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Visual impairment, also known as vision impairment or vision loss, is a decreased ability to see to a degree that causes problems not fixable by usual means, such as
glasses Glasses, also known as eyeglasses or spectacles, are vision eyewear, consisting of glass or hard plastic lenses mounted in a frame that holds them in front of a person's eyes, typically utilizing a bridge over the nose and hinged arms (known ...

glasses
. Some also include those who have a decreased ability to see because they do not have access to glasses or
contact lens One-day disposable contact lenses with blue handling tint in blister-pack packaging Contact lenses, or simply contacts, are thin lenses A lens is a transmissive optics, optical device that focuses or disperses a light beam by means of ...

contact lens
es. Visual impairment is often defined as a best corrected
visual acuity Visual acuity (VA) commonly refers to the clarity of vision Vision or The Vision may refer to: Perception Optical perception * Visual perception Visual perception is the ability to interpret the surrounding environment using light in t ...
of worse than either 20/40 or 20/60. The term blindness is used for complete or nearly complete vision loss. Visual impairment may cause difficulties with normal daily activities such as reading and walking without adaptive training and equipment. The most common causes of visual impairment globally are uncorrected
refractive errors Refractive error, also known as refraction error, is a problem with focusing light Light or visible light is electromagnetic radiation within the portion of the electromagnetic spectrum that can be visual perception, perceived by the human ...

refractive errors
(43%),
cataract A cataract is a cloudy area in the lens A lens is a transmissive optical device that focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens con ...
s (33%), and
glaucoma Glaucoma is a group of eye diseases which result in damage to the optic nerve The optic nerve, also known as cranial nerve II, or simply as CN II, is a paired cranial nerve Cranial nerves are the nerve A nerve is an enclosed, cable-li ...

glaucoma
(2%). Refractive errors include
near-sightedness Near-sightedness, also known as short-sightedness and myopia, is an eye disorder where light focuses in front of, instead of on, the retina The retina (from la, rete) is the innermost, light-sensitive layer of tissue of the eye of most ve ...
,
far-sightedness Far-sightedness, also known as long-sightedness, hypermetropia, or hyperopia, is a condition of the eye where distant objects are seen clearly but near objects appear blurred. This blurred effect is due to incoming light being focused behind, in ...
,
presbyopia Presbyopia is physiological insufficiency of accommodation associated with the aging of the eye that results in progressively worsening ability to focus clearly on close objects. Symptoms include difficulty reading small print, having to hold re ...

presbyopia
, and
astigmatism Astigmatism is a type of refractive error Refractive error, also known as refraction error, is a problem with focus (optics), focusing light accurately on the retina due to the shape of the human eye, eye and or cornea. The most common types ...

astigmatism
. Cataracts are the most common cause of blindness. Other disorders that may cause visual problems include
age-related macular degeneration Macular degeneration, also known as age-related macular degeneration (AMD or ARMD), is a medical condition which may result in blurred or no vision in the center of the visual field. Early on there are often no symptoms. Over time, however, som ...
,
diabetic retinopathy Diabetic retinopathy, also known as diabetic eye disease (DED), is a medical condition in which damage occurs to the retina due to diabetes mellitus. It is a leading cause of blindness Visual impairment, also known as vision impairment or vis ...
, corneal clouding,
childhood blindness Childhood blindness is an important cause contributing to the burden of blindness Visual impairment, also known as vision impairment or vision loss, is a decreased ability to visual perception, see to a degree that causes problems not fixable by ...
, and a number of
infection An infection is the invasion of an organism's body by , their multiplication, and the reaction of tissues to the infectious agents and the s they produce. An infectious disease, also known as a transmissible disease or communicable disease, i ...

infection
s. Visual impairment can also be caused by problems in the
brain A brain is an organ Organ may refer to: Biology * Organ (anatomy) An organ is a group of Tissue (biology), tissues with similar functions. Plant life and animal life rely on many organs that co-exist in organ systems. A given organ's tiss ...

brain
due to
stroke A stroke is a medical condition A disease is a particular abnormal condition that negatively affects the structure or function (biology), function of all or part of an organism, and that is not due to any immediate external injury. Dis ...

stroke
,
premature birth Preterm birth, also known as premature birth, is the birth Birth is the act or process of bearing or bringing forth offspring, also referred to in technical contexts as parturition. In mammals, the process is initiated by hormones which cause ...

premature birth
, or trauma, among others. These cases are known as
cortical visual impairmentCortical visual impairment (CVI) is a form of visual impairment that is caused by a brain problem rather than an eye problem. (The latter is sometimes termed "ocular visual impairment" when discussed in contrast to cortical visual impairment.) Some p ...
. Screening for vision problems in children may improve future vision and educational achievement. Screening adults without symptoms is of uncertain benefit. Diagnosis is by an
eye exam An eye examination is a series of tests performed to assess vision Vision or The Vision may refer to: Perception Optical perception * Visual perception Visual perception is the ability to interpret the surrounding environment using ligh ...

eye exam
. The
World Health Organization The World Health Organization (WHO) is a specialized agency of the United Nations United Nations Specialized Agencies are autonomous organizations working with the United Nations and each other through the co-ordinating machinery of the Unite ...
(WHO) estimates that 80% of visual impairment is either preventable or curable with treatment. This includes cataracts, the infections
river blindness Onchocerciasis, also known as river blindness, is a disease caused by infection with the parasitic worm Parasitic worms, also known as helminths, are large macroparasites; adults can generally be seen with the naked eye. Many are intestinal ...
and
trachoma Trachoma is an infectious disease caused by bacterium ''Chlamydia trachomatis ''Chlamydia trachomatis'' (), commonly known as chlamydia, is a bacterium that causes chlamydia, which can manifest in various ways, including: trachoma, lymphogra ...
, glaucoma, diabetic retinopathy, uncorrected refractive errors, and some cases of childhood blindness. Many people with significant visual impairment benefit from vision rehabilitation, changes in their environment, and assistive devices. As of 2015 there were 940 million people with some degree of vision loss. 246 million had low vision and 39 million were blind. The majority of people with poor vision are in the
developing world Image:Imf-advanced-un-least-developed-2008.svg, 450px, Example of Older Classifications by the International Monetary Fund, IMF and the United Nations, UN from 2008 A developing country is a country with a less developed Industrial sector, i ...
and are over the age of 50 years. Rates of visual impairment have decreased since the 1990s. Visual impairments have considerable economic costs both directly due to the cost of treatment and indirectly due to decreased ability to work.


Classification

The definition of visual impairment is reduced vision not corrected by
glasses Glasses, also known as eyeglasses or spectacles, are vision eyewear, consisting of glass or hard plastic lenses mounted in a frame that holds them in front of a person's eyes, typically utilizing a bridge over the nose and hinged arms (known ...

glasses
or
contact lens One-day disposable contact lenses with blue handling tint in blister-pack packaging Contact lenses, or simply contacts, are thin lenses A lens is a transmissive optics, optical device that focuses or disperses a light beam by means of ...

contact lens
es. The World Health Organization uses the following classifications of visual impairment. When the vision in the better eye with best possible glasses correction is: * 20/30 to 20/60 : is considered mild vision loss, or near-normal vision * 20/70 to 20/160 : is considered moderate visual impairment, or moderate low vision * 20/200 to 20/400 : is considered severe visual impairment, or severe low vision * 20/500 to 20/1,000 : is considered profound visual impairment, or profound low vision * More than 20/1,000 : is considered near-total visual impairment, or near total blindness * No light perception (NLP) : is considered total visual impairment, or total blindness Blindness is defined by the
World Health Organization The World Health Organization (WHO) is a specialized agency of the United Nations United Nations Specialized Agencies are autonomous organizations working with the United Nations and each other through the co-ordinating machinery of the Unite ...
as vision in a person's best eye with best correction of less than 20/500 or a
visual field The visual field is the "spatial array of visual sensations available to observation in introspection Introspection is the examination of one's own conscious thoughts and feelings. In psychology, the process of introspection relies on the observ ...
of less than 10 degrees. This definition was set in 1972, and there is ongoing discussion as to whether it should be altered to officially include uncorrected refractive errors.


United Kingdom

Severely sight impaired * Defined as having central visual acuity of less than 3/60 with normal fields of vision, or gross visual field restriction. * Unable to see at what the normally sighted person sees at . Sight impaired * Able to see at , but not at , what the normally sighted person sees at * Less severe visual impairment is not captured by registration data, and its prevalence is difficult to quantify Low vision * A visual acuity of less than 6/18 but greater than 3/60. * Not eligible to drive and may have difficulty recognising faces across a street, watching television, or choosing clean, unstained, co-ordinated clothing. In the UK, the Certificate of Vision Impairment (CVI) is used to certify patients as severely sight impaired or sight impaired. The accompanying guidance for clinical staff states: "The National Assistance Act 1948 states that a person can be certified as severely sight impaired if they are 'so blind as to be unable to perform any work for which eye sight is essential'". Certification is based on whether a person can do any work for which eyesight is essential, not just one particular job (such as their job before becoming blind). In practice, the definition depends on individuals'
visual acuity Visual acuity (VA) commonly refers to the clarity of vision Vision or The Vision may refer to: Perception Optical perception * Visual perception Visual perception is the ability to interpret the surrounding environment using light in t ...
and the extent to which their
field of vision The visual field is the "spatial array of visual sensations available to observation in introspection Introspection is the examination of one's own conscious , an English Paracelsian physician Consciousness, at its simplest, is " sentience or ...
is restricted. The
Department of Health A health department or health ministry is a part of government A government is the system or group of people governing an organized community, generally a State (polity), state. In the case of its broad associative definition, gover ...

Department of Health
identifies three groups of people who may be classified as severely visually impaired. #Those below 3/60 (equivalent to 20/400 in US notation) (most people below 3/60 are severely sight impaired). #Those better than 3/60 but below 6/60 Snellen (people who have a very contracted field of vision only). #Those 6/60 Snellen or above (people in this group who have a contracted
field of vision The visual field is the "spatial array of visual sensations available to observation in introspection Introspection is the examination of one's own conscious , an English Paracelsian physician Consciousness, at its simplest, is " sentience or ...
especially if the contraction is in the lower part of the field). The Department of Health also state that a person is more likely to be classified as severely visually impaired if their eyesight has failed recently or if they are an older individual, both groups being perceived as less able to adapt to their vision loss.


United States

In the United States, any person with vision that cannot be corrected to better than 20/200 in the best eye, or who has 20
degrees Degree may refer to: As a unit of measurement * Degree symbol (°), a notation used in science, engineering, and mathematics * Degree (angle), a unit of angle measurement * Degree (temperature), any of various units of temperature measurement ...
(
diameter In geometry Geometry (from the grc, γεωμετρία; ''wikt:γῆ, geo-'' "earth", ''wikt:μέτρον, -metron'' "measurement") is, with arithmetic, one of the oldest branches of mathematics. It is concerned with properties of space ...

diameter
) or less of visual field remaining, is considered legally blind or eligible for disability classification and possible inclusion in certain government sponsored programs. In the United States, the terms ''partially sighted'', ''low vision'', ''legally blind'' and ''totally blind'' are used by schools, colleges, and other educational institutions to describe students with visual impairments. They are defined as follows: *''Partially sighted'' indicates some type of visual problem, with a need of person to receive special education in some cases. *''Low vision'' generally refers to a severe visual impairment, not necessarily limited to distance vision. Low vision applies to all individuals with sight who are unable to read the newspaper at a normal viewing distance, even with the aid of eyeglasses or contact lenses. They use a combination of vision and other senses to learn, although they may require adaptations in lighting or the size of print, and, sometimes,
braille Braille ( ; Braille: ⠃⠗⠇; ) is a tactile writing system A writing system is a method of visually representing verbal communication Communication (from Latin ''communicare'', meaning "to share") is the act of developing Semantics, ...

braille
. **''
Myopic Near-sightedness, also known as short-sightedness and myopia, is an eye Eyes are organs of the visual system The visual system comprises the sensory organ (the eye) and parts of the central nervous system (the retina containing photore ...

Myopic
'' – unable to see distant objects clearly, commonly called near-sighted or short-sighted. **''
Hyperopic Far-sightedness, also known as long-sightedness, hypermetropia, or hyperopia, is a condition of the eye where distant objects are seen clearly but near objects appear blurred. This blurred effect is due to incoming light being focused behind, in ...

Hyperopic
'' – unable to see close objects clearly, commonly called far-sighted or long-sighted. *''Legally blind'' indicates that a person has less than 20/200 vision in the better eye after best correction (contact lenses or glasses), or a field of vision of less than 20 degrees in the better eye. *''Totally blind'' students learn via braille or other non-visual media. In 1934, the
American Medical Association The American Medical Association (AMA), founded in 1847 and incorporated in 1897, is the largest association and lobby group Advocacy groups, also known as special interest groups, use various forms of advocacy Advocacy is an activity by a ...
adopted the following definition of blindness:
Central visual acuity of 20/200 or less in the better eye with corrective glasses or central visual acuity of more than 20/200 if there is a visual field defect in which the peripheral field is contracted to such an extent that the widest diameter of the visual field subtends an angular distance no greater than 20 degrees in the better eye.
The
United States Congress The United States Congress is the legislature A legislature is an deliberative assembly, assembly with the authority to make laws for a Polity, political entity such as a Sovereign state, country or city. They are often contrasted with ...

United States Congress
included this definition as part of the Aid to the Blind program in the
Social Security Act The Social Security Act of 1935 is a law enacted by the 74th United States Congress The 74th United States Congress was a meeting of the legislative branch of the United States federal government, composed of the United States Senate ...
passed in 1935. In 1972, the Aid to the Blind program and two others combined under Title XVI of the Social Security Act to form the
Supplemental Security Income Supplemental Security Income (SSI) is a means-tested A means test is a determination of whether an individual or family is eligible for government assistance or welfare Welfare is a type of government support intended to ensure that member ...
program which states:
An individual shall be considered to be blind for purposes of this title if he has central visual acuity of 20/200 or less in the better eye with the use of a correcting lens. An eye which is accompanied by a limitation in the fields of vision such that the widest diameter of the visual field subtends an angle no greater than 20 degrees shall be considered for purposes of the first sentence of this subsection as having a central visual acuity of 20/200 or less. An individual shall also be considered to be blind for purposes of this title if he is blind as defined under a State plan approved under title X or XVI as in effect for October 1972 and received aid under such plan (on the basis of blindness) for December 1973, so long as he is continuously blind as so defined.


Health effects

Visual impairments may take many forms and be of varying degrees. Visual acuity alone is not always a good predictor of the degree of problems a person may have. Someone with relatively good acuity (e.g., 20/40) can have difficulty with daily functioning, while someone with worse acuity (e.g., 20/200) may function reasonably well if their visual demands are not great. The
American Medical Association The American Medical Association (AMA), founded in 1847 and incorporated in 1897, is the largest association and lobby group Advocacy groups, also known as special interest groups, use various forms of advocacy Advocacy is an activity by a ...
has estimated that the loss of one eye equals 25% impairment of the visual system and 24% impairment of the whole person; total loss of vision in both eyes is considered to be 100% visual impairment and 85% impairment of the whole person. Some people who fall into this category can use their considerable residual vision – their remaining sight – to complete daily tasks without relying on alternative methods. The role of a low vision specialist (optometrist or ophthalmologist) is to maximize the functional level of a patient's vision by optical or non-optical means. Primarily, this is by use of magnification in the form of telescopic systems for distance vision and optical or electronic magnification for near tasks. People with significantly reduced acuity may benefit from training conducted by individuals trained in the provision of technical aids. Low vision rehabilitation professionals, some of whom are connected to an agency for the blind, can provide advice on lighting and contrast to maximize remaining vision. These professionals also have access to non-visual aids, and can instruct patients in their uses. The subjects making the most use of rehabilitation instruments, who lived alone, and preserved their own mobility and occupation were the least depressed, with the lowest risk of suicide and the highest level of social integration. Those with worsening sight and the prognosis of eventual blindness are at comparatively high risk of suicide and thus may be in need of supportive services. Many studies have demonstrated how rapid acceptance of the serious visual handicap has led to a better, more productive compliance with rehabilitation programs. Moreover, psychological distress has been reported to be at its highest when sight loss is not complete, but the prognosis is unfavorable. Therefore, early intervention is imperative for enabling successful psychological adjustment.


Associated conditions

Blindness can occur in combination with such conditions as
intellectual disability Intellectual disability (ID), also known as general learning disability and formerly mental retardation (MR),Rosa's Law Rosa's Law is a United States The United States of America (USA), commonly known as the United States (U.S. or US), or ...
,
autism spectrum disorders The autism spectrum encompasses a range of neurodevelopmental conditions, including autism and Asperger syndrome, generally known as autism spectrum disorders (ASD). Individuals on the autism spectrum typically experience difficulties with s ...
,
cerebral palsy Cerebral palsy (CP) is a group of movement disorders Movement disorders are clinical syndromes with either an excess of movement or a paucity of voluntary and involuntary movements, unrelated to weakness or spasticity. Movement disorders are sy ...

cerebral palsy
,
hearing impairment Hearing loss is a partial or total inability to 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 acquire spoken la ...
s, and
epilepsy Epilepsy is a group of non-communicable neurological disorder A neurological disorder is any disorder of the nervous system In Biology, biology, the nervous system is a Complex system, highly complex part of an animal that coordinates it ...

epilepsy
. Blindness in combination with hearing loss is known as
deafblindness Deafblindness is the condition of little or no useful hearing Hearing, or auditory perception, is the ability to perceive sounds by detecting vibrations, changes in the pressure of the surrounding medium through time, through an organ s ...
. It has been estimated that over half of completely blind people have
non-24-hour sleep–wake disorder Non-24-hour sleep–wake disorder (non-24 or N24SWD) is one of several chronic circadian rhythm sleep disorders (CRSDs). It is defined as a "chronic steady pattern comprising ..daily delays in sleep onset and wake times in an individual livi ...
, a condition in which a person's
circadian rhythm A circadian rhythm (), or circadian cycle, is a natural, internal process that regulates the sleep–wake cycle and repeats roughly every 24 hours. It can refer to any process that originates within an organism (i.e., Endogeny (biology), endogenou ...

circadian rhythm
, normally slightly longer than 24 hours, is not entrained (synchronized) to the lightdark cycle.


Cause

The most common causes of visual impairment globally in 2010 were: #
Refractive error Refractive error, also known as refraction error, is a problem with focus FOCUS is a fourth-generation programming language (4GL) computer programming programming language, language and development environment that is used to build database quer ...

Refractive error
(42%) #
Cataract A cataract is a cloudy area in the lens A lens is a transmissive optical device that focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens con ...
(33%) #
Glaucoma Glaucoma is a group of eye diseases which result in damage to the optic nerve The optic nerve, also known as cranial nerve II, or simply as CN II, is a paired cranial nerve Cranial nerves are the nerve A nerve is an enclosed, cable-li ...

Glaucoma
(2%) #
Age-related macular degeneration Macular degeneration, also known as age-related macular degeneration (AMD or ARMD), is a medical condition which may result in blurred or no vision in the center of the visual field. Early on there are often no symptoms. Over time, however, som ...
(1%) #
Corneal opacification The human cornea The cornea is the transparent front part of the eye that covers the iris, pupil The pupil is a black hole located in the center of the Iris (anatomy), iris of the Human eye, eye that allows light to strike the retina.Cassi ...
(1%) #
Diabetic retinopathy Diabetic retinopathy, also known as diabetic eye disease (DED), is a medical condition in which damage occurs to the retina due to diabetes mellitus. It is a leading cause of blindness Visual impairment, also known as vision impairment or vis ...
(1%) #
Childhood blindness Childhood blindness is an important cause contributing to the burden of blindness Visual impairment, also known as vision impairment or vision loss, is a decreased ability to visual perception, see to a degree that causes problems not fixable by ...
#
Trachoma Trachoma is an infectious disease caused by bacterium ''Chlamydia trachomatis ''Chlamydia trachomatis'' (), commonly known as chlamydia, is a bacterium that causes chlamydia, which can manifest in various ways, including: trachoma, lymphogra ...
(1%) #Undetermined (18%) The most common causes of blindness worldwide in 2010 were: #Cataracts (51%) #Glaucoma (8%) #Age-related macular degeneration (5%) #Corneal opacification (4%) #Childhood blindness (4%) #Refractive errors (3%) #Trachoma (3%) #Diabetic retinopathy (1%) #Undetermined (21%) About 90% of people who are visually impaired live in the
developing world Image:Imf-advanced-un-least-developed-2008.svg, 450px, Example of Older Classifications by the International Monetary Fund, IMF and the United Nations, UN from 2008 A developing country is a country with a less developed Industrial sector, i ...
. Age-related macular degeneration, glaucoma, and diabetic retinopathy are the leading causes of blindness in the developed world. Among working-age adults who are newly blind in England and Wales the most common causes in 2010 were: #Hereditary retinal disorders (20.2%) #Diabetic retinopathy (14.4%) #Optic atrophy (14.1%) #Glaucoma (5.9%) #Congenital abnormalities (5.1%) #Disorders of the visual cortex (4.1%) #Cerebrovascular disease (3.2%) #Degeneration of the macula and posterior pole (3.0%) #Myopia (2.8%) #Corneal disorders (2.6%) #Malignant neoplasms of the brain and nervous system (1.5%) #Retinal detachment (1.4%)


Cataracts

Cataracts A cataract is a cloudy area in the lens A lens is a transmissive optical Optics is the branch of physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'natur ...

Cataracts
are the congenital and pediatric pathology that describes the greying or opacity of the crystalline lens, which is most commonly caused by intrauterine infections, metabolic disorders, and genetically transmitted syndromes. Cataracts are the leading cause of child and adult blindness that doubles in prevalence with every ten years after the age of 40. Consequently, today cataracts are more common among adults than in children. That is, people face higher chances of developing cataracts as they age. Nonetheless, cataracts tend to have a greater financial and emotional toll upon children as they must undergo expensive diagnosis, long term rehabilitation, and visual assistance. Also, according to the Saudi Journal for Health Sciences, sometimes patients experience irreversible amblyopia after pediatric cataract surgery because the cataracts prevented the normal maturation of vision prior to operation. Despite the great progress in treatment, cataracts remain a global problem in both economically developed and developing countries. At present, with the variant outcomes as well as the unequal access to cataract surgery, the best way to reduce the risk of developing cataracts is to avoid smoking and extensive exposure to sun light (i.e. UV-B rays).


Glaucoma

Glaucoma Glaucoma is a group of eye diseases which result in damage to the optic nerve The optic nerve, also known as cranial nerve II, or simply as CN II, is a paired cranial nerve Cranial nerves are the nerve A nerve is an enclosed, cable-li ...

Glaucoma
is a congenital and pediatric eye disease characterized by increased pressure within the eye or intraocular pressure (IOP).<>. Glaucoma causes visual field loss as well as severs the optic nerve.Glaucoma Research Foundation. "High Eye Pressure and Glaucoma." Glaucoma Research Foundation. N.p., 5 Sept. 2013. Web.<>. Early diagnosis and treatment of glaucoma in patients is imperative because glaucoma is triggered by non-specific levels of IOP. Also, another challenge in accurately diagnosing glaucoma is that the disease has four causes: 1) inflammatory ocular hypertension syndrome (IOHS); 2) severe uveitic angle closure; 3) corticosteroid-induced; and 4) a heterogonous mechanism associated with structural change and chronic inflammation. In addition, often
pediatric glaucoma Pediatrics ( also spelled paediatrics or pædiatrics) is the branch of medicine that involves the medical care of infants, children, and adolescents. The American Academy of Pediatrics recommends people be under pediatric care through the age ...
differs greatly in cause and management from the glaucoma developed by adults. Currently, the best sign of pediatric glaucoma is an IOP of 21 mm Hg or greater present within a child. One of the most common causes of pediatric glaucoma is cataract removal surgery, which leads to an incidence rate of about 12.2% among infants and 58.7% among 10-year-olds.


Infections

Childhood blindness can be caused by conditions related to pregnancy, such as
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 the ...
and
retinopathy of prematurity Retinopathy of prematurity (ROP), also called retrolental fibroplasia (RLF) and Terry syndrome, is a disease of the eye affecting prematurely born babies generally having received neonatal intensive care, in which oxygen therapy Oxygen thera ...
. Leprosy and onchocerciasis each blind approximately 1 million individuals in the developing world. The number of individuals blind from
trachoma Trachoma is an infectious disease caused by bacterium ''Chlamydia trachomatis ''Chlamydia trachomatis'' (), commonly known as chlamydia, is a bacterium that causes chlamydia, which can manifest in various ways, including: trachoma, lymphogra ...
has decreased in the past 10 years from 6 million to 1.3 million, putting it in seventh place on the list of causes of blindness worldwide. Central corneal ulceration is also a significant cause of monocular blindness worldwide, accounting for an estimated 850,000 cases of corneal blindness every year in the Indian subcontinent alone. As a result, corneal scarring from all causes is now the fourth greatest cause of global blindness.


Injuries

Eye injuries Physiology, Physical or chemical injuries of the human eye, eye can be a serious threat to Visual perception, vision if not treated appropriately and in a timely fashion. The most obvious presentation of ocular (eye) injuries is redness and pain of ...

Eye injuries
, most often occurring in people under 30, are the leading cause of monocular blindness (vision loss in one eye) throughout the
United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country in . It consists of 50 , a , five major , 326 , and some . At , it is the world's . The United States shares significan ...

United States
. Injuries and cataracts affect the eye itself, while abnormalities such as
optic nerve hypoplasia Optic nerve hypoplasia (ONH) is a medical condition arising from the underdevelopment of the optic nerve The optic nerve, also known as cranial nerve II, or simply as CN II, is a paired cranial nerve that transmits visual system, visual inform ...
affect the nerve bundle that sends signals from the eye to the back of the brain, which can lead to decreased visual acuity.
Cortical blindness Cortical blindness is the total or partial loss of vision in a normal-appearing eye caused by damage to the brain's occipital cortex. Cortical blindness can be acquired or congenital, and may also be transient in certain instances. Acquired corti ...
results from injuries to the
occipital lobe The occipital lobe is one of the four major lobes of the cerebral cortex The cerebral cortex, also known as the cerebral mantle, is the outer layer of neural tissue of the cerebrum of the brain in humans and other mammals. The cerebral cortex m ...

occipital lobe
of the
brain A brain is an organ Organ may refer to: Biology * Organ (anatomy) An organ is a group of Tissue (biology), tissues with similar functions. Plant life and animal life rely on many organs that co-exist in organ systems. A given organ's tiss ...

brain
that prevent the brain from correctly receiving or interpreting signals from the
optic nerve The optic nerve, also known as cranial nerve II, or simply as CN II, is a paired cranial nerve Cranial nerves are the nerve A nerve is an enclosed, cable-like bundle of nerve fibers called axon An axon (from Greek ἄξων ''áxōn'' ...

optic nerve
. Symptoms of cortical blindness vary greatly across individuals and may be more severe in periods of exhaustion or stress. It is common for people with cortical blindness to have poorer vision later in the day. Blinding has been used as an act of vengeance and torture in some instances, to deprive a person of a major sense by which they can navigate or interact within the world, act fully independently, and be aware of events surrounding them. An example from the classical realm is
Oedipus Oedipus (, ; grc-gre, wikt:Οἰδίπους, Οἰδίπους "swollen foot") was a mythical Greek king of Ancient Thebes (Boeotia), Thebes. A tragic hero in Greek mythology, Oedipus accidentally fulfilled a prophecy that he would end up kil ...

Oedipus
, who gouges out his own eyes after realizing that he fulfilled the awful prophecy spoken of him. Having crushed the Bulgarians, the Byzantine Emperor
Basil II Basil II Porphyrogenitus ( gr, Βασίλειος πορφυρογέννητος, translit=Basileios porphyrogennētos;) and, most often, the Purple-born ( gr, ὁ πορφυρογέννητος, translit=ho porphyrogennetos).. – 15 Decem ...
blinded as many as 15,000 prisoners taken in the battle, before releasing them. Contemporary examples include the addition of methods such as
acid throwing An acid attack, also called acid throwing, vitriol attack, or vitriolage, is a form of violent Violence is the use of physical force so as to injure, abuse, damage, or destroy. Other definitions are also used, such as the World Health Organ ...
as a form of
disfigurement , which causes much harm to the skin. Disfigurement is the state of having one's appearance deeply and persistently harmed medically, such as from a disease, birth defect A birth defect, also known as a congenital disorder, is a condition presen ...
.


Genetic defects

People with
albinism Albinism is the congenital absence of any pigmentation or colouration in a person, animal or plant, resulting in white hair, feathers, scales and skin and pink eyes in mammals, birds, reptiles, amphibians and fish and invertebrates as well. V ...
often have vision loss to the extent that many are legally blind, though few of them actually cannot see.
Leber congenital amaurosis Leber congenital amaurosis (LCA) is a rare inherited eye disease This is a partial list of publishes a classification of known diseases and injuries, the International Statistical Classification of Diseases and Related Health Problems, or ICD-10 ...
can cause total blindness or severe sight loss from birth or early childhood. Recent advances in
mapping Mapping may refer to: * Mapping (cartography), the process of making a map * Mapping (mathematics), a synonym for a mathematical function and its generalizations ** Mapping (logic), a synonym for functional predicate Types of mapping * Animated ...

mapping
of the
human genome The human genome is a complete set of nucleic acid sequences for humans, encoded as DNA within the 23 chromosome pairs in cell nuclei and in a small DNA molecule found within individual Mitochondrial DNA, mitochondria. These are usually treated se ...

human genome
have identified other genetic causes of
low vision Visual impairment, also known as vision impairment or vision loss, is a decreased ability to see to a degree that causes problems not fixable by usual means, such as glasses Glasses, also known as eyeglasses or spectacles, are vision eyewe ...
or blindness. One such example is
Bardet–Biedl syndrome Bardet–Biedl syndrome (BBS) is a ciliopathic human Humans (''Homo sapiens'') are the most populous and widespread species of primates, characterized by bipedality, opposable thumbs, hairlessness, and intelligence allowing the use of cultur ...
.


Poisoning

Rarely, blindness is caused by the intake of certain chemicals. A well-known example is
methanol Methanol, also known as methyl alcohol, amongst other names, is a chemical A chemical substance is a form of matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. ...

methanol
, which is only mildly toxic and minimally intoxicating, and breaks down into the substances
formaldehyde Formaldehyde ( , also ) (systematic nameA systematic name is a name given in a systematic way to one unique group, organism, object or chemical substance A chemical substance is a form of matter In classical physics and general chemist ...
and
formic acid Formic acid, systematically named methanoic acid, is the simplest carboxylic acid A carboxylic acid is an organic acid An organic acid is an organic compound with acidic properties. The most common organic acids are the carboxylic acid ...

formic acid
which in turn can cause blindness, an array of other health complications, and death. When competing with
ethanol Ethanol (also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic Organic may refer to: * Organic, of or relating to an organism, a living entity * Organic, of or relating to an anatomical organ (anatomy), ...

ethanol
for metabolism, ethanol is metabolized first, and the onset of toxicity is delayed. Methanol is commonly found in methylated spirits, , to avoid paying taxes on selling ethanol intended for human consumption. Methylated spirits are sometimes used by
alcoholics Alcoholism is, broadly, any drinking of alcohol File:Alcohol general.svg, upright=0.8, The bond angle between a hydroxyl group (-OH) and a chain of carbon atoms (R) In chemistry, alcohol is an organic compound that carries at least one ...
as a desperate and cheap substitute for regular ethanol
alcoholic beverages An alcoholic drink is a drink A drink (or beverage) is a liquid A liquid is a nearly incompressible In fluid mechanics or more generally continuum mechanics, incompressible flow (isochoric process, isochoric flow) refers to a fluid f ...

alcoholic beverages
.


Other

*
Amblyopia Amblyopia, also called lazy eye, is a disorder of sight in which the brain A brain is an organ (anatomy), organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It is located in the head, usually ...
: is a category of vision loss or visual impairment that is caused by factors unrelated to refractive errors or coexisting ocular diseases. Amblyopia is the condition when a child's visual systems fail to mature normally because the child either suffers from a premature birth, measles, congenital rubella syndrome, vitamin A deficiency, or meningitis. If left untreated during childhood, amblyopia is currently incurable in adulthood because surgical treatment effectiveness changes as a child matures. Consequently, amblyopia is the world's leading cause of child monocular vision loss, which is the damage or loss of vision in one eye. In the best case scenario, which is very rare, properly treated amblyopia patients can regain 20/40 acuity. *
Corneal opacification The human cornea The cornea is the transparent front part of the eye that covers the iris, pupil The pupil is a black hole located in the center of the Iris (anatomy), iris of the Human eye, eye that allows light to strike the retina.Cassi ...
* Degenerative myopia *
Diabetic retinopathy Diabetic retinopathy, also known as diabetic eye disease (DED), is a medical condition in which damage occurs to the retina due to diabetes mellitus. It is a leading cause of blindness Visual impairment, also known as vision impairment or vis ...
: is one of the manifestation microvascular complications of diabetes, which is characterized by blindness or reduced acuity. That is, diabetic retinopathy describes the retinal and vitreous hemorrhages or retinal capillary blockage caused by the increase of A1C, which a measurement of blood glucose or sugar level. In fact, as A1C increases, people tend to be at greater risk of developing diabetic retinopathy than developing other microvascular complications associated with diabetes (e.g. chronic hyperglycemia, diabetic neuropathy, and diabetic nephropathy). Despite the fact that only 8% of adults 40 years and older experience vision-threatening diabetic retinopathy (e.g. nonproliferative diabetic retinopathy or NPDR and proliferative diabetic retinopathy or PDR), this eye disease accounted for 17% of cases of blindness in 2002. *
Retinitis pigmentosa Retinitis pigmentosa (RP) is a genetic disorder A genetic disorder is a health problem caused by one or more abnormalities in the genome In the fields of molecular biology and genetics Genetics is a branch of biology concerned ...
*
Retinopathy of prematurity Retinopathy of prematurity (ROP), also called retrolental fibroplasia (RLF) and Terry syndrome, is a disease of the eye affecting prematurely born babies generally having received neonatal intensive care, in which oxygen therapy Oxygen thera ...
: The most common cause of blindness in infants worldwide. In its most severe form, ROP causes retinal detachment, with attendant visual loss. Treatment is aimed mainly at prevention, via laser or
Avastin Bevacizumab, sold under the brand name Avastin, is a medication used to treat a number of types of cancer Cancer is a group of diseases involving abnormal cell growth with the potential to invade or Metastasis, spread to other parts of t ...
therapy. * Stargardt's disease *
Uveitis Uveitis () is the inflammation of the uvea, the pigmented layer that lies between the inner retina and the outer fibrous layer composed of the sclera and cornea. The uvea consists of the middle layer of pigmented vascular structures of the eye and ...
: is a group of 30 intraocular inflammatory diseases caused by infections, systemic diseases, organ-specific autoimmune processes, cancer or trauma. That is, uveitis refers to a complex category of ocular diseases that can cause blindness if either left untreated or improperly diagnosed. The current challenge of accurately diagnosing uveitis is that often the cause of a specific ocular inflammation is either unknown or multi-layered. Consequently, about 3–10% uveitis victims in developed countries, and about 25% of victims in the developing countries, become blind from incorrect diagnosis and from ineffectual prescription of drugs, antibiotics or steroids. In addition, uveitis is a diverse category of eye diseases that are subdivided as granulomatous (or tumorous) or non-granulomatous anterior, intermediate, posterior or pan uveitis. In other words, uveitis diseases tend to be classified by their anatomic location in the eye (e.g. uveal tract, retina, or lens), as well as can create complication that can cause cataracts, glaucoma, retinal damage, age-related macular degeneration or diabetic retinopathy. *
Xerophthalmia Xerophthalmia (from Ancient Greek "xērós" (ξηρός) meaning "dry" and "ophthalmos" (οφθαλμός) meaning "eye") is a medical condition in which the human eye, eye fails to produce tears. It may be caused by vitamin A deficiency, which is ...
, often due to
vitamin A deficiency Vitamin A deficiency (VAD) or hypovitaminosis A is a lack of vitamin A in blood and tissue (biology), tissues. It is common in poorer countries, especially among children and women of reproductive age, but is rarely seen in more developed countries. ...
, is estimated to affect 5 million children each year; 500,000 develop active corneal involvement, and half of these go blind.


Diagnosis

It is important that people be examined by someone specializing in low vision care prior to other rehabilitation training to rule out potential medical or surgical correction for the problem and to establish a careful baseline refraction and prescription of both normal and low vision glasses and optical aids. Only a doctor is qualified to evaluate visual functioning of a compromised visual system effectively. The
American Medical Association The American Medical Association (AMA), founded in 1847 and incorporated in 1897, is the largest association and lobby group Advocacy groups, also known as special interest groups, use various forms of advocacy Advocacy is an activity by a ...
provides an approach to evaluating visual loss as it affects an individual's ability to perform activities of daily living. Screening adults who have no symptoms is of uncertain benefit.


Prevention

The World Health Organization estimates that 80% of visual loss is either preventable or curable with treatment. This includes cataracts, onchocerciasis, trachoma, glaucoma, diabetic retinopathy, uncorrected refractive errors, and some cases of childhood blindness. The Center for Disease Control and Prevention estimates that half of blindness in the United States is preventable.


Management


Mobility

Many people with serious visual impairments can travel independently, using a wide range of tools and techniques. Orientation and mobility specialists are professionals who are specifically trained to teach people with visual impairments how to travel safely, confidently, and independently in the home and the community. These professionals can also help blind people to practice travelling on specific routes which they may use often, such as the route from one's house to a convenience store. Becoming familiar with an environment or route can make it much easier for a blind person to navigate successfully. Tools such as the
white cane A white cane is a device used by many people who are visual impairment, blind or visually impaired. A white cane primarily allows its user to scan their surroundings for obstacles or orientation marks, but is also helpful for onlookers in identi ...

white cane
with a red tip – the international symbol of blindness – may also be used to improve mobility. A long cane is used to extend the user's range of touch sensation. It is usually swung in a low sweeping motion, across the intended path of travel, to detect obstacles. However, techniques for cane travel can vary depending on the user and/or the situation. Some visually impaired persons do not carry these kinds of canes, opting instead for the shorter, lighter identification (ID) cane. Still others require a support cane. The choice depends on the individual's vision, motivation, and other factors. A small number of people employ
guide dog Guide dogs (colloquially known in the USA as seeing eye dogs) are assistance dog In general, an assistance dog, known as a service dog in the United States, is a dog trained to aid or assist an individual with a disability. Many are trained ...

guide dog
s to assist in mobility. These dogs are trained to navigate around various obstacles, and to indicate when it becomes necessary to go up or down a step. However, the helpfulness of guide dogs is limited by the inability of dogs to understand complex directions. The human half of the guide dog team does the directing, based upon skills acquired through previous mobility training. In this sense, the handler might be likened to an aircraft's navigator, who must know how to get from one place to another, and the dog to the pilot, who gets them there safely. GPS devices can also be used as a mobility aid. Such software can assist blind people with orientation and navigation, but it is not a replacement for traditional mobility tools such as white canes and guide dogs. Some blind people are skilled at echolocating silent objects simply by producing mouth clicks and listening to the returning echoes. It has been shown that blind echolocation experts use what is normally the "visual" part of their brain to process the echoes. Government actions are sometimes taken to make public places more accessible to blind people. Public transportation is freely available to the blind in many cities.
Tactile paving Tactile paving (also called Tenji blocks, truncated domes, detectable warnings, tactile tiles, tactile ground surface indicators, tactile walking surface indicators, or detectable warning surfaces) is a system of textured ground surface indicator ...

Tactile paving
and audible traffic signals can make it easier and safer for visually impaired pedestrians to cross streets. In addition to making rules about who can and cannot use a cane, some governments mandate the
right-of-way Right of way is "the legal right, established by usage or grant, to pass along a specific route through grounds or property belonging to another", or "a path or thoroughfare subject to such a right". A similar ''right of access'' also exists on ...
be given to users of white canes or guide dogs.


Reading and magnification

Most visually impaired people who are not totally blind read print, either of a regular size or enlarged by magnification devices. Many also read
large-print Large-print (also large-type or large-font) refers to the formatting of a book or other text document in which the typeface A typeface is the design of lettering that can include variations, such as extra bold, bold, regular, light, italic, cond ...
, which is easier for them to read without such devices. A variety of
magnifying glass A magnifying glass is a Lens (optics)#Types of simple lenses, convex lens that is used to produce a magnification, magnified image of an object. The lens (optics), lens is usually mounted in a frame with a handle. A magnifying glass can be used ...

magnifying glass
es, some handheld, and some on desktops, can make reading easier for them. Others read
braille Braille ( ; Braille: ⠃⠗⠇; ) is a tactile writing system A writing system is a method of visually representing verbal communication Communication (from Latin ''communicare'', meaning "to share") is the act of developing Semantics, ...

braille
(or the infrequently used
Moon type The Moon System of Embossed Reading (commonly known as the Moon writing, Moon alphabet, Moon script, Moon type, or Moon code) is a writing system A writing system is a method of visually representing verbal communication Communication (f ...
), or rely on talking books and readers or
reading machineA reading machine is a piece of assistive technology that allows Blindness, blind people to access printed materials. It scans text, converts the image into text by means of optical character recognition and uses a Speech synthesis, speech synthesize ...
s, which convert printed text to speech or
braille Braille ( ; Braille: ⠃⠗⠇; ) is a tactile writing system A writing system is a method of visually representing verbal communication Communication (from Latin ''communicare'', meaning "to share") is the act of developing Semantics, ...

braille
. They use computers with special hardware such as
scanners ''Scanners'' is a 1981 Canadian science-fiction Space exploration, as predicted in August 1958 in the science fiction magazine ''Imagination (magazine)">Imagination.'' Science fiction (sometimes shortened to sci-fi or SF) is a genre of spec ...
and
refreshable braille display#REDIRECT Refreshable braille display A refreshable braille display or braille terminal is an electro-mechanical device for displaying braille Braille ( ; Braille: ⠃⠗⠇; ) is a tactile writing system used by people who are visually impair ...

refreshable braille display
s as well as software written specifically for the blind, such as
optical character recognition Optical character recognition or optical character reader (OCR) is the electronic Electronic may refer to: *Electronics Electronics comprises the physics, engineering, technology and applications that deal with the emission, flow and contro ...
applications and
screen reader A screen reader is a form of assistive technology (AT) that renders text and image content as speech or braille output. Screen readers are essential to people who are blindness, blind, and are useful to people who are visual impairment, visually ...
s. Some people access these materials through agencies for the blind, such as the
National Library Service for the Blind and Physically Handicapped The National Library Service for the Blind and Print Disabled (NLS) is a free library program of braille and audio materials such as books and magazines circulated to eligible borrowers in the United States and American citizens living abroad by pos ...
in the United States, the
National Library for the Blind The National Library for the Blind (NLB) was a public library A library is a curated collection of sources of information and similar resources, made accessible to a defined community for reference or borrowing. It provides physical or electro ...
or the
RNIB The Royal National Institute of Blind People (RNIB) is a UK charity Charity may refer to: Giving * Charitable organization or charity, a non-profit organization whose primary objectives are philanthropy and social well-being * Charity (practice), ...
in the United Kingdom.
Closed-circuit television Closed-circuit television (CCTV), also known as video surveillance, is the use of video camera A video camera is a camera A camera is an optical instrument used to capture an image An SAR radar imaging, radar image acquired b ...
s, equipment that enlarges and contrasts textual items, are a more
high-tech High technology (high tech) or frontier technology (frontier tech) is technology Technology ("science of craft", from Ancient Greek, Greek , ''techne'', "art, skill, cunning of hand"; and , ''wikt:-logia, -logia'') is the sum of Art techni ...

high-tech
alternative to traditional magnification devices. There are also over 100
radio reading service A radio reading service or reading service for the blind is a public service of many universities, community groups and public radio Public broadcasting involves radio Radio is the technology of signaling and communicating Communicatio ...
s throughout the world that provide people with vision impairments with readings from periodicals over the radio. The International Association of Audio Information Services provides links to all of these organizations.


Computers and mobile technology

Access technology such as
screen reader A screen reader is a form of assistive technology (AT) that renders text and image content as speech or braille output. Screen readers are essential to people who are blindness, blind, and are useful to people who are visual impairment, visually ...
s,
screen magnifier A screen magnifier is software that interfaces with a computer's graphical output to present enlarged screen content. By enlarging part (or all) of a screen, people with visual impairments can better see words and images. This type of assistive tec ...
s and
refreshable braille display#REDIRECT Refreshable braille display A refreshable braille display or braille terminal is an electro-mechanical device for displaying braille Braille ( ; Braille: ⠃⠗⠇; ) is a tactile writing system used by people who are visually impair ...

refreshable braille display
s enable the blind to use mainstream computer applications and
mobile phone A mobile phone, cellular phone, cell phone, cellphone, handphone, or hand phone, sometimes shortened to simply mobile, cell or just phone, is a portable telephone A telephone is a telecommunication Telecommunication is the tra ...

mobile phone
s. The availability of assistive technology is increasing, accompanied by concerted efforts to ensure the accessibility of information technology to all potential users, including the blind. Later versions of
Microsoft Windows Microsoft Windows, commonly referred to as Windows, is a group of several proprietary {{Short pages monitor