Near-sightedness, also known as myopia and short-sightedness, is an
eye
Eyes are organs of the visual system. They provide living organisms with vision, the ability to receive and process visual detail, as well as enabling several photo response functions that are independent of vision. Eyes detect light and conv ...
disease where light focuses in front of, instead of on, the
retina
The retina (from la, rete "net") is the innermost, light-sensitive layer of tissue of the eye of most vertebrates and some molluscs. The optics of the eye create a focused two-dimensional image of the visual world on the retina, which then ...
.
[ As a result, distant objects appear ]blurry
"Blurry" is a song by American rock band Puddle of Mudd. It was released on October 16, 2001, as the second single from the band's debut album '' Come Clean'' (2001). It was 2002's most successful rock song in the United States, topping the ''Bi ...
while close objects appear normal.[ Other symptoms may include ]headache
Headache is the symptom of pain in the face, head, or neck. It can occur as a migraine, tension-type headache, or cluster headache. There is an increased risk of depression in those with severe headaches.
Headaches can occur as a result ...
s and eye strain
Eye strain, also known as asthenopia (from Greek ''a-sthen-opia'', grc, ἀσθενωπία, ), is a common eye condition that manifests through non-specific symptoms such as fatigue, pain in or around the eyes, blurred vision, headache, and ...
. Severe near-sightedness is associated with an increased risk of retinal detachment
Retinal detachment is a disorder of the eye in which the retina peels away from its underlying layer of support tissue. Initial detachment may be localized, but without rapid treatment the entire retina may detach, leading to vision loss and blin ...
, cataract
A cataract is a cloudy area in the lens of the eye that leads to a decrease in vision. Cataracts often develop slowly and can affect one or both eyes. Symptoms may include faded colors, blurry or double vision, halos around light, trouble w ...
s, and glaucoma
Glaucoma is a group of eye diseases that result in damage to the optic nerve (or retina) and cause vision loss. The most common type is open-angle (wide angle, chronic simple) glaucoma, in which the drainage angle for fluid within the eye rem ...
.[
The underlying mechanism involves the length of the eyeball growing too long or less commonly the ]lens
A lens is a transmissive optical device which 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 consists of several simple lenses (''elements''), ...
being too strong. It 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 of refractive error are myopia, near-sightedne ...
.[ Diagnosis is by ]eye examination
An eye examination is a series of tests performed to assess vision and ability to focus on and discern objects. It also includes other tests and examinations pertaining to the eyes. Eye examinations are primarily performed by an optometrist, op ...
.[
Tentative evidence indicates that the risk of near-sightedness can be decreased by having young children spend more time outside.] This decrease in risk may be related to natural light
Natural Light, sometimes Natty Light, is an American reduced-calorie light lager brewed by Anheuser-Busch since its introduction on July 31, 1977. Its ingredients are listed as water, barley malt, cereal grains, yeast, and hops. One serving cont ...
exposure. Near-sightedness can be corrected with eyeglass
Glasses, also known as eyeglasses or spectacles, are vision eyewear, with lenses (clear or tinted) 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 as temples or t ...
es, contact lenses
Contact lenses, or simply contacts, are thin lenses placed directly on the surface of the eyes. Contact lenses are ocular prosthetic devices used by over 150 million people worldwide, and they can be worn to correct vision or for cosmetic ...
, or a refractive surgery
Refractive eye surgery is optional eye surgery used to improve the refractive state of the eye and decrease or eliminate dependency on glasses or contact lenses. This can include various methods of surgical remodeling of the cornea (keratomileu ...
.[ Eyeglasses are the easiest and safest method of correction.][ Contact lenses can provide a wider ]field of vision
The visual field is the "spatial array of visual sensations available to observation in introspectionist psychological experiments". Or simply, visual field can be defined as the entire area that can be seen when an eye is fixed straight at a point ...
, but are associated with a risk of infection.[ Refractive surgery permanently changes the shape of the ]cornea
The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. Along with the anterior chamber and lens, the cornea refracts light, accounting for approximately two-thirds of the eye's total optical power ...
.[
Near-sightedness is the most common eye problem and is estimated to affect 1.5 billion people (22% of the world population).] Rates vary significantly in different areas of the world.[ Rates among adults are between 15% to 49%.][ Among children, it affects 1% of rural Nepalese, 4% of South Africans, 12% of people in the US, and 37% in some large Chinese cities.][ In China the proportion of girls is slightly higher than boys.] Rates have increased since the 1950s.[ Uncorrected near-sightedness is one of the most common causes of ]vision impairment
Visual impairment, also known as vision impairment, is a medical definition primarily measured based on an individual's better eye visual acuity; in the absence of treatment such as correctable eyewear, assistive devices, and medical treatment ...
globally along with cataract
A cataract is a cloudy area in the lens of the eye that leads to a decrease in vision. Cataracts often develop slowly and can affect one or both eyes. Symptoms may include faded colors, blurry or double vision, halos around light, trouble w ...
s, 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 ...
, and vitamin A deficiency
Vitamin A deficiency (VAD) or hypovitaminosis A is a lack of vitamin A in blood and tissues. It is common in poorer countries, especially among children and women of reproductive age, but is rarely seen in more developed countries. Nyctalopia (ni ...
.
Etymology
The term ''myopia'' is of Koine Greek
Koine Greek (; Koine el, ἡ κοινὴ διάλεκτος, hē koinè diálektos, the common dialect; ), also known as Hellenistic Greek, common Attic, the Alexandrian dialect, Biblical Greek or New Testament Greek, was the common supra-reg ...
origin: (or ) "short-sight(-ness)", from Ancient Greek
Ancient Greek includes the forms of the Greek language used in ancient Greece and the ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (), Dark Ages (), the Archaic peri ...
"short-sighted (man), (man) with eyes getting shut", from "to shut the eyes" and "eye, look, sight" (GEN
Gen may refer to:
* ''Gen'' (film), 2006 Turkish horror film directed by Togan Gökbakar
* Gen (Street Fighter), a video game character from the ''Street Fighter'' series
* Gen Fu, a video game character from the ''Dead or Alive'' series
* Gen l ...
). The opposite of ''myopia'' in English is ''hyperopia
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, i ...
'' (long-sightedness).
Signs and symptoms
A near-sighted individual can see clearly out to a certain distance (the far point
In visual perception, the far point is the farthest point at which an object can be placed (along the optical axis of the eye) for its image to be focused on the retina within the eye's accommodation. It is sometimes described as the farthest p ...
of the eye), but objects placed beyond this distance appear blurred. If the extent of the myopia is great enough, even standard reading distances can be affected. Upon routine examination of the eyes, the vast majority of myopic eyes appear structurally identical to nonmyopic eyes.
Onset is often in school children, with worsening between the ages of 8 and 15.
Causes
The underlying cause is believed to be a combination of genetic and environmental factors.[ Risk factors include doing work that involves focusing on close objects, greater time spent indoors, urbanization, and a family history of the condition.] It is also associated with a high socioeconomic class and higher level of education.
A 2012 review could not find strong evidence for any single cause, although many theories have been discredited. Twin studies
Twin studies are studies conducted on identical or fraternal twins. They aim to reveal the importance of environmental and genetic influences for traits, phenotypes, and disorders. Twin research is considered a key tool in behavioral genetics a ...
indicate that at least some genetic factors are involved. Myopia has been increasing rapidly throughout the developed world, suggesting environmental factors are involved.
A single-author literature review in 2021 proposed that myopia is the result of corrective lenses interfering with emmetropization.
Genetics
A risk for myopia may be inherited from one's parents. Genetic linkage
Genetic linkage is the tendency of DNA sequences that are close together on a chromosome to be inherited together during the meiosis phase of sexual reproduction. Two genetic markers that are physically near to each other are unlikely to be separ ...
studies have identified 18 possible loci on 15 different chromosomes that are associated with myopia, but none of these loci is part of the candidate genes that cause myopia. Instead of a simple one-gene locus controlling the onset of myopia, a complex interaction of many mutated proteins acting in concert may be the cause. Instead of myopia being caused by a defect in a structural protein, defects in the control of these structural proteins might be the actual cause of myopia. A collaboration of all myopia studies worldwide identified 16 new loci for refractive error in individuals of European ancestry, of which 8 were shared with Asians. The new loci include candidate genes with functions in neurotransmission, ion transport, retinoic acid metabolism, extracellular matrix remodeling and eye development. The carriers of the high-risk genes have a tenfold increased risk of myopia. Aberrant genetic recombination and gene splicing in the OPNLW1 and OPNMW1 genes that code for two retinal cone photopigment proteins can produce high myopia by interfering with refractive development of the eye.
Human population studies suggest that contribution of genetic factors accounts for 60–90% of variance in refraction. However, the currently identified variants account for only a small fraction of myopia cases, suggesting the existence of a large number of yet unidentified low-frequency or small-effect variants, which underlie the majority of myopia cases.
Environmental factors
Environmental factors which increase the risk of nearsightedness include insufficient light exposure, low physical activity, near work, and increased year of education.[
One hypothesis is that a lack of normal visual stimuli causes improper development of the eyeball. Under this hypothesis, "normal" refers to the environmental stimuli that the eyeball evolved to.][Lieberman, Daniel E. (2013) ''The Story of the Human Body: Evolution, Health, and Disease''. New York: Pantheon Books.] Modern humans who spend most of their time indoors, in dimly or fluorescently lit buildings may be at risk of development of myopia.
People, and children especially, who spend more time doing physical exercise
Exercise is a body activity that enhances or maintains physical fitness and overall health and wellness.
It is performed for various reasons, to aid growth and improve strength, develop muscles and the cardiovascular system, hone athletic ...
and outdoor play have lower rates of myopia, suggesting the increased magnitude and complexity of the visual stimuli encountered during these types of activities decrease myopic progression. There is preliminary evidence that the protective effect of outdoor activities on the development of myopia is due, at least in part, to the effect of long hours of exposure to daylight
Daylight is the combination of all direct and indirect sunlight during the daytime. This includes direct sunlight, diffuse sky radiation, and (often) both of these reflected by Earth and terrestrial objects, like landforms and buildings. Sunligh ...
on the production and the release of retinal dopamine
Dopamine (DA, a contraction of 3,4-dihydroxyphenethylamine) is a neuromodulatory molecule that plays several important roles in cells. It is an organic compound, organic chemical of the catecholamine and phenethylamine families. Dopamine const ...
.
Myopia can be induced with minus spherical lenses, and overminus in prescription lenses can induce myopia progression.[https://www.optometrystudents.com/pearl/over-minus-you-probably-do-itstop-it/] Overminus during refraction can be avoided through various techniques and tests, such as fogging, plus to blur, and the duochrome test
A duochrome test is a test commonly used to refine the final sphere in refraction, which makes use of the longitudinal chromatic aberration of the eye. Because of the chromatic aberration of the eye, the shorter wavelengths (green) are focused in ...
.
The near work hypothesis, also referred to as the "use-abuse theory" states that spending time involved in near work strains the intraocular and extraocular muscles. Some studies support the hypothesis, while other studies do not.[ While an association is present, it is not clearly causal.][
Nearsightedness is also more common in children with ]diabetes
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 ...
, childhood arthritis
Childhood arthritis (also known as juvenile arthritis, JRA, Juvenile Idiopathic Disease or Arthritis, or Still's Disease) is any form of chronic arthritis or chronic arthritis-related conditions which affects individuals under the age of 16. Most ...
, uveitis
Uveitis () is inflammation of the uvea, the pigmented layer of the eye 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 ...
, and 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 ...
.[
]
Mechanism
Because myopia is a refractive error, the physical cause of myopia is comparable to any optical system that is out of focus. Borish and Duke-Elder classified myopia by these physical causes:[Borish, Irvin M. (1949). ''Clinical Refraction''. Chicago: The Professional Press.][Duke-Elder, Sir Stewart (1969). ''The Practice of Refraction'' (8th ed.). St. Louis: The C.V. Mosby Company. .]
* Axial myopia is attributed to an increase in the eye's axial length
* Refractive myopia is attributed to the condition of the refractive
In physics, refraction is the redirection of a wave as it passes from one medium to another. The redirection can be caused by the wave's change in speed or by a change in the medium. Refraction of light is the most commonly observed phenomeno ...
elements of the eye. Borish further subclassified refractive myopia:
** Curvature myopia is attributed to excessive, or increased, curvature of one or more of the refractive surfaces of the eye, especially the cornea. In those with Cohen syndrome
Cohen syndrome (also known as Pepper syndrome or Cervenka syndrome) is a very rare autosomal recessive genetic disorder with varied expression, characterised by obesity, intellectual disability, distinct craniofacial abnormalities and potential ocu ...
, myopia appears to result from high corneal and lenticular power.
** Index myopia is attributed to variation in the index of refraction
In optics, the refractive index (or refraction index) of an optical medium is a dimensionless number that gives the indication of the light bending ability of that medium.
The refractive index determines how much the path of light is bent, or ...
of one or more of the ocular media.
As with any optical system experiencing a defocus aberration
In optics, defocus is the aberration in which an image is simply out of focus. This aberration is familiar to anyone who has used a camera, videocamera, microscope, telescope, or binoculars. Optically, defocus refers to a translation of the ...
, the effect can be exaggerated or masked by changing the aperture size. In the case of the eye, a large pupil emphasizes refractive error and a small pupil masks it. This phenomenon can cause a condition in which an individual has a greater difficulty seeing in low-illumination areas, even though there are no symptoms in bright light, such as daylight.
Under rare conditions, edema of the ciliary body
The ciliary body is a part of the eye that includes the ciliary muscle, which controls the shape of the lens, and the ciliary epithelium, which produces the aqueous humor. The aqueous humor is produced in the non-pigmented portion of the ciliar ...
can cause an anterior displacement of the lens, inducing a myopia shift in refractive error.
Diagnosis
A diagnosis of myopia is typically made by an eye care professional
An eye care professional (ECP) is an individual who provides a service related to the eyes or vision. It is any healthcare worker involved in eye care, from one with a small amount of post-secondary training to practitioners with a doctoral level ...
, usually an optometrist
Optometry is a specialized health care profession that involves examining the eyes and related structures for defects or abnormalities. Optometrists are health care professionals who typically provide comprehensive primary eye care.
In the Uni ...
or ophthalmologist
Ophthalmology ( ) is a surgery, surgical subspecialty within medicine that deals with the diagnosis and treatment of eye disorders.
An ophthalmologist is a physician who undergoes subspecialty training in medical and surgical eye care. Followin ...
. During a refraction
In physics, refraction is the redirection of a wave as it passes from one medium to another. The redirection can be caused by the wave's change in speed or by a change in the medium. Refraction of light is the most commonly observed phenomeno ...
, an autorefractor
An autorefractor or automated refractor is a computer-controlled machine used during an eye examination to provide an objective measurement of a person's refractive error and prescription for glasses or contact lenses. This is achieved by measuri ...
or retinoscope
Retinoscopy (Ret) is a technique to obtain an objective measurement of the refractive error of a patient's eyes. The examiner uses a retinoscope to shine light into the patient's eye and observes the reflection (reflex) off the patient's retina. W ...
is used to give an initial objective assessment of the refractive status of each eye, then a phoropter
A phoropter or refractor is an ophthalmic testing device. It is commonly used by eye care professionals during an eye examination, and contains different lenses used for refraction of the eye during sight testing, to measure an individual's ref ...
is used to subjectively refine the patient's eyeglass prescription
An eyeglass prescription is an order written by an eyewear prescriber, such as an optometrist, that specifies the value of all parameters the prescriber has deemed necessary to construct and/or dispense corrective lenses appropriate for a patie ...
. Other types 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 of refractive error are myopia, near-sightedne ...
are hyperopia
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, i ...
, astigmatism
Astigmatism is a type of refractive error due to rotational asymmetry in the eye's refractive power. This results in distorted or blurred vision at any distance. Other symptoms can include eyestrain, headaches, and trouble driving at nig ...
, and 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. Also known as age-related farsightedness (or age-related long si ...
.
Types
Various forms of myopia have been described by their clinical appearance:
* Simple myopia: Myopia in an otherwise normal eye, typically less than 4.00 to 6.00 diopters
A dioptre (British spelling) or diopter (American spelling) is a unit of measurement with dimension of reciprocal length, equivalent to one reciprocal metre, 1 dioptre = 1 m−1. It is normally used to express the optical power of a lens or curv ...
. This is the most common form of myopia.
* Degenerative myopia, also known as malignant, pathological, or progressive myopia, is characterized by marked fundus changes, such as posterior staphyloma
A staphyloma is an abnormal protrusion of the uveal tissue through a weak point in the eyeball. The protrusion is generally black in colour, due to the inner layers of the eye. It occurs due to weakening of outer layer of eye (cornea or sclera) by ...
, and associated with a high 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 of refractive error are myopia, near-sightedne ...
and subnormal visual acuity after correction. This form of myopia gets progressively worse over time. Degenerative myopia has been reported as one of the main causes of visual impairment
Visual impairment, also known as vision impairment, is a medical definition primarily measured based on an individual's better eye visual acuity; in the absence of treatment such as correctable eyewear, assistive devices, and medical treatment ...
.
* Pseudomyopia
Pseudomyopia occurs when a spasm of the ciliary muscle prevents the eye from focusing in the distance, sometimes intermittently; this is different from myopia which is caused by the eye's shape or other basic anatomy. Pseudomyopia may be either org ...
is the blurring of distance vision brought about by spasm
A spasm is a sudden involuntary contraction of a muscle, a group of muscles, or a hollow organ such as the bladder.
A spasmodic muscle contraction may be caused by many medical conditions, including dystonia. Most commonly, it is a muscle c ...
of the accommodation system.[Cassin, B. and Solomon, S. (2001) ''Dictionary of Eye Terminology''. Gainesville, Florida: Triad Publishing Company. .]
* Nocturnal myopia: Without adequate stimulus for accurate accommodation, the accommodation system partially engages, pushing distance objects out of focus.
* Nearwork-induced transient myopia (NITM): short-term myopic far point shift immediately following a sustained near visual task. Some authors argue for a link between NITM and the development of permanent myopia.
* Instrument myopia
Instrument myopia, is a form of myopia that occurs when someone is looking into an optical instrument such as a microscope. The person focusses their eyes closer than needed for the image produced by the instrument.
Characteristics
Ordinarily, wh ...
: over-accommodation when looking into an instrument such as a microscope
A microscope () is a laboratory instrument used to examine objects that are too small to be seen by the naked eye. Microscopy is the science of investigating small objects and structures using a microscope. Microscopic means being invisibl ...
.
* Induced myopia, also known as acquired myopia, results from various medications, increases in glucose
Glucose is a simple sugar with the molecular formula . Glucose is overall the most abundant monosaccharide, a subcategory of carbohydrates. Glucose is mainly made by plants and most algae during photosynthesis from water and carbon dioxide, using ...
levels, nuclear sclerosis
Nuclear sclerosis is an age-related change in the density of the crystalline lens
The lens, or crystalline lens, is a transparent biconvex structure in the eye that, along with the cornea, helps to refract light to be focused on the retina. ...
, oxygen toxicity
Oxygen toxicity is a condition resulting from the harmful effects of breathing molecular oxygen () at increased partial pressures. Severe cases can result in cell damage and death, with effects most often seen in the central nervous system, lu ...
(e.g., from diving or from oxygen and hyperbaric therapy) or other anomalous conditions. Sulphonamide
In organic chemistry, the sulfonamide functional group (also spelled sulphonamide) is an organosulfur group with the structure . It consists of a sulfonyl group () connected to an amine group (). Relatively speaking this group is unreactive. ...
therapy can cause ciliary body edema, resulting in anterior
Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position prov ...
displacement of the lens, pushing the eye out of focus. Elevation of blood-glucose levels can also cause edema (swelling) of the crystalline lens
The lens, or crystalline lens, is a transparent biconvex structure in the eye that, along with the cornea, helps to refract light to be focused on the retina. By changing shape, it functions to change the focal length of the eye so that it can ...
as a result of sorbitol
Sorbitol (), less commonly known as glucitol (), is a sugar alcohol with a sweet taste which the human body metabolizes slowly. It can be obtained by reduction of glucose, which changes the converted aldehyde group (−CHO) to a primary alcohol g ...
accumulating in the lens. This edema often causes temporary myopia. Scleral buckle
A scleral buckle is one of several ophthalmologic procedures that can be used to repair a retinal detachment. Retinal detachments are usually caused by retinal tears, and a scleral buckle can be used to close the retinal break, both for acute and ...
s, used in the repair of retinal detachment
Retinal detachment is a disorder of the eye in which the retina peels away from its underlying layer of support tissue. Initial detachment may be localized, but without rapid treatment the entire retina may detach, leading to vision loss and blin ...
s may induce myopia by increasing the axial length of the eye.
* Index myopia is attributed to variation in the index of refraction of one or more of the ocular media. Cataracts may lead to index myopia.
* Form deprivation myopia occurs when the eyesight is deprived by limited illumination and vision range, or the eye is modified with artificial lenses or deprived of clear form vision. In lower vertebrates, this kind of myopia seems to be reversible within short periods of time. Myopia is often induced this way in various animal models to study the pathogenesis
Pathogenesis is the process by which a disease or disorder develops. It can include factors which contribute not only to the onset of the disease or disorder, but also to its progression and maintenance. The word comes from Greek πάθος ''pat ...
and mechanism of myopia development.
Degree
The degree of myopia is described in terms of the power
Power most often refers to:
* Power (physics), meaning "rate of doing work"
** Engine power, the power put out by an engine
** Electric power
* Power (social and political), the ability to influence people or events
** Abusive power
Power may a ...
of the ideal correction
Correction may refer to:
* A euphemism for punishment
* Correction (newspaper), the posting of a notice of a mistake in a past issue of a newspaper
* Correction (stock market), in financial markets, a short-term price decline
* Correction (novel), ...
, which is measured in diopters
A dioptre (British spelling) or diopter (American spelling) is a unit of measurement with dimension of reciprocal length, equivalent to one reciprocal metre, 1 dioptre = 1 m−1. It is normally used to express the optical power of a lens or curv ...
:
* Myopia between −0.00 and −0.50 diopters is usually classified as emmetropia
Emmetropia is the state of vision in which a faraway object at infinity is in sharp focus with the ciliary muscle in a relaxed state. That condition of the normal eye is achieved when the refractive power of the cornea and eye lens and the axia ...
.
* Low myopia usually describes myopia between −0.50 and −3.00 diopters.
* Moderate myopia usually describes myopia between −3.00 and −6.00 diopters
A dioptre (British spelling) or diopter (American spelling) is a unit of measurement with dimension of reciprocal length, equivalent to one reciprocal metre, 1 dioptre = 1 m−1. It is normally used to express the optical power of a lens or curv ...
. Those with moderate amounts of myopia are more likely to have pigment dispersion syndrome
Pigment dispersion syndrome (PDS) is an eye disorder that can lead to a form of glaucoma known as pigmentary glaucoma. It takes place when pigment cells slough off from the back of the iris and float around in the aqueous humor. Over time, these ...
or pigmentary glaucoma
Pigment dispersion syndrome (PDS) is an eye disorder that can lead to a form of glaucoma known as pigmentary glaucoma. It takes place when pigment cells slough off from the back of the iris and float around in the aqueous humor. Over time, these ...
.
* High myopia usually describes myopia of −6.00 or more. People with high myopia are more likely to have retinal detachment
Retinal detachment is a disorder of the eye in which the retina peels away from its underlying layer of support tissue. Initial detachment may be localized, but without rapid treatment the entire retina may detach, leading to vision loss and blin ...
s and primary open angle glaucoma
Glaucoma is a group of eye diseases that result in damage to the optic nerve (or retina) and cause vision loss. The most common type is open-angle (wide angle, chronic simple) glaucoma, in which the drainage angle for fluid within the eye rem ...
. They are also more likely to experience floater
Floaters or eye floaters are sometimes visible deposits (e.g., the shadows of tiny structures of protein or other cell debris projected onto the retina) within the eye's vitreous humour ("the vitreous"), which is normally transparent, or betwee ...
s, shadow-like shapes which appear in the field of vision
The visual field is the "spatial array of visual sensations available to observation in introspectionist psychological experiments". Or simply, visual field can be defined as the entire area that can be seen when an eye is fixed straight at a point ...
. In addition to this, high myopia is linked to macular degeneration, cataracts, and significant visual impairment.
Age at onset
Myopia is sometimes classified by the age at onset:
* Congenital myopia, also known as infantile myopia, is present at birth and persists through infancy.
* Youth onset myopia occurs in early childhood or teenage, and the ocular power can keep varying until the age of 21, before which any form of corrective surgery is usually not recommended by ophthalmic specialists around the world.
* School myopia appears during childhood, particularly the school-age years. This form of myopia is attributed to the use of the eyes for close work during the school years. A 2004-2015 Singapore-Sydney study of children of Chinese descent found that time spent on outdoor activities was a factor.
* Adult onset myopia
:* Early adult onset myopia occurs between ages 20 and 40.
:*Late adult onset myopia occurs after age 40.
Prevention
Various methods have been employed in an attempt to decrease the progression of myopia, although studies show mixed results. Many myopia treatment studies have a number of design drawbacks: small numbers, lack of adequate control
Control may refer to:
Basic meanings Economics and business
* Control (management), an element of management
* Control, an element of management accounting
* Comptroller (or controller), a senior financial officer in an organization
* Controllin ...
group, and failure to mask examiners from knowledge of treatments used. Among myopia specialists, mydriatic
Mydriasis is the dilation of the pupil, usually having a non-physiological cause, or sometimes a physiological pupillary response. Non-physiological causes of mydriasis include disease, trauma, or the use of certain types of drugs.
Normally, as ...
eyedrops are the most favored approach, applied by almost 75% in North America and more than 80% in Australia. A 2015 review suggested that increased outdoor time protects young children from myopia. A 2020 study of global practice patterns used by paediatric ophthalmologists to decrease the progression of myopia showed behavioral intervention (counseling to spend more time outdoors and less time with near-work) to be favored by 25% of specialists, usually in addition to medications.
Glasses and contacts
The use of reading glasses when doing close work may improve vision by reducing or eliminating the need to accommodate. Altering the use of eyeglasses between full-time, part-time, and not at all does not appear to alter myopia progression. The American Optometric Association's Clinical Practice Guidelines found evidence of effectiveness of bifocal lenses and recommends it as the method for "myopia control". In some studies, bifocal and progressive lens
Progressive lenses, also called multifocal lenses, progressive addition lenses (PAL), varifocal lenses, progressive power lenses, graduated prescription lenses, or progressive spectacle lenses are corrective lenses used in eyeglasses to correct pr ...
es have not shown differences in altering the progression of myopia compared to placebo.
In 2019 contact lenses to prevent the worsening of nearsightedness in children were approved for use in the United States. This "MiSight" type claims to work by focusing peripheral light in front of the retina.
Medication
Anti-muscarinic topical medications in children under 18 years of age may slow the worsening of myopia. These treatments include pirenzepine gel, cyclopentolate eye drops, and atropine
Atropine is a tropane alkaloid and anticholinergic medication used to treat certain types of nerve agent and pesticide poisonings as well as some types of slow heart rate, and to decrease saliva production during surgery. It is typically given i ...
eye drops
Eye drops or eyedrops are liquid drops applied directly to the surface of the eye usually in small amounts such as a single drop or a few drops. Eye drops usually contain saline to match the salinity of the eye. Drops containing only saline ...
. While these treatments were shown to be effective in slowing the progression of myopia, side effects included light sensitivity and near blur.
Other methods
Scleral reinforcement surgery
Scleral reinforcement is a surgical procedure used to reduce or stop further macular damage caused by high myopia, which can be degenerative.
High myopia
Myopia is one of the leading causes of blindness in the world.Grossniklaus, H.E. and W.R. ...
is aimed to cover the thinning posterior pole with a supportive material to withstand intraocular pressure and prevent further progression of the posterior staphyloma. The strain is reduced, although damage from the pathological process cannot be reversed. By stopping the progression of the disease, vision may be maintained or improved.
Treatment
The National Institutes of Health says there is no known way of preventing myopia, and the use of glasses or contact lenses does not affect its progression, unless the glasses or contact lenses are too strong of a prescription. There is no universally accepted method of preventing myopia and proposed methods need additional study to determine their effectiveness. Optical correction using glasses
Glasses, also known as eyeglasses or spectacles, are vision eyewear, with lenses (clear or tinted) 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 as temples or ...
or contact lens
Contact lenses, or simply contacts, are thin lenses placed directly on the surface of the eyes. Contact lenses are ocular prosthetic devices used by over 150 million people worldwide, and they can be worn to correct vision or for cosmetic ...
es is the most common treatment; other approaches include orthokeratology
Orthokeratology, also referred to as Night lenses, Ortho-K, OK, Overnight Vision Correction, Corneal Refractive Therapy (CRT), Accelerated Orthokeretology, Cornea Corrective Contacts, Eccentricity Zero Molding, and Gentle Vision Shaping System (GVS ...
, and refractive surgery
Refractive eye surgery is optional eye surgery used to improve the refractive state of the eye and decrease or eliminate dependency on glasses or contact lenses. This can include various methods of surgical remodeling of the cornea (keratomileu ...
. Medications (mostly atropine
Atropine is a tropane alkaloid and anticholinergic medication used to treat certain types of nerve agent and pesticide poisonings as well as some types of slow heart rate, and to decrease saliva production during surgery. It is typically given i ...
) and vision therapy
Vision therapy (VT), or behavioral optometry, is an umbrella term for alternative medicine treatments using eye exercises, based around the pseudoscientific claim that vision problems are the true underlying cause of learning disabilities, parti ...
can be effective in addressing the various forms of pseudomyopia
Pseudomyopia occurs when a spasm of the ciliary muscle prevents the eye from focusing in the distance, sometimes intermittently; this is different from myopia which is caused by the eye's shape or other basic anatomy. Pseudomyopia may be either org ...
.
Glasses and contacts
Corrective lens
A corrective lens is a lens (i.e. a transmissive optical device) that is typically worn in front of the eye to improve daily vision. The most common use is to treat refractive errors: myopia, hypermetropia, astigmatism, and presbyopia. Glass ...
es bend the light entering the eye in a way that places a focused image accurately onto the retina. The power of any lens system can be expressed in diopter
A dioptre (British spelling) or diopter (American spelling) is a unit of measurement with dimension of reciprocal length, equivalent to one reciprocal metre, 1 dioptre = 1 m−1. It is normally used to express the optical power of a lens or cur ...
s, the reciprocal
Reciprocal may refer to:
In mathematics
* Multiplicative inverse, in mathematics, the number 1/''x'', which multiplied by ''x'' gives the product 1, also known as a ''reciprocal''
* Reciprocal polynomial, a polynomial obtained from another pol ...
of its focal length
The focal length of an optical system is a measure of how strongly the system converges or diverges light; it is the inverse of the system's optical power. A positive focal length indicates that a system converges light, while a negative foca ...
in meters. Corrective lenses for myopia have negative powers because a divergent lens is required to move the far point
In visual perception, the far point is the farthest point at which an object can be placed (along the optical axis of the eye) for its image to be focused on the retina within the eye's accommodation. It is sometimes described as the farthest p ...
of focus out to the distance. More severe myopia needs lens powers further from zero (more negative). However, strong eyeglass prescriptions create distortions such as prismatic movement and chromatic aberration
In optics, chromatic aberration (CA), also called chromatic distortion and spherochromatism, is a failure of a lens to focus all colors to the same point. It is caused by dispersion: the refractive index of the lens elements varies with the wave ...
. Strongly near-sighted wearers of contact lens
Contact lenses, or simply contacts, are thin lenses placed directly on the surface of the eyes. Contact lenses are ocular prosthetic devices used by over 150 million people worldwide, and they can be worn to correct vision or for cosmetic ...
es do not experience these distortions because the lens moves with the cornea, keeping the optic axis in line with the visual axis and because the vertex distance has been reduced to zero.
Surgery
Refractive surgery
Refractive eye surgery is optional eye surgery used to improve the refractive state of the eye and decrease or eliminate dependency on glasses or contact lenses. This can include various methods of surgical remodeling of the cornea (keratomileu ...
includes procedures which alter the corneal curvature of some structure of the eye or which add additional refractive means inside the eye.
Photorefractive keratectomy
Photorefractive keratectomy (PRK) involves ablation of corneal tissue from the corneal surface using an excimer laser
An excimer laser, sometimes more correctly called an exciplex laser, is a form of ultraviolet laser which is commonly used in the production of microelectronic devices, semiconductor based integrated circuits or "chips", eye surgery, and microm ...
. The amount of tissue ablation corresponds to the amount of myopia. While PRK is a relatively safe procedure for up to 6 dioptres of myopia, the recovery phase post-surgery is usually painful.
LASIK
In a LASIK
LASIK or Lasik (''laser-assisted in situ keratomileusis''), commonly referred to as laser eye surgery or laser vision correction, is a type of refractive surgery for the correction of myopia, hyperopia, and an actual cure for astigmatism, since ...
pre-procedure, a corneal flap is cut into the cornea and lifted to allow the excimer laser beam access to the exposed corneal tissue. After that, the excimer laser ablates the tissue according to the required correction. When the flap again covers the cornea, the change in curvature generated by the laser ablation proceeds to the corneal surface. Though LASIK is usually painless and involves a short rehabilitation period post-surgery, it can potentially result in flap complications and loss of corneal stability (post-LASIK keratectasia Corneal ectatic disorders or corneal ectasia are a group of uncommon, noninflammatory, eye disorders characterised by bilateral thinning of the central, paracentral, or peripheral cornea. Types
* Keratoconus, a progressive, noninflammatory, bilater ...
).
Phakic intra-ocular lens
Instead of modifying the corneal surface, as in laser vision correction (LVC), this procedure involves implanting an additional lens inside the eye (i.e., in addition to the already existing natural lens). While it usually results in good control of the refractive change, it can induce potential serious long-term complications such as glaucoma, cataract and endothelial decompensation.
Orthokeratology
Orthokeratology or simply Ortho-K is a temporary corneal reshaping process using rigid gas permeable (RGP) contact lenses. Overnight wearing of specially designed contact lenses will temporarily reshape cornea, so patients may see clearly without any lenses in daytime. Orthokeratology can correct myopia up to -6D. Several studies shown that Ortho-K can reduce myopia progression also. Risk factors of using Ortho-K lenses include microbial keratitis, corneal edema, etc. Other contact lens related complications like corneal aberration, photophobia, pain, irritation, redness etc. are usually temporary conditions, which may be eliminated by proper usage of lenses.
Intrastromal corneal ring segment
The Intrastromal corneal ring segment
An intrastromal corneal ring segment (ICRS) (also known as intrastromal corneal ring, corneal implant or corneal insert) is a small device implanted in the eye to correct vision. The procedure involves an ophthalmologist who makes a small incisio ...
(ICRS), commonly used in keratoconus
Keratoconus (KC) is a disorder of the eye that results in progressive thinning of the cornea. This may result in blurry vision, double vision, nearsightedness, irregular astigmatism, and light sensitivity leading to poor quality-of-life. Usu ...
treatment now, was originally designed to correct mild to moderate myopia. The thickness is directly related to flattening and the diameter of the ring is proportionally inverse to the flattening of cornea. So, if diameter is smaller or thickness is greater, resulting myopia correction will be greater.
Alternative medicine
A number of alternative therapies have been claimed to improve myopia, including vision therapy
Vision therapy (VT), or behavioral optometry, is an umbrella term for alternative medicine treatments using eye exercises, based around the pseudoscientific claim that vision problems are the true underlying cause of learning disabilities, parti ...
, "behavioural optometry", various eye exercises and relaxation techniques, and the Bates method
The Bates method is an ineffective and potentially dangerous alternative therapy aimed at improving eyesight. Eye-care physician William Horatio Bates (1860–1931) held the erroneous belief that the extraocular muscles effected changes in ...
.[Bates, Wm H (1920]
Sight Without Glasses
. Ch. 10, p. 106. . Scientific reviews have concluded that there was "no clear scientific evidence" that eye exercises are effective in treating near-sightedness and as such they "cannot be advocated".
Epidemiology
Global refractive errors have been estimated to affect 800 million to 2.3 billion. The incidence of myopia within sampled population often varies with age, country, sex, race
Race, RACE or "The Race" may refer to:
* Race (biology), an informal taxonomic classification within a species, generally within a sub-species
* Race (human categorization), classification of humans into groups based on physical traits, and/or s ...
, ethnicity
An ethnic group or an ethnicity is a grouping of people who identify with each other on the basis of shared attributes that distinguish them from other groups. Those attributes can include common sets of traditions, ancestry, language, history, ...
, occupation, environment, and other factors. Variability in testing and data collection methods makes comparisons of prevalence and progression difficult.
The prevalence of myopia has been reported as high as 70–90% in some Asia
Asia (, ) is one of the world's most notable geographical regions, which is either considered a continent in its own right or a subcontinent of Eurasia, which shares the continental landmass of Afro-Eurasia with Africa. Asia covers an area ...
n countries, 30–40% in Europe and the United States, and 10–20% in Africa. Myopia is about twice as common in Jewish people
Jews ( he, יְהוּדִים, , ) or Jewish people are an ethnoreligious group and nation originating from the Israelites Israelite origins and kingdom: "The first act in the long drama of Jewish history is the age of the Israelites""Th ...
than in people of non-Jewish ethnicity.[ Jensen, A.R. (1998) '' The g Factor''. Westport, Connecticut: Praeger Publishers, ] Myopia is less common in African
African or Africans may refer to:
* Anything from or pertaining to the continent of Africa:
** People who are native to Africa, descendants of natives of Africa, or individuals who trace their ancestry to indigenous inhabitants of Africa
*** Ethn ...
people and associated diaspora. In Americans between the ages of 12 and 54, myopia has been found to affect African Americans
African Americans (also referred to as Black Americans and Afro-Americans) are an ethnic group consisting of Americans with partial or total ancestry from sub-Saharan Africa. The term "African American" generally denotes descendants of ens ...
less than Caucasians.
Asia
In some parts of Asia
Asia (, ) is one of the world's most notable geographical regions, which is either considered a continent in its own right or a subcontinent of Eurasia, which shares the continental landmass of Afro-Eurasia with Africa. Asia covers an area ...
, myopia is very common.
* Singapore
Singapore (), officially the Republic of Singapore, is a sovereign island country and city-state in maritime Southeast Asia. It lies about one degree of latitude () north of the equator, off the southern tip of the Malay Peninsula, borde ...
is believed to have the highest prevalence of myopia in the world; up to 80% of people there have myopia, but the accurate figure is unknown.
* China
China, officially the People's Republic of China (PRC), is a country in East Asia. It is the world's most populous country, with a population exceeding 1.4 billion, slightly ahead of India. China spans the equivalent of five time zones and ...
's myopia rate is 31%: 400 million of its 1.3 billion people are myopic. The prevalence of myopia in high school
A secondary school describes an institution that provides secondary education and also usually includes the building where this takes place. Some secondary schools provide both '' lower secondary education'' (ages 11 to 14) and ''upper seconda ...
in China is 77%, and in college
A college (Latin: ''collegium'') is an educational institution or a constituent part of one. A college may be a degree-awarding tertiary educational institution, a part of a collegiate or federal university, an institution offering ...
is more than 80%.
* In some areas, such as China and Malaysia
Malaysia ( ; ) is a country in Southeast Asia. The federation, federal constitutional monarchy consists of States and federal territories of Malaysia, thirteen states and three federal territories, separated by the South China Sea into two r ...
, up to 41% of the adult population is myopic to 1.00 dpt, and up to 80% to 0.5 dpt.
* A study of Jordanian adults aged 17 to 40 found over half (54%) were myopic.
* Some research suggests the prevalence of myopia in Indian children is less than 15%.
Europe
* In first-year undergraduate students in the United Kingdom 50% of British
British may refer to:
Peoples, culture, and language
* British people, nationals or natives of the United Kingdom, British Overseas Territories, and Crown Dependencies.
** Britishness, the British identity and common culture
* British English, ...
whites and 53% of British Asians were myopic.
* A recent review found 27% of Western Europeans aged 40 or over have at least −1.00 diopters of myopia and 5% have at least −5.00 diopters.
North America
Myopia is common in 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 primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territorie ...
, with research suggesting this condition has increased dramatically in recent decades. In 1971–1972, the National Health and Nutrition Examination Survey provided the earliest nationally representative estimates for myopia prevalence in the U.S., and found the prevalence in persons aged 12–54 was 25%. Using the same method, in 1999–2004, myopia prevalence was estimated to have climbed to 42%.
A study of 2,523 children in grades 1 to 8 (age, 5–17 years) found nearly one in 10 (9%) have at least −0.75 diopters of myopia. In this study, 13% had at least +1.25 D hyperopia
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, i ...
(farsightedness), and 28% had at least 1.00-D difference between the two principal meridians (cycloplegic autorefraction) of astigmatism
Astigmatism is a type of refractive error due to rotational asymmetry in the eye's refractive power. This results in distorted or blurred vision at any distance. Other symptoms can include eyestrain, headaches, and trouble driving at nig ...
. For myopia, Asians had the highest prevalence (19%), followed by Hispanics (13%). Caucasian children had the lowest prevalence of myopia (4%), which was not significantly different from African Americans (7%).
A recent review found 25% of Americans aged 40 or over have at least −1.00 diopters of myopia and 5% have at least −5.00 diopters.
Australia
In Australia
Australia, officially the Commonwealth of Australia, is a Sovereign state, sovereign country comprising the mainland of the Australia (continent), Australian continent, the island of Tasmania, and numerous List of islands of Australia, sma ...
, the overall prevalence of myopia (worse than −0.50 diopters) has been estimated to be 17%. In one recent study, less than one in 10 (8%) Australian children between the ages of four and 12 were found to have myopia greater than −0.50 diopters. A recent review found 16% of Australians aged 40 or over have at least −1.00 diopters of myopia and 3% have at least −5.00 diopters.
South America
In Brazil
Brazil ( pt, Brasil; ), officially the Federative Republic of Brazil (Portuguese: ), is the largest country in both South America and Latin America. At and with over 217 million people, Brazil is the world's fifth-largest country by area ...
, a 2005 study estimated 6% of Brazilians between the ages of 12 and 59 had −1.00 diopter of myopia or more, compared with 3% of the indigenous people in northwestern Brazil. Another found nearly 1 in 8 (13%) of the students in the city of Natal
NATAL or Natal may refer to:
Places
* Natal, Rio Grande do Norte, a city in Brazil
* Natal, South Africa (disambiguation), a region in South Africa
** Natalia Republic, a former country (1839–1843)
** Colony of Natal, a former British colony ( ...
were myopic.
History
The difference between the near-sighted and far-sighted people was noted already by Aristotle
Aristotle (; grc-gre, Ἀριστοτέλης ''Aristotélēs'', ; 384–322 BC) was a Greek philosopher and polymath during the Classical period in Ancient Greece. Taught by Plato, he was the founder of the Peripatetic school of phil ...
. Graeco-Roman physician Galen
Aelius Galenus or Claudius Galenus ( el, Κλαύδιος Γαληνός; September 129 – c. AD 216), often Anglicized as Galen () or Galen of Pergamon, was a Greek physician, surgeon and philosopher in the Roman Empire. Considered to be one of ...
first used the term "myopia" (from Greek words "myein" meaning "to close or shut" and "ops" (gen. opos) meaning "eye") for near-sightedness. The first spectacles for correcting myopia were invented by a German cardinal in the year 1451. Johannes Kepler
Johannes Kepler (; ; 27 December 1571 – 15 November 1630) was a German astronomer, mathematician, astrologer, natural philosopher and writer on music. He is a key figure in the 17th-century Scientific Revolution, best known for his laws ...
in his ''Clarification of Ophthalmic Dioptrics'' (1604) first demonstrated that near-sightedness was due to the incident light focusing in front of the retina. Kepler also showed that near-sightedness could be corrected by concave lenses. In 1632, Vopiscus Fortunatus Plempius Vopiscus may refer to:
* Vopiscus (praenomen), a Latin praenomen, or personal name
*Vopiscus Julius Iulus, a Roman consul in 473 BC
*Gaius Julius Caesar Strabo Vopiscus (c. 130 — 87 BC), an orator and tragedian
*Marcus Manilius Vopiscus, a Rom ...
examined a myopic eye and confirmed that myopia was due to a lengthening of its axial diameter.
Society and culture
The terms "myopia" and "myopic" (or the common terms "short-sightedness" or "short-sighted", respectively) have been used metaphor
A metaphor is a figure of speech that, for rhetorical effect, directly refers to one thing by mentioning another. It may provide (or obscure) clarity or identify hidden similarities between two different ideas. Metaphors are often compared wit ...
ically to refer to cognitive thinking and decision making that is narrow in scope or lacking in foresight or in concern for wider interests or for longer-term consequences. It is often used to describe a decision that may be beneficial in the present, but detrimental in the future, or a viewpoint that fails to consider anything outside a very narrow and limited range. Hyperopia
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, i ...
, the biological opposite of myopia, may also be used metaphorically for a value system
In ethics and social sciences, value denotes the degree of importance of something or action, with the aim of determining which actions are best to do or what way is best to live (normative ethics in ethics), or to describe the significance of dif ...
or motivation
Motivation is the reason for which humans and other animals initiate, continue, or terminate a behavior at a given time. Motivational states are commonly understood as forces acting within the agent that create a disposition to engage in goal-dire ...
that exhibits "farsighted" or possibly visionary
A visionary, defined broadly, is one who can envision the future. For some groups, this can involve the supernatural.
The visionary state is achieved via meditation, lucid dreams, daydreams, or art. One example is Hildegard of Bingen, a 12th-c ...
thinking and behavior; that is, emphasizing long-term interests at the apparent expense of near-term benefit.
Correlations
Numerous studies have found correlations between myopia, on the one hand, and intelligence and academic achievement, on the other; it is not clear whether there is a causal relationship
Causality (also referred to as causation, or cause and effect) is influence by which one event, process, state, or object (''a'' ''cause'') contributes to the production of another event, process, state, or object (an ''effect'') where the cau ...
.
Myopia is also correlated with increased microsaccade
Microsaccades are a kind of fixational eye movement. They are small, jerk-like, involuntary eye movements, similar to miniature versions of voluntary saccades. They typically occur during prolonged visual fixation (of at least several seconds), ...
amplitude, suggesting that blurred vision from myopia might cause instability in fixational eye movements
Fixation or visual fixation is the maintaining of the gaze on a single location. An animal can exhibit visual fixation if it possess a fovea in the anatomy of their eye. The fovea is typically located at the center of the retina and is the poi ...
.
See also
* Myopia in animals
Some animals suffer from shortsightedness and have poor eyesight. In domestic animals, myopia, with or without Astigmatism (eye), astigmatism, occurs frequently.
In cats
Evolved for low-light hunting, cats' eyes are proportionally enormous. Th ...
* Myopic crescent
References
External links
{{Portal bar, Medicine
Disorders of ocular muscles, binocular movement, accommodation and refraction
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