lead to amyloidosis. Individuals with MWS often have episodic fever, chills, and joint pain. As a result, MWS is ...
, a rare inherited
autoinflammatory
Periodic fever syndromes are a set of disorders characterized by recurrent episodes of systemic and organ-specific inflammation. Unlike autoimmune disorders such as systemic lupus erythematosus, in which the disease is caused by abnormalities of ...
disorder, can lead to hearing loss.
*
Autoimmune disease
An autoimmune disease is a condition arising from an abnormal immune response to a functioning body part. At least 80 types of autoimmune diseases have been identified, with some evidence suggesting that there may be more than 100 types. Nearly a ...
: although probably rare, it is possible for autoimmune processes to target the cochlea specifically, without symptoms affecting other organs.
Granulomatosis with polyangiitis
Granulomatosis with polyangiitis (GPA), previously known as Wegener's granulomatosis (WG), is a rare long-term systemic disorder that involves the formation of granulomas and inflammation of blood vessels (vasculitis). It is a form of vasculitis ...
, an autoimmune condition, may precipitate hearing loss.
Congenital
* Infections:
**
Congenital rubella syndrome
Congenital rubella syndrome (CRS) can occur in a developing fetus of a pregnant woman who has contracted rubella, usually in the first trimester. If infection occurs 0–28 days before conception, the infant has a 43% risk of being affected. If ...
, CRS, results from transplacental transmission of the
rubella virus
Rubella virus (RuV) is the pathogenic agent of the disease rubella, transmitted only between humans via the respiratory route, and is the main cause of congenital rubella syndrome when infection occurs during the first weeks of pregnancy.
Rube ...
during pregnancy. CRS has been controlled by universal vaccination (
MMR or
MMRV vaccine
The MMRV vaccine combines the attenuated virus MMR (measles, mumps, rubella) vaccine with the addition of the chickenpox vaccine or varicella vaccine (V stands for varicella). The MMRV vaccine is typically given to children between one and two ...
).
**
Cytomegalovirus
''Cytomegalovirus'' (''CMV'') (from ''cyto-'' 'cell' via Greek - 'container' + 'big, megalo-' + -''virus'' via Latin 'poison') is a genus of viruses in the order ''Herpesvirales'', in the family ''Herpesviridae'', in the subfamily ''Betaherpe ...
(CMV) infection is the most common cause of progressive sensorineural hearing loss in children. It is a common viral infection contracted by contact with infected bodily fluids such as saliva or urine and easily transmitted in nurseries and thus from toddlers to expectant mothers. CMV infection during pregnancy can affect the developing foetus and lead to learning difficulties as well as hearing loss.
**
Toxoplasmosis
Toxoplasmosis is a parasitic disease caused by ''Toxoplasma gondii'', an apicomplexan. Infections with toxoplasmosis are associated with a variety of neuropsychiatric and behavioral conditions. Occasionally, people may have a few weeks or months ...
, a parasitic disease affecting 23% of the population in the U.S., can cause sensorineural deafness to the fetus in utero.
*
Hypoplastic
Hypoplasia (from Ancient Greek ὑπo- ''hypo-'' 'under' + πλάσις ''plasis'' 'formation'; adjective form ''hypoplastic'') is underdevelopment or incomplete development of a tissue or organ.Pendred syndrome
Pendred syndrome is a genetic disorder leading to congenital bilateral (both sides) sensorineural hearing loss and goitre with euthyroid or mild hypothyroidism (decreased thyroid gland function). There is no specific treatment, other than support ...
,
branchio-oto-renal syndrome
Branchio-oto-renal syndrome (BOR) is an autosomal dominant genetic disorder involving the kidneys, ears, and neck. It often has also been described as Melnick-Fraser syndrome.
Signs and symptoms
The signs and symptoms of branchio-oto-renal syndrom ...
,
CHARGE syndrome
CHARGE syndrome (formerly known as CHARGE association) is a rare syndrome caused by a genetic disorder. First described in 1979, the acronym "CHARGE" came into use for newborn children with the congenital features of coloboma of the eye, heart ...
*
GATA2 deficiency
GATA2 deficiency is a grouping of several disorders caused by common defect, viz., familial or sporadic inactivating mutations in one of the two parental ''GATA2'' genes. Being the gene haploinsufficient, mutations that cause a reduction in the ce ...
, a grouping of several disorders caused by common defect, viz., familial or sporadic
inactivating mutations in one of the two parental ''
GATA2
GATA2 or GATA-binding factor 2 is a transcription factor, i.e. a nuclear protein which regulates the expression of genes. It regulates many genes that are critical for the embryonic development, self-renewal, maintenance, and functionality of b ...
''
gene
In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a ba ...
s. These
autosomal dominant
In genetics, dominance is the phenomenon of one variant (allele) of a gene on a chromosome masking or overriding the effect of a different variant of the same gene on the other copy of the chromosome. The first variant is termed dominant and t ...
mutations cause a reduction, i.e. a
haploinsufficiency
Haploinsufficiency in genetics describes a model of dominant gene action in diploid organisms, in which a single copy of the wild-type allele at a locus in heterozygous combination with a variant allele is insufficient to produce the wild-type ...
, in the cellular levels of the gene's product,
GATA2
GATA2 or GATA-binding factor 2 is a transcription factor, i.e. a nuclear protein which regulates the expression of genes. It regulates many genes that are critical for the embryonic development, self-renewal, maintenance, and functionality of b ...
. The GATA2
protein
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respo ...
is a
transcription factor
In molecular biology, a transcription factor (TF) (or sequence-specific DNA-binding factor) is a protein that controls the rate of transcription of genetic information from DNA to messenger RNA, by binding to a specific DNA sequence. The fu ...
critical for the
embryonic development
An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male sperm ...
, maintenance, and functionality of
blood-forming
Blood is a body fluid in the circulatory system of humans and other vertebrates that delivers necessary substances such as nutrients and oxygen to the cells, and transports metabolic waste products away from those same cells. Blood in the c ...
,
lympathic-forming, and other tissue-forming
stem cell
In multicellular organisms, stem cells are undifferentiated or partially differentiated cells that can differentiate into various types of cells and proliferate indefinitely to produce more of the same stem cell. They are the earliest type o ...
s. In consequence of these mutations, cellular levels of GATA2 are deficient and individuals develop over time hematological, immunological, lymphatic, and/or other disorders. GATA2 deficiency-induced abnormalities in the lymphatic system are proposed to be responsible for a failure in generating the
perilymphatic space
Perilymph is an extracellular fluid located within the inner ear. It is found within the scala tympani and scala vestibuli of the cochlea. The ionic composition of perilymph is comparable to that of plasma and cerebrospinal fluid. The major ca ...
around the inner ear's
semicircular canals
The semicircular canals or semicircular ducts are three semicircular, interconnected tubes located in the innermost part of each ear, the inner ear. The three canals are the horizontal, superior and posterior semicircular canals.
Structure
The ...
, which in turn underlies the development of sensorineural hearing loss.
Presbycusis
Progressive age-related loss of hearing acuity or sensitivity can start as early as age 18, primarily affecting the high frequencies, and men more than women. Such losses may not become apparent until much later in life.
Presbycusis
Presbycusis (also spelled presbyacusis, from Greek πρέσβυς ''presbys'' "old" + ἄκουσις ''akousis'' "hearing"), or age-related hearing loss, is the cumulative effect of aging on hearing. It is a progressive and irreversible bilateral ...
is by far the dominant cause of sensorineural hearing loss in industrialized societies. A study conducted in Sudan, with a population free from loud noise exposures, found significantly less cases of hearing loss when compared with age-matched cases from an industrialized country. Similar findings were reported by a study conducted of a population from Easter island, which reported worse hearing among those that spent time in industrialized countries when compared with those that never left the island. Researchers have argued that factors other than differences in noise exposure, such as genetic make up, might also have contributed to the findings. Hearing loss that worsens with age but is caused by factors other than normal aging, such as noise-induced hearing loss, is not presbycusis, although differentiating the individual effects of multiple causes of hearing loss can be difficult. One in three persons have significant hearing loss by age 65; by age 75, one in two. Age-related hearing loss is neither preventable nor reversible.
Noise
Most people living in modern society have some degree of progressive sensorineural (i.e. permanent) noise-induced hearing loss (NIHL) resulting from overloading and damaging the sensory or neural apparatus of hearing in the inner ear. NIHL is typically a drop-out or notch centered at 4000 Hz. Both intensity (SPL) and duration of exposure, and repetitive exposure to unsafe levels of noise contribute to cochlear damage that results in hearing loss. The louder the noise is, the shorter the safe amount of exposure is. NIHL can be either permanent or temporary, called a threshold shift. Unsafe levels of noise can be as little as 70 dB (about twice as loud as normal conversation) if there is prolonged (24-hour) or continuous exposure. 125 dB (a loud rock concert is ~120 dB) is the pain level; sounds above this level cause instant and permanent ear damage.
Noise and ageing are the primary causes of
presbycusis
Presbycusis (also spelled presbyacusis, from Greek πρέσβυς ''presbys'' "old" + ἄκουσις ''akousis'' "hearing"), or age-related hearing loss, is the cumulative effect of aging on hearing. It is a progressive and irreversible bilateral ...
, or age-related hearing loss, the most common kind of hearing loss in industrial society.
The dangers of environmental and occupational noise exposure are widely recognized. Numerous national and international organizations have established standards for safe levels of exposure to noise in industry, the environment, military, transportation, agriculture, mining and other areas.
[A few prominent ones are American National Standards Institute (ANSI), International Organization for Standardization (ISO), Deutsches Institut für Normung (DIN), Swedish Standards Institute (SSI), Canadian Standards Association (CSA), British Standards Institute (BSI), Austrian Standards International(ÖNORM), and in the United States, Environmental Protection Agency (EPA), Occupational Safety and Health Administration (OSHA) and numerous state agencies, and Department of Defense (DOD) among others.] Sound intensity or sound pressure level (SPL) is measured in decibels (dB). For reference:
An increase of 6 dB represents a doubling of the SPL, or energy of the sound wave, and therefore its propensity to cause ear damage. Because human ears hear logarithmically, not linearly, it takes an increase of 10 dB to produce a sound that is perceived to be twice as loud. Ear damage due to noise is proportional to sound intensity, not perceived loudness, so it's misleading to rely on subjective perception of loudness as an indication of the risk to hearing, i.e. it can significantly underestimate the danger.
While the standards differ moderately in levels of intensity and duration of exposure considered safe, some guidelines can be derived.
[The various standards quantify nose exposure with a set of specified measures, usually with respect to a reference exposure time of 8 hours, a typical working day. The measures include, a weighting scale (usually A) with a sample time, a threshold value in dB, a criterion sound pressure level in dB with an exposure time usually in hours, and an exchange rate in dB. A weighted SPL is denoted dB(X) where X is a weighting scale, usually A, but sometimes C. (A) refers to ]A-weighting
A-weighting is the most commonly used of a family of curves defined in the International standard IEC 61672:2003 and various national standards relating to the measurement of sound pressure level. A-weighting is applied to instrument-measured ...
of SPL, which is an adjustment to measured SPL to compensate for the frequency response of the human ear, which is less sensitive to low frequencies. The criterion level is the average sound pressure level permitted over the exposure time. The threshold sound pressure level is the level above which sound will be integrated into the average. The sample time (fast, slow or impulse) is the rate of sampling — a slow sample time is 1 second; a fast sample time is 1/8 second, and impulse sample time is 35 milliseconds. The effect of a slower sample time means that very short duration sounds may not be fully sampled (or even sampled at all in rare cases), so the noise exposure may be underestimated. The exchange rate is the amount by which the permitted sound level may increase if the exposure time is halved.
The safe amount of exposure is reduced by a factor of 2 for every exchange rate (3 dB for NIOSH standard or 5 dB for
OSHA
OSHA or Osha may refer to:
Work
* Occupational Safety and Health Administration, a federal agency of the United States that regulates workplace safety and health
* Occupational Safety and Health Act (United States) of 1970, a federal law in the Un ...
standard) increase in SPL. For example, the safe daily exposure amount at 85 dB (90 dB for OSHA) is 8 hours, while the safe exposure at 94 dB(A) (nightclub level) is only 1 hour. Noise trauma can also cause a reversible hearing loss, called a temporary threshold shift. This typically occurs in individuals who are exposed to gunfire or firecrackers, and hear ringing in their ears after the event (
tinnitus
Tinnitus is the perception of sound when no corresponding external sound is present. Nearly everyone experiences a faint "normal tinnitus" in a completely quiet room; but it is of concern only if it is bothersome, interferes with normal hearin ...
).
* Ambient environmental noise: Populations living near airports, railyards and train stations, freeways and industrial areas are exposed to levels of noise typically in the 65 to 75 dBA range. If lifestyles include significant outdoor or open window conditions, these exposures over time can degrade hearing.
U.S. Dept. of Housing and Urban Development sets standards for noise impact in residential and commercial construction zones. HUD's noise standards may be found in 24 CFR Part 51, Subpart B. Environmental noise above 65 dB defines a noise-impacted area.
* Personal audio electronics: Personal audio equipment such as
iPods
The iPod is a discontinued series of portable media players and multi-purpose mobile devices designed and marketed by Apple Inc. The first version was released on October 23, 2001, about months after the Macintosh version of iTunes ...
(iPods often reach 115 decibels or higher), can produce powerful enough sound to cause significant NIHL.
* Acoustic trauma: Exposure to a single event of extremely loud noise (such as explosions) can also cause temporary or permanent hearing loss. A typical source of acoustic trauma is a too-loud music concert.
* Workplace noise: The OSHA standards 1910.95 General Industry Occupational Noise Exposure and 1926.52 Construction Industry Occupational Noise Exposure identify the level of 90 dB(A) for 8 hour exposure as the level necessary to protect workers from hearing loss.
Disease or disorder
*
Inflammatory
** Suppurative
labyrinthitis
Labyrinthitis is inflammation of the labyrinth – a maze of fluid-filled channels in the inner ear. Vestibular neuritis is inflammation of the vestibular nerve – the nerve in the inner ear that sends messages related to motion and position t ...
or otitis interna (inflammation of the inner ear)
*
Diabetes mellitus
Diabetes, also known as diabetes mellitus, is a group of metabolic disorders characterized by a high blood sugar level ( hyperglycemia) over a prolonged period of time. Symptoms often include frequent urination, increased thirst and increased ap ...
A recent study found that hearing loss is twice as common in people with diabetes as it is in those who don't have the disease. Also, of the 86 million adults in the U.S. who have prediabetes, the rate of hearing loss is 30 percent higher than in those with normal blood glucose. It has not been established how diabetes is related to hearing loss. It is possible that the high blood glucose levels associated with diabetes cause damage to the small blood vessels in the inner ear, similar to the way in which diabetes can damage the eyes and the kidneys. Similar studies have shown a possible link between that hearing loss and neuropathy (nerve damage).
*
Tumor
A neoplasm () is a type of abnormal and excessive growth of tissue. The process that occurs to form or produce a neoplasm is called neoplasia. The growth of a neoplasm is uncoordinated with that of the normal surrounding tissue, and persists ...
** Cerebellopontine angle tumour (junction of the
pons
The pons (from Latin , "bridge") is part of the brainstem that in humans and other bipeds lies inferior to the midbrain, superior to the medulla oblongata and anterior to the cerebellum.
The pons is also called the pons Varolii ("bridge of Va ...
and
cerebellum
The cerebellum (Latin for "little brain") is a major feature of the hindbrain of all vertebrates. Although usually smaller than the cerebrum, in some animals such as the mormyrid fishes it may be as large as or even larger. In humans, the cerebel ...
) – The cerebellopontine angle is the exit site of both the
facial nerve
The facial nerve, also known as the seventh cranial nerve, cranial nerve VII, or simply CN VII, is a cranial nerve that emerges from the pons of the brainstem, controls the muscles of facial expression, and functions in the conveyance of tas ...
(CN7) and the
vestibulocochlear nerve
The vestibulocochlear nerve or auditory vestibular nerve, also known as the eighth cranial nerve, cranial nerve VIII, or simply CN VIII, is a cranial nerve that transmits sound and equilibrium (balance) information from the inner ear to the bra ...
(CN8). Patients with these tumors often have signs and symptoms corresponding to compression of both nerves.
***
Acoustic neuroma
A vestibular schwannoma (VS), also called acoustic neuroma, is a benign tumor that develops on the vestibulocochlear nerve that passes from the inner ear to the brain. The tumor originates when Schwann cells that form the insulating myelin sheath o ...
(vestibular schwannoma) – benign neoplasm of
Schwann cell
Schwann cells or neurolemmocytes (named after German physiologist Theodor Schwann) are the principal glia of the peripheral nervous system (PNS). Glial cells function to support neurons and in the PNS, also include satellite cells, olfactory ensh ...
s affecting the vestibulocochlear nerve
***
Meningioma
Meningioma, also known as meningeal tumor, is typically a slow-growing tumor that forms from the meninges, the membranous layers surrounding the brain and spinal cord. Symptoms depend on the location and occur as a result of the tumor pressing o ...
– benign tumour of the
pia and
arachnoid mater
The arachnoid mater (or simply arachnoid) is one of the three meninges, the protective membranes that cover the brain and spinal cord. It is so named because of its resemblance to a spider web. The arachnoid mater is a derivative of the neural c ...
*
Ménière's disease
Ménière's disease (MD) is a disease of the inner ear that is characterized by potentially severe and incapacitating episodes of vertigo, tinnitus, hearing loss, and a feeling of fullness in the ear. Typically, only one ear is affected initial ...
– causes sensorineural hearing loss in the low frequency range (125 Hz to 1000 Hz). Ménière's disease is characterized by sudden attacks of vertigo, lasting minutes to hours preceded by
tinnitus
Tinnitus is the perception of sound when no corresponding external sound is present. Nearly everyone experiences a faint "normal tinnitus" in a completely quiet room; but it is of concern only if it is bothersome, interferes with normal hearin ...
, aural fullness, and fluctuating hearing loss. It is relatively rare and commonly over diagnosed.
* Bacterial
meningitis
Meningitis is acute or chronic inflammation of the protective membranes covering the brain and spinal cord, collectively called the meninges. The most common symptoms are fever, headache, and neck stiffness. Other symptoms include confusion or ...
e.g. pneumococcal, meningococcal, haemophilus influenzae may damage the
cochlea
The cochlea is the part of the inner ear involved in hearing. It is a spiral-shaped cavity in the bony labyrinth, in humans making 2.75 turns around its axis, the modiolus. A core component of the cochlea is the Organ of Corti, the sensory org ...
– Hearing loss is one of the most common after-effects of bacterial meningitis. It has been estimated that 30% of bacterial meningitis cases result in mild to profound hearing loss. Children are most at risk: seventy percent of all bacterial meningitis occurs in young children under the age of five.
* Viral
**
AIDS
Human immunodeficiency virus infection and acquired immunodeficiency syndrome (HIV/AIDS) is a spectrum of conditions caused by infection with the human immunodeficiency virus (HIV), a retrovirus. Following initial infection an individual m ...
and
ARC
ARC may refer to:
Business
* Aircraft Radio Corporation, a major avionics manufacturer from the 1920s to the '50s
* Airlines Reporting Corporation, an airline-owned company that provides ticket distribution, reporting, and settlement services
* ...
patients frequently experience auditory system anomalies.
**
Mumps
MUMPS ("Massachusetts General Hospital Utility Multi-Programming System"), or M, is an imperative, high-level programming language with an integrated transaction processing key–value database. It was originally developed at Massachusetts Gener ...
(epidemic parotitis) may result in profound sensorineural hearing loss (90
dB or more), unilaterally (one ear) or bilaterally (both ears).
**
Measles
Measles is a highly contagious infectious disease caused by measles virus. Symptoms usually develop 10–12 days after exposure to an infected person and last 7–10 days. Initial symptoms typically include fever, often greater than , cough, ...
may result in
auditory nerve
The cochlear nerve (also auditory nerve or acoustic nerve) is one of two parts of the vestibulocochlear nerve, a cranial nerve present in amniotes, the other part being the vestibular nerve. The cochlear nerve carries auditory sensory information ...
damage but more commonly gives a mixed (sensorineural plus conductive) hearing loss, and can be bilaterally.
**
Ramsay Hunt syndrome type II
Inflammation of the geniculate ganglion of the facial nerve is a late consequence of ''varicella zoster'' virus (VZV) known as Ramsay Hunt syndrome (RHS), commonly known as herpes zoster oticus. In regards with the frequency, less than 1% of vari ...
(herpes zoster oticus)
* Bacterial
**
Syphilis
Syphilis () is a sexually transmitted infection caused by the bacterium ''Treponema pallidum'' subspecies ''pallidum''. The signs and symptoms of syphilis vary depending in which of the four stages it presents (primary, secondary, latent, an ...
is commonly transmitted from pregnant women to their fetuses, and about a third of the infected children will eventually become deaf.
Ototoxic and neurotoxic drugs and chemicals
Some over-the-counter as well as prescription drugs and certain industrial chemicals are ototoxic. Exposure to these can result in temporary or permanent hearing loss.
Some medications cause irreversible damage to the ear, and are limited in their use for this reason. The most important group is the
aminoglycoside
Aminoglycoside is a medicinal and bacteriologic category of traditional Gram-negative antibacterial medications that inhibit protein synthesis and contain as a portion of the molecule an amino-modified glycoside (sugar). The term can also refer ...
s (main member
gentamicin
Gentamicin is an antibiotic used to treat several types of bacterial infections. This may include bone infections, endocarditis, pelvic inflammatory disease, meningitis, pneumonia, urinary tract infections, and sepsis among others. It is not e ...
). A rare mitochondrial mutation, m.1555A>G, can increase an individual's susceptibility to the ototoxic effect of aminoglycosides. Long term
hydrocodone
Hydrocodone, also known as dihydrocodeinone, is an opioid used to treat pain and as a cough suppressant. It is taken by mouth. Typically it is dispensed as the combination acetaminophen/hydrocodone or ibuprofen/hydrocodone for pain severe en ...
(Vicodin) abuse is known to cause rapidly progressing sensorineural hearing loss, usually without vestibular symptoms.
Methotrexate
Methotrexate (MTX), formerly known as amethopterin, is a chemotherapy agent and immune-system suppressant. It is used to treat cancer, autoimmune diseases, and ectopic pregnancies. Types of cancers it is used for include breast cancer, leuke ...
, a chemotherapy agent, is also known to cause hearing loss. In most cases hearing loss does not recover when the drug is stopped. Paradoxically, methotrexate is also used in the treatment of autoimmune-induced inflammatory hearing loss.
Various other medications may reversibly degrade hearing. This includes loop
diuretic
A diuretic () is any substance that promotes diuresis, the increased production of urine. This includes forced diuresis. A diuretic tablet is sometimes colloquially called a water tablet. There are several categories of diuretics. All diuretics in ...
s,
sildenafil
Sildenafil, sold under the brand name Viagra, among others, is a medication used to treat erectile dysfunction and pulmonary arterial hypertension. It is unclear if it is effective for treating sexual dysfunction in women. It is taken by ...
(Viagra), high or sustained dosing of
NSAID
Non-steroidal anti-inflammatory drugs (NSAID) are members of a therapeutic drug class which reduces pain, decreases inflammation, decreases fever, and prevents blood clots. Side effects depend on the specific drug, its dose and duration of ...
s (
aspirin
Aspirin, also known as acetylsalicylic acid (ASA), is a nonsteroidal anti-inflammatory drug (NSAID) used to reduce pain, fever, and/or inflammation, and as an antithrombotic. Specific inflammatory conditions which aspirin is used to treat inc ...
,
ibuprofen
Ibuprofen is a nonsteroidal anti-inflammatory drug (NSAID) that is used for treating pain, fever, and inflammation. This includes painful menstrual periods, migraines, and rheumatoid arthritis. It may also be used to close a patent ductus arte ...
,
naproxen
Naproxen is a nonsteroidal anti-inflammatory drug (NSAID) used to treat pain, menstrual cramps, inflammatory diseases such as rheumatoid arthritis, gout and fever. It is taken orally. It is available in immediate and delayed release formulati ...
, and various prescription drugs:
celecoxib
Celecoxib, sold under the brand name Celebrex among others, is a COX-2 inhibitor and nonsteroidal anti-inflammatory drug (NSAID). It is used to treat the pain and inflammation in osteoarthritis, acute pain in adults, rheumatoid arthritis, ankyl ...
, etc.),
quinine
Quinine is a medication used to treat malaria and babesiosis. This includes the treatment of malaria due to ''Plasmodium falciparum'' that is resistant to chloroquine when artesunate is not available. While sometimes used for nocturnal leg cr ...
, and
macrolide
The Macrolides are a class of natural products that consist of a large macrocyclic lactone ring to which one or more deoxy sugars, usually cladinose and desosamine, may be attached. The lactone rings are usually 14-, 15-, or 16-membered. Macrol ...
antibiotics (
erythromycin
Erythromycin is an antibiotic used for the treatment of a number of bacterial infections. This includes respiratory tract infections, skin infections, chlamydia infections, pelvic inflammatory disease, and syphilis. It may also be used duri ...
, etc.). Cytotoxic agents such as carboplatinum, used to treat malignancies can give rise to a dose dependent SNHL, as can drugs such as desferrioxamine, used for haematological disorders such as thalassaemia; patients prescribed these drugs need to have hearing monitored.
Prolonged or repeated environmental or work-related exposure to ototoxic chemicals can also result in sensorineural hearing loss. Some of these chemicals are:
*
butyl nitrite
Butyl nitrite is the organic compound with the formula CH3(CH2)3ONO. It is an alkyl nitrite made from n-Butanol, ''n''-butanol. Butyl nitrite is used recreationally as poppers. Synonyms include ''1-butyl nitrite'', n''-butyl nitrite'' and ''nitr ...
– chemical used recreationally known as '
poppers
Popper is a slang term given broadly to drugs of the chemical class called alkyl nitrites that are inhaled. Most widely sold products include the original isoamyl nitrite or isopentyl nitrite, and isopropyl nitrite. Isobutyl nitrite is also ...
'
*
carbon disulfide
Carbon disulfide (also spelled as carbon disulphide) is a neurotoxic, colorless, volatile liquid with the formula and structure . The compound is used frequently as a building block in organic chemistry as well as an industrial and chemical non ...
– a solvent used as a building block in many organic reactions
*
styrene
Styrene () is an organic compound with the chemical formula C6H5CH=CH2. This derivative of benzene is a colorless oily liquid, although aged samples can appear yellowish. The compound evaporates easily and has a sweet smell, although high concen ...
, an industrial chemical precursor of
polystyrene
Polystyrene (PS) is a synthetic polymer made from monomers of the aromatic hydrocarbon styrene. Polystyrene can be solid or foamed. General-purpose polystyrene is clear, hard, and brittle. It is an inexpensive resin per unit weight. It is a ...
, a plastic
*
carbon monoxide
Carbon monoxide (chemical formula CO) is a colorless, poisonous, odorless, tasteless, flammable gas that is slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the simple ...
, a poisonous gas resulting from incomplete
combustion
Combustion, or burning, is a high-temperature exothermic redox chemical reaction between a fuel (the reductant) and an oxidant, usually atmospheric oxygen, that produces oxidized, often gaseous products, in a mixture termed as smoke. Combusti ...
* heavy metals:
tin
Tin is a chemical element with the symbol Sn (from la, stannum) and atomic number 50. Tin is a silvery-coloured metal.
Tin is soft enough to be cut with little force and a bar of tin can be bent by hand with little effort. When bent, t ...
,
lead
Lead is a chemical element with the symbol Pb (from the Latin ) and atomic number 82. It is a heavy metal that is denser than most common materials. Lead is soft and malleable, and also has a relatively low melting point. When freshly cu ...
,
manganese
Manganese is a chemical element with the symbol Mn and atomic number 25. It is a hard, brittle, silvery metal, often found in minerals in combination with iron. Manganese is a transition metal with a multifaceted array of industrial alloy use ...
,
mercury
Mercury commonly refers to:
* Mercury (planet), the nearest planet to the Sun
* Mercury (element), a metallic chemical element with the symbol Hg
* Mercury (mythology), a Roman god
Mercury or The Mercury may also refer to:
Companies
* Merc ...
*
hexane
Hexane () is an organic compound, a straight-chain alkane with six carbon atoms and has the molecular formula C6H14.
It is a colorless liquid, odorless when pure, and with boiling points approximately . It is widely used as a cheap, relatively ...
, an industrial solvent and one of the significant constituents of
gasoline
Gasoline (; ) or petrol (; ) (see ) is a transparent, petroleum-derived flammable liquid that is used primarily as a fuel in most spark-ignited internal combustion engines (also known as petrol engines). It consists mostly of organic co ...
*
ethylbenzene
Ethylbenzene is an organic compound with the formula . It is a highly flammable, colorless liquid with an odor similar to that of gasoline. This monocyclic aromatic hydrocarbon is important in the petrochemical industry as an reaction intermediat ...
, an industrial solvent used in the production of styrene
*
toluene
Toluene (), also known as toluol (), is a substituted aromatic hydrocarbon. It is a colorless, water-insoluble liquid with the smell associated with paint thinners. It is a mono-substituted benzene derivative, consisting of a methyl group (CH3) at ...
and
xylene
In organic chemistry, xylene or xylol (; IUPAC name: dimethylbenzene) are any of three organic compounds with the formula . They are derived from the substitution of two hydrogen atoms with methyl groups in a benzene ring; which hydrogens are sub ...
, highly poisonous petrochemical solvents. Toluene is a component of high-octane gasoline; xylene is used in the production of polyester fibers and resins.
*
trichloroethylene
The chemical compound trichloroethylene is a halocarbon commonly used as an industrial solvent. It is a clear, colourless non-flammable liquid with a chloroform-like sweet smell. It should not be confused with the similar 1,1,1-trichloroethane, ...
, an industrial degreasing solvent
*
Organophosphate pesticide
In organic chemistry, organophosphates (also known as phosphate esters, or OPEs) are a class of organophosphorus compounds with the general structure , a central phosphate molecule with alkyl or aromatic substituents. They can be considered a ...
s
Head trauma
There can be damage either to the ear itself or to the central auditory pathways that process the information conveyed by the ears. People who sustain head injury are susceptible to hearing loss or tinnitus, either temporary or permanent. Contact sports like football (U.S. NFL), hockey and cricket have a notable incidence of head injuries (concussions). In one survey of retired NFL players, all of whom reported one or more concussions during their playing careers, 25% had hearing loss and 50% had tinnitus.
Perinatal conditions
These are much more common in premature babies, particularly those under 1500 g at birth.
Premature birth
Preterm birth, also known as premature birth, is the birth of a baby at fewer than 37 weeks gestational age, as opposed to full-term delivery at approximately 40 weeks. Extreme preterm is less than 28 weeks, very early preterm birth is between 2 ...
can be associated with problems that result in sensorineural hearing loss such as anoxia or hypoxia (poor oxygen levels), jaundice, intracranial haemorrhages, meningitis.
Fetal alcohol syndrome is reported to cause hearing loss in up to 64% of infants born to
alcoholic
Alcoholism is, broadly, any drinking of alcohol that results in significant mental or physical health problems. Because there is disagreement on the definition of the word ''alcoholism'', it is not a recognized diagnostic entity. Predomin ...
mothers, from the
ototoxic effect on the developing fetus, plus malnutrition during pregnancy from the excess
alcohol
Alcohol most commonly refers to:
* Alcohol (chemistry), an organic compound in which a hydroxyl group is bound to a carbon atom
* Alcohol (drug), an intoxicant found in alcoholic drinks
Alcohol may also refer to:
Chemicals
* Ethanol, one of sev ...
intake.
Iodine deficiency / Hypothyroidism
Iodine deficiency
Iodine deficiency is a lack of the trace element iodine, an essential nutrient in the diet. It may result in metabolic problems such as goiter, sometimes as an endemic goiter as well as congenital iodine deficiency syndrome due to untreated cong ...
and endemic
hypothyroidism
Hypothyroidism (also called ''underactive thyroid'', ''low thyroid'' or ''hypothyreosis'') is a disorder of the endocrine system in which the thyroid gland does not produce enough thyroid hormone. It can cause a number of symptoms, such as po ...
are associated with hearing loss. If a pregnant mother has insufficient iodine intake during pregnancy it affects the development of the inner ear in the foetus leading to sensorineural deafness. This occurs in certain areas of the world, such as the Himalayas, where iodine is deficient in the soil and thus the diet. In these areas there is a high incidence of endemic goitre. This cause of deafness is prevented by adding iodine to salt.
Brain stroke
Brain stroke
A stroke is a medical condition in which poor blood flow to the brain causes cell death. There are two main types of stroke: ischemic, due to lack of blood flow, and hemorrhagic, due to bleeding. Both cause parts of the brain to stop functionin ...
in a region affecting auditory function such as a
posterior circulation infarct
Posterior circulation infarct (POCI) is a type of cerebral infarction affecting the posterior circulation supplying one side of the brain.
Posterior circulation stroke syndrome (POCS) refers to the symptoms of a patient who clinically appears t ...
has been associated with deafness.
Pathophysiology
Sensory hearing loss is caused by abnormal structure or function of the
hair cells
Hair cells are the sensory receptors of both the auditory system and the vestibular system in the ears of all vertebrates, and in the lateral line organ of fishes. Through mechanotransduction, hair cells detect movement in their environment.
...
of the
organ of Corti
The organ of Corti, or spiral organ, is the receptor organ for hearing and is located in the mammalian cochlea. This highly varied strip of epithelial cells allows for transduction of auditory signals into nerve impulses' action potential. Transd ...
in the
cochlea
The cochlea is the part of the inner ear involved in hearing. It is a spiral-shaped cavity in the bony labyrinth, in humans making 2.75 turns around its axis, the modiolus. A core component of the cochlea is the Organ of Corti, the sensory org ...
. Neural hearing impairments are consequent upon damage to the eighth
cranial nerve
Cranial nerves are the nerves that emerge directly from the brain (including the brainstem), of which there are conventionally considered twelve pairs. Cranial nerves relay information between the brain and parts of the body, primarily to and ...
(the
vestibulocochlear nerve
The vestibulocochlear nerve or auditory vestibular nerve, also known as the eighth cranial nerve, cranial nerve VIII, or simply CN VIII, is a cranial nerve that transmits sound and equilibrium (balance) information from the inner ear to the bra ...
) or the auditory tracts of the
brainstem
The brainstem (or brain stem) is the posterior stalk-like part of the brain that connects the cerebrum with the spinal cord. In the human brain the brainstem is composed of the midbrain, the pons, and the medulla oblongata. The midbrain is cont ...
. If higher levels of the auditory tract are affected this is known as
central deafness
Central is an adjective usually referring to being in the center of some place or (mathematical) object.
Central may also refer to:
Directions and generalised locations
* Central Africa, a region in the centre of Africa continent, also known as ...
. Central deafness may present as sensorineural deafness but should be distinguishable from the history and audiological testing.
Cochlear dead regions in sensory hearing loss
Hearing impairment may be associated with damage to the hair cells in the cochlea. Sometimes there may be complete loss of function of inner hair cells (IHCs) over a certain region of the cochlea; this is called a "dead region". The region can be defined in terms of the range of characteristic frequencies (CFs) of the IHCs and/or neurons immediately adjacent to the dead region.
Cochlear hair cells
![Cochlea-crosssection](https://upload.wikimedia.org/wikipedia/commons/c/cb/Cochlea-crosssection.svg)
Outer
hair cells
Hair cells are the sensory receptors of both the auditory system and the vestibular system in the ears of all vertebrates, and in the lateral line organ of fishes. Through mechanotransduction, hair cells detect movement in their environment.
...
(OHCs) contribute to the structure of the
Organ of Corti
The organ of Corti, or spiral organ, is the receptor organ for hearing and is located in the mammalian cochlea. This highly varied strip of epithelial cells allows for transduction of auditory signals into nerve impulses' action potential. Transd ...
, which is situated between the
basilar membrane
The basilar membrane is a stiff structural element within the cochlea of the inner ear which separates two liquid-filled tubes that run along the coil of the cochlea, the scala media and the scala tympani. The basilar membrane moves up and down in ...
and the
tectorial membrane
The tectoria membrane (TM) is one of two acellular membranes in the cochlea of the inner ear, the other being the basilar membrane (BM). "Tectorial" in anatomy means forming a cover. The TM is located above the spiral limbus and the spiral organ ...
within the cochlea (See Figure 3). The tunnel of corti, which runs through the Organ of Corti, divides the OHCs and the inner hair cells (IHCs). OHCs are connected to the reticular laminar and the Deiters’ cells. There are roughly twelve thousand OHCs in each human ear, and these are arranged in up to five rows. Each OHC has small tufts of 'hairs', or cilia, on their upper surface known as
stereocilia
Stereocilia (or stereovilli or villi) are non-motile apical cell modifications. They are distinct from cilia and microvilli, but are closely related to microvilli. They form single "finger-like" projections that may be branched, with normal cel ...
, and these are also arranged into rows which are graded in height. There are approximately 140 stereocilia on each OHC.
[Gelfand SA. Hearing: An Introduction to Psychological and Physiological Acoustics. 4th ed. New York: Marcel Dekker; 2004.]
The fundamental role of the OHCs and the IHCs is to function as
sensory receptors
Sensory neurons, also known as afferent neurons, are neurons in the nervous system, that convert a specific type of stimulus, via their receptors, into action potentials or graded potentials. This process is called sensory transduction. The cell ...
. The main function of the IHCs is to transmit sound information via
afferent neurons
Sensory neurons, also known as afferent neurons, are neurons in the nervous system, that convert a specific type of stimulus, via their receptors, into action potentials or graded potentials. This process is called sensory transduction. The cel ...
. They do this by transducing mechanical movements or signals into neural activity. When stimulated, the stereocilia on the IHCs move, causing a flow of electric current to pass through the hair cells. This electric current creates
action potentials
An action potential occurs when the membrane potential of a specific cell location rapidly rises and falls. This depolarization then causes adjacent locations to similarly depolarize. Action potentials occur in several types of animal cells, c ...
within the connected afferent neurons.
OHCs are different in that they actually contribute to the active mechanism of the cochlea. They do this by receiving mechanical signals or vibrations along the basilar membrane, and transducing them into electrochemical signals. The stereocilia found on OHCs are in contact with the tectorial membrane. Therefore, when the basilar membrane moves due to vibrations, the stereocilia bend. The direction in which they bend, dictates the firing rate of the auditory neurons connected to the OHCs.
[Moore BCJ. Cochlear Hearing Loss. London: Whurr Publishers; 1998.]
The bending of the stereocilia towards the
basal body
A basal body (synonymous with basal granule, kinetosome, and in older cytological literature with blepharoplast) is a protein structure found at the base of a eukaryotic undulipodium (cilium or flagellum). The basal body was named by Theodor W ...
of the OHC causes excitation of the hair cell. Thus, an increase in firing rate of the auditory neurons connected to the hair cell occurs. On the other hand, the bending of the stereocilia away from the basal body of the OHC causes inhibition of the hair cell. Thus, a decrease in firing rate of the auditory neurons connected to the hair cell occurs. OHCs are unique in that they are able to contract and expand (electromotility). Therefore, in response to the electrical stimulations provided by the efferent nerve supply, they can alter in length, shape and stiffness. These changes influence the response of the basilar membrane to sound.
It is therefore clear that the OHCs play a major role in the active processes of the cochlea.
The main function of the active mechanism is to finely tune the basilar membrane, and provide it with a high sensitivity to quiet sounds. The active mechanism is dependent on the cochlea being in good physiological condition. However, the cochlea is very susceptible to damage.
=Hair cell damage
=
SNHL is most commonly caused by damage to the OHCs and the IHCs. There are two methods by which they might become damaged. Firstly, the entire hair cell might die. Secondly, the stereocilia might become distorted or destroyed. Damage to the cochlea can occur in several ways, for example by viral infection, exposure to ototoxic chemicals, and intense noise exposure. Damage to the OHCs results in either a less effective active mechanism, or it may not function at all. OHCs contribute to providing a high sensitivity to quiet sounds at a specific range of frequencies (approximately 2–4 kHz). Thus, damage to the OHCs results in the reduction of sensitivity of the basilar membrane to weak sounds. Amplification to these sounds is therefore required, in order for the basilar membrane to respond efficiently. IHCs are less susceptible to damage in comparison to the OHCs. However, if they become damaged, this will result in an overall loss of sensitivity.
Neural tuning curves
Frequency selectivity
![Tuning curve](https://upload.wikimedia.org/wikipedia/commons/5/52/Tuning_curve.svg)
The
traveling wave
In physics, mathematics, and related fields, a wave is a propagating dynamic disturbance (change from equilibrium) of one or more quantities. Waves can be periodic, in which case those quantities oscillate repeatedly about an equilibrium (res ...
along the basilar membrane peaks at different places along it, depending on whether the sound is low or high frequency. Due to the mass and
stiffness
Stiffness is the extent to which an object resists deformation in response to an applied force.
The complementary concept is flexibility or pliability: the more flexible an object is, the less stiff it is.
Calculations
The stiffness, k, of a b ...
of the basilar membrane, low frequency waves peak in the apex, while high frequency sounds peak in the basal end of the cochlea.
Therefore, each position along the basilar membrane is finely tuned to a particular frequency. These specifically tuned frequencies are referred to as characteristic frequencies (CF).
If a sound entering the ear is displaced from the characteristic frequency, then the strength of response from the basilar membrane will progressively lessen. The fine tuning of the basilar membrane is created by the input of two separate mechanisms. The first mechanism being a linear passive mechanism, which is dependent on the mechanical structure of the basilar membrane and its surrounding structures. The second mechanism is a non-linear active mechanism, which is primarily dependent on the functioning of the OHCs, and also the general physiological condition of the cochlea itself. The base and apex of the basilar membrane differ in stiffness and width, which cause the basilar membrane to respond to varying frequencies differently along its length. The base of the basilar membrane is narrow and stiff, resulting in it responding best to high frequency sounds. The apex of the basilar membrane is wider and much less stiff in comparison to the base, causing it to respond best to low frequencies.
This selectivity to certain frequencies can be illustrated by neural tuning curves. These demonstrate the frequencies a fiber responds to, by showing threshold levels
(dB SPL) of
auditory nerve
The cochlear nerve (also auditory nerve or acoustic nerve) is one of two parts of the vestibulocochlear nerve, a cranial nerve present in amniotes, the other part being the vestibular nerve. The cochlear nerve carries auditory sensory information ...
fibers as a function of different frequencies. This demonstrates that auditory nerve fibers respond best, and hence have better thresholds at the fiber's characteristic frequency and frequencies immediately surrounding it. The basilar membrane is said to be ‘sharply tuned’ due to the sharp ‘V’ shaped curve, with its ‘tip’ centered at the auditory fibers characteristic frequency. This shape shows how few frequencies a fiber responds to. If it were a broader ‘V’ shape, it would be responding to more frequencies (See Figure 4).
IHC vs OHC hearing loss
![Tuning curve of OHC front row loss and IHC loss](https://upload.wikimedia.org/wikipedia/commons/b/ba/Tuning_curve_of_OHC_front_row_loss_and_IHC_loss.svg)
A normal neural tuning curve is characterised by a broadly tuned low frequency ‘tail’, with a finely tuned middle frequency ‘tip’. However, where there is partial or complete damage to the OHCs, but with unharmed IHCs, the resulting tuning curve would show the elimination of sensitivity at the quiet sounds. I.e. where the neural tuning curve would normally be most sensitive (at the ‘tip’) (See Figure 5).
Where both the OHCs and the IHCs are damaged, the resulting neural tuning curve would show the elimination of sensitivity at the ‘tip'. However, due to IHC damage, the whole tuning curve becomes raised, giving a loss of sensitivity across all frequencies (See Figure 6). It is only necessary for the first row of OHCs to be damaged for the elimination of the finely tuned ‘tip’ to occur. This supports the idea that the incidence of OHC damage and thus a loss of sensitivity to quiet sounds, occurs more than IHC loss.
When the IHCs or part of the basilar membrane are damaged or destroyed, so that they no longer function as transducers, the result is a ‘dead region’. Dead regions can be defined in terms of the characteristic frequencies of the IHC, related to the specific place along the basilar membrane where the dead region occurs. Assuming that there has been no shift in the characteristic frequencies relating to certain regions of the basilar membrane, due to the damage of OHCs. This often occurs with IHC damage. Dead regions can also be defined by the anatomical place of the non-functioning IHC (such as an “apical dead region”), or by the characteristic frequencies of the IHC adjacent to the dead region.
Dead region audiometry
Pure tone audiometry (PTA)
Dead regions affect audiometric results, but perhaps not in the way expected. For example, it may be expected that thresholds would not be obtained at the frequencies within the dead region, but would be obtained at frequencies adjacent to the dead region. Therefore, assuming normal hearing exists around the dead region, it would produce an
audiogram
An audiogram is a graph that shows the audible threshold for standardized frequencies as measured by an audiometer. The Y axis represents intensity measured in decibels and the X axis represents frequency measured in hertz. The threshold of hear ...
that has a dramatically steep slope between the frequency where a threshold is obtained, and the frequency where a threshold cannot be obtained due to the dead region.
![Basilar membrane 2B](https://upload.wikimedia.org/wikipedia/commons/a/ad/Basilar_membrane_2B.jpg)
However, it appears that this is not the case. Dead regions cannot be clearly found via PTA
audiogram
An audiogram is a graph that shows the audible threshold for standardized frequencies as measured by an audiometer. The Y axis represents intensity measured in decibels and the X axis represents frequency measured in hertz. The threshold of hear ...
s. This may be because although the neurons innervating the dead region, cannot react to vibration at their characteristic frequency. If the basilar membrane vibration is large enough, neurons tuned to different characteristic frequencies such as those adjacent to the dead region, will be stimulated due to the spread of excitation. Therefore, a response from the patient at the test frequency will be obtained. This is referred to as “off-place listening”, and is also known as ‘off-frequency listening’. This will lead to a false threshold being found. Thus, it appears a person has better hearing than they actually do, resulting in a dead region being missed. Therefore, using PTA alone, it is impossible to identify the extent of a dead region (See Figure 7 and 8).
Consequently, how much is an audiometric threshold affected by a tone with its frequency within a dead region? This depends on the location of the dead region. Thresholds at low frequency dead regions, are more inaccurate than those at higher frequency dead regions. This has been attributed to the fact that excitation due to vibration of the basilar membrane spreads upwards from the apical regions of the basilar membrane, more than excitation spreads downwards from higher frequency basal regions of the cochlea. This pattern of the spread of excitation is similar to the ‘upward spread of masking’ phenomenon. If the tone is sufficiently loud to produce enough excitation at the normally functioning area of the cochlea, so that it is above that areas threshold. The tone will be detected, due to off-frequency listening which results in a misleading threshold.
To help to overcome the issue of PTA producing inaccurate thresholds within dead regions, masking of the area beyond the dead region that is being stimulated can be used. This means that the threshold of the responding area is sufficiently raised, so that it cannot detect the spread of excitation from the tone. This technique has led to the suggestion that a low frequency dead region may be related to a loss of 40-50 dB. However, as one of the aims of PTA is to determine whether or not there is a dead region, it may be difficult to assess which frequencies to mask without the use of other tests.
Based on research it has been suggested that a low frequency dead region may produce a relatively flat loss, or a very gradually sloping loss towards the higher frequencies. As the dead region will be less detectable due to the upward spread of excitation. Whereas, there may be a more obvious steeply sloping loss at high frequencies for a high frequency dead region. Although it is likely that the slope represents the less pronounced downward spread of excitation, rather than accurate thresholds for those frequencies with non-functioning hair cells. Mid-frequency dead regions, with a small range, appear to have less effect on the patient's ability to hear in everyday life, and may produce a notch in the PTA thresholds.
Although it is clear that PTA is not the best test to identify a dead region.
Psychoacoustic tuning curves (PTC) and threshold equalizing noise (TEN) tests
Although some debate continues regarding the reliability of such tests,
it has been suggested that psychoacoustic tuning curves (PTCs) and threshold-equalising noise (TEN) results may be useful in detecting dead regions, rather than PTA. PTCs are similar to neural tuning curves. They illustrate the level of a masker (dB SPL) tone at threshold, as a function of deviation from center frequency (Hz).
They are measured by presenting a fixed low intensity pure tone while also presenting a narrow-band masker, with a varying center frequency. The masker level is varied, so that the level of masker needed to just mask the test signal is found for the masker at each center frequency. The tip of the PTC is where the masker level needed to just mask the test signal is the lowest. For normal hearing people this is when the masker center frequency is closest to the frequency of the test signal (See Figure 9).
In the case of dead regions, when the test signal lies within the boundaries of a dead region, the tip of the PTC will be shifted to the edge of the dead region, to the area that is still functioning and detecting the spread of excitation from the signal. In the case of a low frequency dead region, the tip is shifted upwards indicating a low frequency dead region starting at the tip of the curve. For a high frequency dead region, the tip is shifted downwards from the signal frequency to the functioning area below the dead region.
However, the traditional method of obtaining PTCs is not practical for clinical use, and it has been argued that TENs are not accurate enough.
A fast method for finding PTCs has been developed and it may provide the solution. However, more research to validate this method is required, before it can be accepted clinically.
Perceptual consequences of a dead region
Audiogram
An audiogram is a graph that shows the audible threshold for standardized frequencies as measured by an audiometer. The Y axis represents intensity measured in decibels and the X axis represents frequency measured in hertz. The threshold of hear ...
configurations are not good indicators of how a dead region will affect a person functionally, mainly due to individual differences.
For example, a sloping audiogram is often present with a dead region, due to the spread of excitation. However, the individual may well be affected differently from someone with a corresponding sloped audiogram caused by partial damage to hair cells rather than a dead region. They will perceive sounds differently, yet the audiogram suggests that they have the same degree of loss. Huss and Moore investigated how hearing impaired patients perceive pure tones, and found that they perceive tones as noisy and distorted, more (on average) than a person without a hearing impairment. However, they also found that the perception of tones as being like noise, was not directly related to frequencies within the dead regions, and was therefore not an indicator of a dead region. This therefore suggests that audiograms, and their poor representation of dead regions, are inaccurate predictors of a patient's perception of pure tone quality.
Research by Kluk and Moore has shown that dead regions may also affect the patient's perception of frequencies beyond the dead regions. There is an enhancement in the ability to distinguish between tones that differ very slightly in frequency, in regions just beyond the dead regions compared to tones further away. An explanation for this may be that cortical re-mapping has occurred. Whereby, neurons which would normally be stimulated by the dead region, have been reassigned to respond to functioning areas near it. This leads to an over-representation of these areas, resulting in an increased perceptual sensitivity to small frequency differences in tones.
Vestibulocochlear nerve pathology
* congenital deformity of the internal auditory canal,
* neoplastic and pseudo-neoplastic lesions, with special detailed emphasis on schwannoma of the eighth cranial nerve (acoustic neuroma),
* non-neoplastic Internal Auditory Canal/CerebelloPontine Angle pathology, including vascular loops,
Diagnosis
Case history
Before examination, a case history provides guidance about the context of the hearing loss.
* major concern
* pregnancy and childbirth information
* medical history
* development history
* family history
Otoscopy
Direct examination of the external canal and tympanic membrane (ear drum) with an
otoscope
An otoscope or auriscope is a medical device which is used to look into the ears. Health care providers use otoscopes to screen for illness during regular check-ups and also to investigate ear symptoms. An otoscope potentially gives a view of ...
, a medical device inserted into the ear canal that uses light to examine the condition of the external ear and tympanic membrane, and
middle ear through the semi-translucent membrane.
Differential testing
Differential testing is most useful when there is unilateral hearing loss, and distinguishes conductive from sensorineural loss. These are conducted with a low frequency tuning fork, usually 512 Hz, and contrast measures of air and bone conducted sound transmission.
*
Weber test
Weber (, or ; German: ) is a surname of German origin, derived from the noun meaning " weaver". In some cases, following migration to English-speaking countries, it has been anglicised to the English surname 'Webber' or even 'Weaver'.
Notable pe ...
, in which a tuning fork is touched to the midline of the forehead, localizes to the normal
ear
An ear is the organ that enables hearing and, in mammals, body balance using the vestibular system. In mammals, the ear is usually described as having three parts—the outer ear, the middle ear and the inner ear. The outer ear consists of ...
in people with unilateral sensorineural hearing loss.
*
Rinne test Rinne may refer to:
People
*Aleksi Rinne (1899–1974), Finnish smallholder and politician
*Antti Rinne (b. 1962), Finnish politician
*Esa Rinne (b. 1943), Finnish athlete
*Fanny Rinne (b. 1980), field hockey midfielder from Germany
*Fred Rinne (b. ...
, which tests air conduction ''vs.'' bone conduction is positive, because both bone and air conduction are reduced equally.
* less common
Bing
Bing most often refers to:
* Bing Crosby (1903–1977), American singer
* Microsoft Bing, a web search engine
Bing may also refer to:
Food and drink
* Bing (bread), a Chinese flatbread
* Bing (soft drink), a UK brand
* Bing cherry, a variety ...
and Schwabach variants of the Rinne test.
* absolute bone conduction (ABC) test.
''Table 1''. A table comparing sensorineural to
conductive hearing loss
Conductive hearing loss (CHL) occurs when there is a problem transferring sound waves anywhere along the pathway through the outer ear, tympanic membrane (eardrum), or middle ear ( ossicles). If a conductive hearing loss occurs in conjunction wi ...
Other, more complex, tests of auditory function are required to distinguish the different types of hearing loss. Bone conduction thresholds can differentiate sensorineural hearing loss from conductive hearing loss. Other tests, such as oto-acoustic emissions, acoustic stapedial reflexes, speech audiometry and evoked response audiometry are needed to distinguish sensory, neural and auditory processing hearing impairments.
Tympanometry
A
tympanogram
Tympanometry is an acoustic evaluation of the condition of the middle ear eardrum (tympanic membrane) and the conduction bones by creating variations of air pressure in the ear canal.
Tympanometry is an objective test of middle-ear function. It is ...
is the result of a test with a tympanometer. It tests the function of the middle ear and mobility of the eardrum. It can help identify conductive hearing loss due to disease of the middle ear or eardrum from other kinds of hearing loss including SNHL.
Audiometry
An
audiogram
An audiogram is a graph that shows the audible threshold for standardized frequencies as measured by an audiometer. The Y axis represents intensity measured in decibels and the X axis represents frequency measured in hertz. The threshold of hear ...
is the result of a hearing test. The most common type of hearing test is
pure tone audiometry
Pure-tone audiometry is the main hearing test used to identify hearing threshold levels of an individual, enabling determination of the degree, type and configuration of a hearing loss and thus providing a basis for diagnosis and management. Pur ...
(PTA). It charts the thresholds of hearing sensitivity at a selection of standard frequencies between 250 and 8000 Hz. There is also high frequency pure tone audiometry which tests frequencies from 8000-20,000 Hz. PTA can be used to differentiate between conductive hearing loss, sensorineural hearing loss and mixed hearing loss. A hearing loss can be described by its degree i.e. mild, moderate, severe or profound, or by its shape i.e. high frequency or sloping, low frequency or rising, notched, U-shaped or 'cookie-bite', peaked or flat.
There are also other kinds of audiometry designed to test hearing acuity rather than sensitivity (speech audiometry), or to test
auditory neural pathway transmission (evoked response audiometry).
Magnetic resonance imaging
MRI scans can be used to identify gross structural causes of hearing loss. They are used for congenital hearing loss when changes to the shape of the inner ear or nerve of hearing may help diagnosis of the cause of the hearing loss. They are also useful in cases where a tumour is suspected or to determine the degree of damage in a hearing loss caused by bacterial infection or auto-immune disease. Scanning is of no value in age-related deafness.
Prevention
Presbycusis is the leading cause of SNHL and is progressive and nonpreventable, and at this time, we do not have either somatic or gene therapy to counter heredity-related SNHL. But other causes of acquired SNHL are largely preventable, especially nosocusis type causes. This would involve avoiding environmental noise, and traumatic noise such as rock concerts and nightclubs with loud music. Use of noise attenuation measures like
ear plugs
An earplug is a device that is inserted in the ear canal to protect the user's ears from loud noises, intrusion of water, foreign bodies, dust or excessive wind. Since they reduce the sound volume, earplugs are often used to help prevent hearing ...
is an alternative, as well as learning about the noise levels one is exposed to. Currently, severa
accurate sound level measurementapps exist.
can also help manage risk from loud exposures.
Treatment
Treatment modalities fall into three categories: pharmacological, surgical, and management. As SNHL is a physiologic degradation and considered permanent, there are as of this time, no approved or recommended treatments.
There have been significant advances in identification of human deafness genes and elucidation of their cellular mechanisms as well as their physiological function in mice. Nevertheless, pharmacological treatment options are very limited and clinically unproven. Such pharmaceutical treatments as are employed are palliative rather than curative, and addressed to the underlying cause if one can be identified, in order to avert progressive damage.
Profound or total hearing loss may be amenable to management by
cochlear implant
A cochlear implant (CI) is a surgically implanted neuroprosthesis that provides a person who has moderate-to-profound sensorineural hearing loss with sound perception. With the help of therapy, cochlear implants may allow for improved speech und ...
s, which stimulate
cochlear nerve
The cochlear nerve (also auditory nerve or acoustic nerve) is one of two parts of the vestibulocochlear nerve, a cranial nerve present in amniotes, the other part being the vestibular nerve. The cochlear nerve carries auditory sensory information ...
endings directly. A cochlear implant is surgical implantation of a battery powered electronic medical device in the inner ear. Unlike
hearing aid
A hearing aid is a device designed to improve hearing by making sound audible to a person with hearing loss. Hearing aids are classified as medical devices in most countries, and regulated by the respective regulations. Small audio amplifiers su ...
s, which make sounds louder, cochlear implants do the work of damaged parts of the inner ear (cochlea) to provide sound signals to the brain. These consist of both internal implanted electrodes and magnets and external components. The quality of sound is different than natural hearing but may enable the recipient to better recognize speech and environmental sounds.
Because of risk and expense, such surgery is reserved for cases of severe and disabling hearing impairment
Management of sensorineural hearing loss involves employing strategies to support existing hearing such as lip-reading, enhanced communication etc. and amplification using
hearing aid
A hearing aid is a device designed to improve hearing by making sound audible to a person with hearing loss. Hearing aids are classified as medical devices in most countries, and regulated by the respective regulations. Small audio amplifiers su ...
s. Hearing aids are specifically tuned to the individual hearing loss to give maximum benefit.
Research
Pharmaceuticals
*
Antioxidant
Antioxidants are compounds that inhibit oxidation, a chemical reaction that can produce free radicals. This can lead to polymerization and other chain reactions. They are frequently added to industrial products, such as fuels and lubricant ...
vitamin
A vitamin is an organic molecule (or a set of molecules closely related chemically, i.e. vitamers) that is an Nutrient#Essential nutrients, essential micronutrient that an organism needs in small quantities for the proper functioning of its ...
s – Researchers at the University of Michigan report that a combination of high doses of vitamins A, C, and E, and Magnesium, taken one hour before noise exposure and continued as a once-daily treatment for five days, was very effective at preventing permanent noise-induced hearing loss in animals.
*
Tanakan
''Ginkgo biloba'', commonly known as ginkgo or gingko ( ), also known as the maidenhair tree, is a species of tree native to China. It is the last living species in the order Ginkgoales, which first appeared over 290 million years ago. Fossils ...
– a brand name for an international prescription drug extract of Ginkgo biloba. It is classified as a vasodilator. Among its research uses is treatment of sensorineural deafness and tinnitus presumed to be of vascular origin.
*
Coenzyme Q10
Coenzyme Q, also known as ubiquinone and marketed as CoQ10, is a coenzyme family that is ubiquitous in animals and most bacteria (hence the name ubiquinone). In humans, the most common form is coenzyme Q10 or ubiquinone-10.
It is a 1,4-benzoq ...
– a substance similar to a vitamin, with antioxidant properties. It is made in the body, but levels fall with age.
[Coenzyme Q10(CoQ10) supports mitochondrial function and has significant antioxidant properties (Quinzii 2010). Animal studies have found that supplementation with CoQ10 reduced noise-induced hearing loss and the death of hair cells (Hirose 2008; Fetoni 2009, 2012). Human studies have also yielded promising results, as 160-600 mg of CoQ10 daily was found to reduce hearing loss in people with sudden sensorineural hearing loss and presbycusis (Ahn 2010; Salami 2010; Guastini 2011). Also, a small preliminary trial found that CoQ10 supplementation alleviated tinnitus in those whose CoQ10 blood levels were initially low (Khan 2007). Another small trial found CoQ10 may slow progression of hearing loss associated with a mitochondrial genetic mutation (Angeli 2005).]
*
Ebselen
Ebselen (also called PZ 51, DR3305, and SPI-1005), is a synthetic organoselenium drug molecule with anti-inflammatory, anti-oxidant and cytoprotective activity. It acts as a mimic of glutathione peroxidase and can also react with peroxynitrite. It ...
, a synthetic drug molecule that mimics
glutathione peroxidase
Glutathione peroxidase (GPx) () is the general name of an enzyme family with peroxidase activity whose main biological role is to protect the organism from oxidative damage. The biochemical function of glutathione peroxidase is to reduce lipid h ...
(GPx), a critical enzyme in the inner ear that protects it from damage caused by loud sounds or noise
Stem cell and gene therapy
Hair cell regeneration using
stem cell
In multicellular organisms, stem cells are undifferentiated or partially differentiated cells that can differentiate into various types of cells and proliferate indefinitely to produce more of the same stem cell. They are the earliest type o ...
and
gene therapy
Gene therapy is a medical field which focuses on the genetic modification of cells to produce a therapeutic effect or the treatment of disease by repairing or reconstructing defective genetic material. The first attempt at modifying human DN ...
is years or decades away from being clinically feasible. However, studies are currently underway on the subject, with the first
FDA
The United States Food and Drug Administration (FDA or US FDA) is a federal agency of the Department of Health and Human Services. The FDA is responsible for protecting and promoting public health through the control and supervision of food s ...
-approved trial beginning in February 2012.
Sudden sensorineural hearing loss
Sudden sensorineural hearing loss (SSHL or SSNHL), commonly known as sudden deafness, occurs as an unexplained, rapid loss of hearing—usually in one ear—either at once or over several days. Nine out of ten people with SSHL lose hearing in only one ear. It should be considered a medical emergency. Delaying diagnosis and treatment may render treatment less effective or ineffective.
Experts estimate that SSHL strikes one person per 100 every year, typically adults in their 40s and 50s. The actual number of new cases of SSHL each year could be much higher because the condition often goes undiagnosed.
Presentation
Many people notice that they have SSHL when they wake up in the morning. Others first notice it when they try to use the deafened ear, such as when they use a phone. Still others notice a loud, alarming "pop" just before their hearing disappears. People with sudden deafness often become dizzy, have ringing in their ears (tinnitus), or both.
Diagnosis
SSHL is diagnosed via pure tone audiometry. If the test shows a loss of at least 30 dB in three adjacent frequencies, the hearing loss is diagnosed as SSHL. For example, a hearing loss of 30 dB would make conversational speech sound more like a whisper.
Causes
Only 10 to 15 percent of the cases diagnosed as SSHL have an identifiable cause. Most cases are classified as
idiopathic
An idiopathic disease is any disease with an unknown cause or mechanism of apparent wikt:spontaneous, spontaneous origin. From Ancient Greek, Greek ἴδιος ''idios'' "one's own" and πάθος ''pathos'' "suffering", ''idiopathy'' means approxi ...
, also called sudden idiopathic hearing loss (SIHL) and idiopathic sudden sensorineural hearing loss (ISSHL or ISSNHL) The majority of evidence points to some type of inflammation in the inner ear as the most common cause of SSNHL.
*
Viral
Viral means "relating to viruses" (small infectious agents).
Viral may also refer to:
Viral behavior, or virality
Memetic behavior likened that of a virus, for example:
* Viral marketing, the use of existing social networks to spread a marke ...
– The swelling may be due to a virus. A herpes type virus is believed to be the most common cause of sudden sensorineural hearing loss. The herpes virus lies dormant in our bodies and reactivates for an unknown reason.
* Vascular
ischemia
Ischemia or ischaemia is a restriction in blood supply to any tissue, muscle group, or organ of the body, causing a shortage of oxygen that is needed for cellular metabolism (to keep tissue alive). Ischemia is generally caused by problems wi ...
of the
inner ear
The inner ear (internal ear, auris interna) is the innermost part of the vertebrate ear. In vertebrates, the inner ear is mainly responsible for sound detection and balance. In mammals, it consists of the bony labyrinth, a hollow cavity in the ...
or cranial nerve VIII (CN8)
*
Perilymph fistula
A labyrinthine fistula is an abnormal opening in the inner ear. This can result in leakage of the perilymph into the middle ear. This includes specifically a perilymph fistula (PLF), an abnormal connection between the fluid of the inner ear and the ...
, usually due to a rupture of the
round
Round or rounds may refer to:
Mathematics and science
* The contour of a closed curve or surface with no sharp corners, such as an ellipse, circle, rounded rectangle, cant, or sphere
* Rounding, the shortening of a number to reduce the number ...
or
oval
An oval () is a closed curve in a plane which resembles the outline of an egg. The term is not very specific, but in some areas (projective geometry, technical drawing, etc.) it is given a more precise definition, which may include either one or ...
windows and the leakage of
perilymph
Perilymph is an extracellular fluid located within the inner ear. It is found within the scala tympani and scala vestibuli of the cochlea. The ionic composition of perilymph is comparable to that of plasma and cerebrospinal fluid. The major ca ...
. The patient will usually also experience
vertigo
Vertigo is a condition where a person has the sensation of movement or of surrounding objects moving when they are not. Often it feels like a spinning or swaying movement. This may be associated with nausea, vomiting, sweating, or difficulties w ...
or
imbalance
A balance disorder is a disturbance that causes an individual to feel unsteady, for example when standing or walking. It may be accompanied by feelings of giddiness, or wooziness, or having a sensation of movement, spinning, or floating. Balance ...
. A history of trauma is usually present and changes to hearing or vertigo occur with alteration in intracranial pressure such as with straining; lifting, blowing etc.
*
Autoimmune
In immunology, autoimmunity is the system of immune responses of an organism against its own healthy cells, tissues and other normal body constituents. Any disease resulting from this type of immune response is termed an "autoimmune disease". P ...
– can be due to an autoimmune illness such as
systemic lupus erythematosus
Lupus, technically known as systemic lupus erythematosus (SLE), is an autoimmune disease in which the body's immune system mistakenly attacks healthy tissue in many parts of the body. Symptoms vary among people and may be mild to severe. Comm ...
,
granulomatosis with polyangiitis
Granulomatosis with polyangiitis (GPA), previously known as Wegener's granulomatosis (WG), is a rare long-term systemic disorder that involves the formation of granulomas and inflammation of blood vessels (vasculitis). It is a form of vasculitis ...
Treatment
Hearing loss completely recovers in around 35-39% of patients with SSNHL, usually within one to two weeks from onset. Eighty-five percent of those who receive treatment from an
otolaryngologist
Otorhinolaryngology ( , abbreviated ORL and also known as otolaryngology, otolaryngology–head and neck surgery (ORL–H&N or OHNS), or ear, nose, and throat (ENT)) is a surgical subspeciality within medicine that deals with the surgical a ...
(sometimes called an ENT surgeon) will recover some of their hearing.
*
vitamin
A vitamin is an organic molecule (or a set of molecules closely related chemically, i.e. vitamers) that is an Nutrient#Essential nutrients, essential micronutrient that an organism needs in small quantities for the proper functioning of its ...
s and
antioxidant
Antioxidants are compounds that inhibit oxidation, a chemical reaction that can produce free radicals. This can lead to polymerization and other chain reactions. They are frequently added to industrial products, such as fuels and lubricant ...
s
*
vasodilator
Vasodilation is the widening of blood vessels. It results from relaxation of smooth muscle cells within the vessel walls, in particular in the large veins, large arteries, and smaller arterioles. The process is the opposite of vasoconstriction, ...
s
*
betahistine
Betahistine, sold under the brand name Serc among others, is an anti-vertigo medication. It is commonly prescribed for balance disorders or to alleviate vertigo symptoms. It was first registered in Europe in 1970 for the treatment of Ménière' ...
(Betaserc), an anti-
vertigo
Vertigo is a condition where a person has the sensation of movement or of surrounding objects moving when they are not. Often it feels like a spinning or swaying movement. This may be associated with nausea, vomiting, sweating, or difficulties w ...
drug
*
hyperbaric oxygen
Hyperbaric medicine is medical treatment in which an ambient pressure greater than sea level atmospheric pressure is a necessary component. The treatment comprises hyperbaric oxygen therapy (HBOT), the medical use of oxygen at an ambient pressure ...
*
rheologic agents that reduce blood viscosity (such as
hydroxyethyl starch
Hydroxyethyl starch (HES/HAES), sold under the brand name Voluven among others, is a nonionic starch derivative, used as a volume expander in intravenous therapy. The use of HES on critically ill patients is associated with an increased risk of d ...
,
dextran
Dextran is a complex branched glucan (polysaccharide derived from the condensation of glucose), originally derived from wine. IUPAC defines dextrans as "Branched poly-α-d-glucosides of microbial origin having glycosidic bonds predominantly C-1 ...
and
pentoxifylline
Pentoxifylline, also known as oxpentifylline, is a xanthine derivative used as a drug to treat muscle pain in people with peripheral artery disease. It is generic and sold under many brand names worldwide.Drugs.codrugs.com international listings ...
)
*
anti-inflammatory
Anti-inflammatory is the property of a substance or treatment that reduces inflammation or swelling. Anti-inflammatory drugs, also called anti-inflammatories, make up about half of analgesics. These drugs remedy pain by reducing inflammation as o ...
agents, primarily oral
corticosteroid
Corticosteroids are a class of steroid hormones that are produced in the adrenal cortex of vertebrates, as well as the synthetic analogues of these hormones. Two main classes of corticosteroids, glucocorticoids and mineralocorticoids, are involv ...
s such as
prednisone
Prednisone is a glucocorticoid medication mostly used to immunosuppressive drug, suppress the immune system and decrease inflammation in conditions such as asthma, COPD, and rheumatologic diseases. It is also used to treat high blood calcium ...
,
methylprednisone
* Intratympanic administration – Gel formulations are under investigation to provide more consistent drug delivery to the inner ear.
Local drug delivery can be accomplished through intratympanic administration, a minimally invasive procedure where the ear drum is anesthetized and a drug is administered into the middle ear. From the middle ear, a drug can diffuse across the round window membrane into the inner ear.
Intratympanic administration of steroids may be effective for sudden sensorineural hearing loss for some patients, but high quality clinical data has not been generated. Intratympanic administration of an anti-apoptotic peptide (JNK inhibitor) is currently being evaluated in late-stage clinical development.
Epidemiology
General hearing loss affects close to 10% of the global population. In the United States alone, it is expected that 13.5 million Americans have sensorineural hearing loss. Of those with sensorineural hearing loss, approximately 50% are
congenital
A birth defect, also known as a congenital disorder, is an abnormal condition that is present at birth regardless of its cause. Birth defects may result in disabilities that may be physical, intellectual, or developmental. The disabilities can ...
ly related. The other 50% are due to maternal or fetal infections, post-natal infections, viral infections due to rubella or
cytomegalovirus
''Cytomegalovirus'' (''CMV'') (from ''cyto-'' 'cell' via Greek - 'container' + 'big, megalo-' + -''virus'' via Latin 'poison') is a genus of viruses in the order ''Herpesvirales'', in the family ''Herpesviridae'', in the subfamily ''Betaherpe ...
,
ototoxic
Ototoxicity is the property of being toxic to the ear (''oto-''), specifically the cochlea or auditory nerve and sometimes the vestibular system, for example, as a side effect of a drug. The effects of ototoxicity can be reversible and temporary, ...
drugs, exposure to loud sounds, severe head trauma, and premature births
Of the genetically related sensorineural hearing loss cases, 75% are
autosomal recessive
In genetics, dominance is the phenomenon of one variant (allele) of a gene on a chromosome masking or overriding the effect of a different variant of the same gene on the other copy of the chromosome. The first variant is termed dominant and t ...
, 15-20%
autosomal dominant
In genetics, dominance is the phenomenon of one variant (allele) of a gene on a chromosome masking or overriding the effect of a different variant of the same gene on the other copy of the chromosome. The first variant is termed dominant and t ...
, and 1-3% sex-linked. While the specific gene and protein is still unknown, mutations in the connexin 26 gene near the DFNB1 locus of chromosome 13
are thought to account for most of the autosomal recessive genetic-related sensorineural hearing loss
At least 8.5 per 1000 children younger than age 18 have sensorineural hearing loss. General hearing loss is proportionally related to age. At least 314 per 1000 people older than age 65 have hearing loss. Several risk factors for sensorineural hearing loss have been studied over the past decade. Osteoporosis,
stapedectomy surgery, pneumococcal vaccinations, mobile phone users, and
hyperbilirubinemia
Bilirubin (BR) (Latin for "red bile") is a red-orange compound that occurs in the normal catabolic pathway that breaks down heme in vertebrates. This catabolism is a necessary process in the body's clearance of waste products that arise from the ...
at birth are among some of the known risk factors.
See also
*
Conductive hearing loss
Conductive hearing loss (CHL) occurs when there is a problem transferring sound waves anywhere along the pathway through the outer ear, tympanic membrane (eardrum), or middle ear ( ossicles). If a conductive hearing loss occurs in conjunction wi ...
, hearing loss caused primarily by conditions in the middle ear
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Cortical deafness
Cortical deafness is a rare form of sensorineural hearing loss caused by damage to the primary auditory cortex. Cortical deafness is an auditory disorder where the patient is unable to hear sounds but has no apparent damage to the anatomy of the ea ...
, another kind of nerve deafness
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Hearing loss
Hearing loss is a partial or total inability to Hearing, hear. Hearing loss may be present at birth or acquired at any time afterwards. Hearing loss may occur in one or both ears. In children, hearing problems can affect the ability to Language ...
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Inner ear
The inner ear (internal ear, auris interna) is the innermost part of the vertebrate ear. In vertebrates, the inner ear is mainly responsible for sound detection and balance. In mammals, it consists of the bony labyrinth, a hollow cavity in the ...
, the innermost portion of the ear containing the sensorineural apparatus of hearing
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Otosclerosis
Otosclerosis is a condition of the middle ear where portions of the dense enchondral layer of the bony labyrinth remodel into one or more lesions of irregularly-laid spongy bone. As the lesions reach the stapes the bone is resorbed, then hardene ...
, a sometimes associated or predecessor
conductive hearing loss
Conductive hearing loss (CHL) occurs when there is a problem transferring sound waves anywhere along the pathway through the outer ear, tympanic membrane (eardrum), or middle ear ( ossicles). If a conductive hearing loss occurs in conjunction wi ...
condition of the middle ear
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Tinnitus
Tinnitus is the perception of sound when no corresponding external sound is present. Nearly everyone experiences a faint "normal tinnitus" in a completely quiet room; but it is of concern only if it is bothersome, interferes with normal hearin ...
, ringing in the ears, a common accompaniment of SNHL
Notes
References
38.Ghazavi H,Kargoshaei A-A,Jamshidi-Koohsari M,"Investigation of vitamin D levels in patients with Sudden Sensory-Neural Hearing Loss and its effect on treatment",American journal of otolaryngology and head and neck medicine and suegery,November 2019
https://doi.org/10.1016/j.amjoto.2019.102327
External links
Hearing Loss Web
{{DEFAULTSORT:Sensorineural Hearing Loss
Hearing loss