Myopia
Overview
 
Myopia is a refractive defect
Refractive error
A refractive error, or refraction error, is an error in the focusing of light by the eye and a frequent reason for reduced visual acuity.-Classification:...

 of the eye
Human eye
The human eye is an organ which reacts to light for several purposes. As a conscious sense organ, the eye allows vision. Rod and cone cells in the retina allow conscious light perception and vision including color differentiation and the perception of depth...

 in which collimated light
Collimated light
Collimated light is light whose rays are parallel, and therefore will spread slowly as it propagates. The word is related to "collinear" and implies light that does not disperse with distance , or that will disperse minimally...

 produces image focus
Focus (optics)
In geometrical optics, a focus, also called an image point, is the point where light rays originating from a point on the object converge. Although the focus is conceptually a point, physically the focus has a spatial extent, called the blur circle. This non-ideal focusing may be caused by...

 in front of the retina
Retina
The vertebrate retina is a light-sensitive tissue lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical...

 under conditions of accommodation
Accommodation (eye)
Accommodation is the process by which the vertebrate eye changes optical power to maintain a clear image on an object as its distance changes....

. In simpler terms, myopia is a condition of the eye where the light that comes in does not directly focus on the retina but in front of it. This causes the image that one sees when looking at a distant object to be out of focus but in focus when looking at a close object.

Eye care professional
Eye care professional
An eye care professional is an individual who provides a service related to the eyes or vision. It is a general term that can refer to any healthcare worker involved in eye care, from one with a small amount of post-secondary training to practitioners with a doctoral level of education.-Current...

s most commonly correct myopia through the use of corrective lens
Corrective lens
A corrective lens is a lens worn in front of the eye, mainly used to treat myopia, hyperopia, astigmatism, and presbyopia. Glasses or "spectacles" are worn on the face a short distance in front of the eye. Contact lenses are worn directly on the surface of the eye...

es, such as glasses
Glasses
Glasses, also known as eyeglasses , spectacles or simply specs , are frames bearing lenses worn in front of the eyes. They are normally used for vision correction or eye protection. Safety glasses are a kind of eye protection against flying debris or against visible and near visible light or...

 or contact lens
Contact lens
A contact lens, or simply contact, is a lens placed on the eye. They are considered medical devices and can be worn to correct vision, for cosmetic or therapeutic reasons. In 2004, it was estimated that 125 million people use contact lenses worldwide, including 28 to 38 million in the United...

es.
Encyclopedia
Myopia is a refractive defect
Refractive error
A refractive error, or refraction error, is an error in the focusing of light by the eye and a frequent reason for reduced visual acuity.-Classification:...

 of the eye
Human eye
The human eye is an organ which reacts to light for several purposes. As a conscious sense organ, the eye allows vision. Rod and cone cells in the retina allow conscious light perception and vision including color differentiation and the perception of depth...

 in which collimated light
Collimated light
Collimated light is light whose rays are parallel, and therefore will spread slowly as it propagates. The word is related to "collinear" and implies light that does not disperse with distance , or that will disperse minimally...

 produces image focus
Focus (optics)
In geometrical optics, a focus, also called an image point, is the point where light rays originating from a point on the object converge. Although the focus is conceptually a point, physically the focus has a spatial extent, called the blur circle. This non-ideal focusing may be caused by...

 in front of the retina
Retina
The vertebrate retina is a light-sensitive tissue lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical...

 under conditions of accommodation
Accommodation (eye)
Accommodation is the process by which the vertebrate eye changes optical power to maintain a clear image on an object as its distance changes....

. In simpler terms, myopia is a condition of the eye where the light that comes in does not directly focus on the retina but in front of it. This causes the image that one sees when looking at a distant object to be out of focus but in focus when looking at a close object.

Eye care professional
Eye care professional
An eye care professional is an individual who provides a service related to the eyes or vision. It is a general term that can refer to any healthcare worker involved in eye care, from one with a small amount of post-secondary training to practitioners with a doctoral level of education.-Current...

s most commonly correct myopia through the use of corrective lens
Corrective lens
A corrective lens is a lens worn in front of the eye, mainly used to treat myopia, hyperopia, astigmatism, and presbyopia. Glasses or "spectacles" are worn on the face a short distance in front of the eye. Contact lenses are worn directly on the surface of the eye...

es, such as glasses
Glasses
Glasses, also known as eyeglasses , spectacles or simply specs , are frames bearing lenses worn in front of the eyes. They are normally used for vision correction or eye protection. Safety glasses are a kind of eye protection against flying debris or against visible and near visible light or...

 or contact lens
Contact lens
A contact lens, or simply contact, is a lens placed on the eye. They are considered medical devices and can be worn to correct vision, for cosmetic or therapeutic reasons. In 2004, it was estimated that 125 million people use contact lenses worldwide, including 28 to 38 million in the United...

es. It may also be corrected by refractive surgery
Refractive surgery
Refractive eye surgery is any 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 or cataract surgery. The most common methods today use excimer lasers to...

, though there are cases of associated side effects. The corrective lenses
Lens (optics)
A lens is an optical device with perfect or approximate axial symmetry which transmits and refracts light, converging or diverging the beam. A simple lens consists of a single optical element...

 have a negative optical power
Optical power
Optical power is the degree to which a lens, mirror, or other optical system converges or diverges light. It is equal to the reciprocal of the focal length of the device. The dioptre is the most common unit of measurement of optical power...

 (i.e. are concave
Concave
The word concave means curving in or hollowed inward, as opposed to convex. The former may be used in reference to:* Concave lens, a lens with inward-curving surfaces.* Concave polygon, a polygon which is not convex....

) which compensates for the excessive positive diopters of the myopic eye. Myopia is partly hereditary
Heredity
Heredity is the passing of traits to offspring . This is the process by which an offspring cell or organism acquires or becomes predisposed to the characteristics of its parent cell or organism. Through heredity, variations exhibited by individuals can accumulate and cause some species to evolve...

.

By cause

Borish and Duke-Elder classified myopia by cause:
  • Axial myopia is attributed to an increase in the eye's axial length.
  • Refractive myopia is attributed to the condition of the refractive 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
    Cohen syndrome is believed to be a gene mutation at locus 8q22 gene COH1. Cohen syndrome has several characteristics such as obesity, mental retardation and craniofacial dysmorphism...

    , myopia appears to result from high corneal and lenticular power.
  • Index myopia is attributed to variation in the index of refraction of one or more of the ocular media.


Elevation of blood-glucose levels can also cause edema (swelling) of the crystalline lens (hyperphacosorbitomyopicosis
Hyperphacosorbitomyopicosis
Hyperphacosorbitomyopicosis is an edema of the crystalline lens in the eye commonly resulting in a myopia, or near-sightedness. This edema is caused by the accumulation of sorbitol, a sugar alcohol. This may occur when blood glucose levels are above normal, such as in diabetes mellitus....

) as a result of sorbitol
Sorbitol
Sorbitol, also known as glucitol, Sorbogem® and Sorbo®, is a sugar alcohol that the human body metabolizes slowly. It can be obtained by reduction of glucose, changing the aldehyde group to a hydroxyl group. Sorbitol is found in apples, pears, peaches, and prunes...

 (sugar alcohol
Sugar alcohol
A sugar alcohol is a hydrogenated form of carbohydrate, whose carbonyl group has been reduced to a primary or secondary hydroxyl group . Sugar alcohols have the general formula Hn+1H, whereas sugars have HnHCO...

) accumulating in the lens. This edema often causes temporary myopia (nearsightedness). A common sign of hyperphacosorbitomyopicosis is blurring of distance vision while near vision remains adequate.

Clinical entity

Various forms of myopia have been described by their clinical appearance:
  • Simple myopia, more common than other types of myopia, is characterized by an eye that is too long for its optical power
    Optical power
    Optical power is the degree to which a lens, mirror, or other optical system converges or diverges light. It is equal to the reciprocal of the focal length of the device. The dioptre is the most common unit of measurement of optical power...

     (which is determined by the cornea
    Cornea
    The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. Together with the lens, the cornea refracts light, with the cornea accounting for approximately two-thirds of the eye's total optical power. In humans, the refractive power of the cornea is...

     and crystalline lens
    Lens (anatomy)
    The 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. The lens, by changing shape, functions to change the focal distance of the eye so that it can focus on objects at various distances, thus allowing a...

    ) or optically too powerful for its axial length. Both genetic and environmental factors, particularly significant amounts of near work, are thought to contribute to the development of simple myopia.
  • Degenerative myopia, also known as malignant, pathological, or progressive myopia, is characterized by marked fundus
    Fundus (eye)
    The fundus of the eye is the interior surface of the eye, opposite the lens, and includes the retina, optic disc, macula and fovea, and posterior pole. The fundus can be viewed with an ophthalmoscope. The term may also be inclusive of Bruch's membrane and the choroid.The color of the fundus varies...

     changes, such as posterior staphyloma
    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...

    , and associated with a high refractive error 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
    Visual impairment is vision loss to such a degree as to qualify as an additional support need through a significant limitation of visual capability resulting from either disease, trauma, or congenital or degenerative conditions that cannot be corrected by conventional means, such as refractive...

    .
  • Nocturnal myopia, also known as night or twilight myopia, is a condition in which the eye has a greater difficulty seeing in low-illumination areas, even though its daytime vision is normal. Essentially, the eye's far point of an individual's focus varies with the level of light. Night myopia is believed to be caused by pupils dilating to let more light in, which adds aberrations, resulting in becoming more nearsighted. A stronger prescription for myopic night drivers is often needed. Younger people are more likely to be affected by night myopia than the elderly.
  • Pseudomyopia
    Pseudomyopia
    Pseudomyopia refers to an intermittent and temporary shift in refraction of the eye towards myopia, in which the focusing of light in front of the retina is due to a transient spasm of the ciliary muscle causing an increase in the refractive power of the eye...

    is the blurring of distance vision brought about by spasm of the ciliary muscle
    Ciliary muscle
    The ciliary muscle is a ring of striated smooth muscle in the eye's middle layer that controls accommodation for viewing objects at varying distances and regulates the flow of aqueous humour into Schlemm's canal. It changes the shape of the lens within the eye not the size of the pupil which is...

    .
  • Induced myopia, also known as acquired myopia, results from exposure to various pharmaceuticals, increases in glucose
    Glucose
    Glucose is a simple sugar and an important carbohydrate in biology. Cells use it as the primary source of energy and a metabolic intermediate...

     levels, nuclear sclerosis
    Nuclear sclerosis
    Nuclear sclerosis is an age-related change in the density of the crystalline lens nucleus that occurs in all older animals. It is caused by compression of older lens fibers in the nucleus by new fiber formation. The denser construction of the nucleus causes it to scatter light...

    , oxygen toxicity
    Oxygen toxicity
    Oxygen toxicity is a condition resulting from the harmful effects of breathing molecular oxygen at elevated partial pressures. It is also known as oxygen toxicity syndrome, oxygen intoxication, and oxygen poisoning...

     (e.g., from diving or from oxygen and hyperbaric therapy) or other anomalous conditions. The encircling bands used in the repair of retinal detachments 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 and mechanism of myopia development.
  • Nearwork-induced transient myopia (NITM) is defined as 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.

Degree

Myopia, which is measured in diopters by the strength or optical power
Optical power
Optical power is the degree to which a lens, mirror, or other optical system converges or diverges light. It is equal to the reciprocal of the focal length of the device. The dioptre is the most common unit of measurement of optical power...

 of a corrective lens that focuses distant images on the retina, has also been classified by degree or severity:
  • Low myopia usually describes myopia of −3.00 diopters or less (i.e. closer to 0.00).
  • Medium myopia usually describes myopia between −3.00 and −6.00 diopters. Those with moderate amounts of myopia are more likely to have pigment dispersion syndrome
    Pigment dispersion syndrome
    Pigment dispersion syndrome is an affliction of the eye 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...

     or pigmentary glaucoma
    Glaucoma
    Glaucoma is an eye disorder in which the optic nerve suffers damage, permanently damaging vision in the affected eye and progressing to complete blindness if untreated. It is often, but not always, associated with increased pressure of the fluid in the eye...

    .
  • High myopia usually describes myopia of −6.00 or more. People with high myopia are more likely to have retinal detachment
    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 blindness. It is a medical emergency.The retina is a...

    s and primary open angle glaucoma
    Glaucoma
    Glaucoma is an eye disorder in which the optic nerve suffers damage, permanently damaging vision in the affected eye and progressing to complete blindness if untreated. It is often, but not always, associated with increased pressure of the fluid in the eye...

    . They are also more likely to experience floater
    Floater
    Floaters are deposits of various size, shape, consistency, refractive index, and motility within the eye's vitreous humour, which is normally transparent. At young age the vitreous is perfectly transparent, but during life imperfections gradually develop. The common type of floater, which is...

    s, shadow-like shapes which appear singly or in clusters in the field of vision
    Visual field
    The term visual field is sometimes used as a synonym to field of view, though they do not designate the same thing. The visual field is the "spatial array of visual sensations available to observation in introspectionist psychological experiments", while 'field of view' "refers to the physical...

    . Roughly 30% of myopes have high myopia.

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 prior to age 20.
  • 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.
  • Adult onset myopia
  • Early adult onset myopia occurs between ages 20 and 40.
  • Late adult onset myopia occurs after age 40.

Signs and symptoms

Myopia presents with blurry distance vision, but generally gives good near vision. In high myopia, even near vision is affected, and patients cannot read without their glasses for distance.

Diagnosis

A diagnosis of myopia is typically confirmed during an eye examination
Eye examination
An eye examination is a battery of tests performed by an ophthalmologist, optometrist, or orthoptist assessing vision and ability to focus on and discern objects, as well as other tests and examinations pertaining to the eyes....

 by an ophthalmologist, optometrist or orthoptist. Frequently an autorefractor or retinoscope is used to give an initial objective assessment of the refractive status of each eye, then a phoropter
Phoropter
A phoropter is an instrument commonly used by eye care professionals during an eye examination, containing different lenses used for refraction of the eye during sight testing, to measure an individual's refractive error and determine his or her eyeglass prescription.Typically, the patient sits...

 is used to subjectively refine the patient's eyeglass prescription
Eyeglass prescription
An eyeglass prescription is an order written by an eyewear prescriber, such as an optometrist or ophthalmologist, that specifies the value of all parameters the prescriber has deemed necessary to construct and/or dispense corrective lenses appropriate for a patient.If an examination indicates that...

.

Prevention

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.

There is no universally accepted method of preventing myopia. Commonly attempted preventive methods include wearing reading glasses, eye drops and participating in more outdoor activities as described below. Some clinicians and researchers recommend plus power (convex) lenses in the form of reading glasses when engaged in close work or reading instead of using single focal concave lens glasses commonly prescribed. The reasoning behind a convex lens's possible effectiveness in preventing myopia is simple to understand: Convex lenses' refractive property of converging light are used in reading glasses to help reduce the accommodation
Accommodation (eye)
Accommodation is the process by which the vertebrate eye changes optical power to maintain a clear image on an object as its distance changes....

 needed when reading and doing close work. Although accommodation is irrelevant in Medina's quantitative model of myopia, it reaches the same conclusion. The model teaches a very simple method to prevent myopia.

For people with presbyopia
Presbyopia
Presbyopia is a condition where the eye exhibits a progressively diminished ability to focus on near objects with age. Presbyopia’s exact mechanisms are not known with certainty; the research evidence most strongly supports a loss of elasticity of the crystalline lens, although changes in the...

, whose eyes' lenses can not accommodate enough for very near focus, reading glasses help converge the light before it enters the eye to complement the refractive power of the eye lens, so near objects focus clearly on the retina. By reducing the focusing effort needed (accommodation
Accommodation (eye)
Accommodation is the process by which the vertebrate eye changes optical power to maintain a clear image on an object as its distance changes....

), reading glasses or convex lenses essentially relax the focusing ciliary muscles and may consequently reduce chances of developing myopia. Inexpensive nonprescription reading glasses are commonly sold in drug stores and dollar stores. Alternatively, reading glasses fitted by optometrists have a wider range of styles and lens choices.

A recent Malaysian study reported in New Scientist
New Scientist
New Scientist is a weekly non-peer-reviewed English-language international science magazine, which since 1996 has also run a website, covering recent developments in science and technology for a general audience. Founded in 1956, it is published by Reed Business Information Ltd, a subsidiary of...

suggested undercorrection of myopia caused more rapid progression of myopia. However, the reliability of these data has been called into question. Many myopia treatment studies suffer from any of a number of design drawbacks: small numbers
Sample size
Sample size determination is the act of choosing the number of observations to include in a statistical sample. The sample size is an important feature of any empirical study in which the goal is to make inferences about a population from a sample...

, lack of adequate control group, failure to mask examiners from knowledge of treatments used, etc.

Pirenzepine
Pirenzepine
Pirenzepine is used in the treatment of peptic ulcers, as it reduces gastric acid secretion and reduces muscle spasm. It is in a class of drugs known as muscarinic receptor antagonists - acetylcholine being the neurotransmitter of the parasympathetic nervous system which initiates the...

 eyedrops had a limited effect on retarding myopic progression in a recent, placebo-controlled, double-blind, prospective-controlled study.

Researchers at the University of Cambridge have found that a lack of outdoor play could be linked to myopia.

Orthokeratology
Orthokeratology
Orthokeratology , marketed under brand names like "", "MiracLens", "DreamLens", "i-GO OVC", "GOV", "Wake and See", "CRT" and "Emerald", is the use of rigid gas-permeable contact lenses, normally worn only at night, to improve vision through the reshaping of the cornea...

 ("Ortho-K"), where special contact lenses are worn overnight but removed after awakening, has been shown to reduce myopic progression compared to conventional contact lenses.

Near work causes the lens of the eye to focus (accommodate) excessively, leading to a spasm of the ciliary muscles surrounding the lens of the eye. Prolonged ciliary muscle spasms eventually lead to the elongation of the eye resulting in myopia. Some claim that wearing a plus lens during near work greatly reduces the eyes need for accommodation and therefore prevents ciliary spasm, and the elongation of the eye. Near work can also be eliminated almost completely by working at the computer from a distance of around 1.5 meters and reading electronic versions of the books (on a computer in a distance).

Management

Eyeglasses, contact lenses, and refractive surgery
Refractive surgery
Refractive eye surgery is any 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 or cataract surgery. The most common methods today use excimer lasers to...

 are the primary options to treat the visual symptoms of those with myopia. Lens implants are now available offering an alternative to glasses or contact lenses for myopics for whom laser surgery is not an option. Orthokeratology
Orthokeratology
Orthokeratology , marketed under brand names like "", "MiracLens", "DreamLens", "i-GO OVC", "GOV", "Wake and See", "CRT" and "Emerald", is the use of rigid gas-permeable contact lenses, normally worn only at night, to improve vision through the reshaping of the cornea...

 is the practice of using special rigid contact lenses to flatten the cornea to reduce myopia. Occasionally, pinhole glasses
Pinhole glasses
Pinhole glasses, also known as stenopeic glasses, are eyeglasses with a series of pinhole-sized perforations filling an opaque sheet of plastic in place of each lens. Similar to the workings of a pinhole camera, each perforation allows only a very narrow beam of light to enter the eye which reduces...

 are used by patients with low-level myopia. These work by reducing the blur circle formed on the retina, but their adverse effects on peripheral vision, contrast and brightness make them unsuitable in most situations.

Chromatic aberration of strong eyeglasses

For people with a high degree of myopia, very strong eyeglass prescriptions are needed to correct the focus error. However, strong eyeglass prescriptions have a negative side effect in that off-axis viewing of objects away from the center of the lens results in prismatic movement and separation of colors, known as chromatic aberration
Chromatic aberration
In optics, chromatic aberration is a type of distortion in which there is a failure of a lens to focus all colors to the same convergence point. It occurs because lenses have a different refractive index for different wavelengths of light...

. This prismatic distortion is visible to the wearer as color fringes around strongly contrasting colors. The fringes move around as the wearer's gaze through the lenses changes, and the prismatic shifting reverses on either side, above, and below the exact center of the lenses. Color fringing can make accurate drawing and painting difficult for users of strong eyeglass prescriptions.

Strongly nearsighted wearers of contact lenses do not experience chromatic aberration because the lens moves with the cornea and always stays centered in the middle of the wearer's gaze.

Eye exercises and biofeedback

Practitioners and advocates of alternative therapies
Alternative medicine
Alternative medicine is any healing practice, "that does not fall within the realm of conventional medicine." It is based on historical or cultural traditions, rather than on scientific evidence....

 often recommend eye exercises and relaxation techniques, such as the Bates method
Bates Method
The Bates method is an alternative therapy aimed at improving eyesight. Eye-care physician William Horatio Bates attributed nearly all sight problems to habitual strain of the eyes, and felt that glasses were harmful and never necessary...

. However, the efficacy of these practices is disputed by scientists and eye care practitioners. A 2005 review of scientific papers on the subject concluded that there was "no clear scientific evidence" that eye exercises were effective in treating myopia.

In the 1980s and 1990s, biofeedback
Biofeedback
Biofeedback is the process of becoming aware of various physiological functions using instruments that provide information on the activity of those same systems, with a goal of being able to manipulate them at will...

 created a flurry of interest as a possible treatment for myopia. A 1997 review of this biofeedback research concluded "controlled studies to validate such methods ... have been rare and contradictory." One study found that myopes could improve their visual acuity with biofeedback training, but that this improvement was "instrument-specific" and did not generalise to other measures or situations. In another study, an "improvement" in visual acuity was found, but the authors concluded this could be a result of subjects learning the task. Finally, in an evaluation of a training system designed to improve acuity, "no significant difference was found between the control and experimental subjects".

Myopia control

Various methods have been employed in an attempt to decrease the progression of myopia. Dr Chua Weihan and his team at National Eye Centre Singapore have conducted large scale studies on the effect of Atropine of varying strength in stabilizing, and in some case, reducing myopia. The use of reading glasses when doing close work may provide success 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 for Myopia refers to numerous studies which indicated the effectiveness of bifocal lenses and recommends it as the method for "Myopia Control". In some studies, bifocal and progressive lenses
Progressive lenses
Progressive spectacle lenses, also called progressive addition lenses , progressive power lenses, graduated prescription lenses, and varifocal or multifocal lenses, are corrective lenses used in eyeglasses to correct presbyopia and other disorders of accommodation...

 have not shown significant differences in altering the progression of myopia. More recently robust studies on children have shown that Orthokeratology
Orthokeratology
Orthokeratology , marketed under brand names like "", "MiracLens", "DreamLens", "i-GO OVC", "GOV", "Wake and See", "CRT" and "Emerald", is the use of rigid gas-permeable contact lenses, normally worn only at night, to improve vision through the reshaping of the cornea...

 and Centre Distance bifocal contact lenses may arrest myopic development.

By region

The global prevalence of refractive errors has been estimated from 800 million to 2.3 billion. The incidence of myopia within sampled population often varies with age, country, sex, race, ethnicity, occupation, environment, and other factors. Variability in testing and data collection methods makes comparisons of prevalence and progression difficult.

Asia

In some parts of Asia
Asia
Asia is the world's largest and most populous continent, located primarily in the eastern and northern hemispheres. It covers 8.7% of the Earth's total surface area and with approximately 3.879 billion people, it hosts 60% of the world's current human population...

, myopia is the norm.

Singapore
Singapore
Singapore , officially the Republic of Singapore, is a Southeast Asian city-state off the southern tip of the Malay Peninsula, north of the equator. An island country made up of 63 islands, it is separated from Malaysia by the Straits of Johor to its north and from Indonesia's Riau Islands by the...

 is believed to have the highest prevalence of myopia in the world; up to 80 percent of people there have myopia.

China
China
Chinese civilization may refer to:* China for more general discussion of the country.* Chinese culture* Greater China, the transnational community of ethnic Chinese.* History of China* Sinosphere, the area historically affected by Chinese culture...

 has one of the highest myopia rates in the world: 400 million of its 1.3 billion people are myopic. The prevalence of myopia in high school
High school
High school is a term used in parts of the English speaking world to describe institutions which provide all or part of secondary education. The term is often incorporated into the name of such institutions....

 in China is 77.3%, and in college
College
A college is an educational institution or a constituent part of an educational institution. Usage varies in English-speaking nations...

 is more than 80%.

In some areas, such as China, India
India
India , officially the Republic of India , is a country in South Asia. It is the seventh-largest country by geographical area, the second-most populous country with over 1.2 billion people, and the most populous democracy in the world...

 and Malaysia, up to 41% of the adult population is myopic to −1.00 dpt, up to 80% to −0.5dpt.

A study of Jordanian adults aged 17 to 40 found over half (53.7%) were myopic.

However, some research suggests the prevalence of myopia in India in the general population is only 6.9%.

Europe

A recent study involving first-year undergraduate students in the United Kingdom found that 50% of British
United Kingdom
The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...

 whites and 53.4% of British Asians were myopic.

In Greece
Greece
Greece , officially the Hellenic Republic , and historically Hellas or the Republic of Greece in English, is a country in southeastern Europe....

, the prevalence of myopia among 15 to 18 year old students was found to be 36.8%.

A recent review found that 26.6% of Western Europeans aged 40 or over have at least −1.00 diopters of myopia and 4.6% have at least −5.00 diopters.

United States

Myopia is common in the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...

, 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 US myopia prevalence, and found the prevalence of myopia in persons aged 12-54 was 25.0%. Using the same method, in 1999-2004, myopia prevalence was estimated to have climbed to 41.6%.

A study of 2523 children in grades 1 to 8 (age, 5-17 years) found nearly 1 in 10 (9.2%) have at least − 0.75 diopters of myopia . A recent review found 25.4% of Americans aged 40 or over have at least −1.00 diopters of myopia and 4.5% have at least −5.00 diopters.

Australia

In Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...

, the overall prevalence of myopia (worse than −0.50 diopters) has been estimated to be 17%. In one recent study, less than 1 in 10 (8.4%) Australian children between the ages of 4 and 12 were found to have myopia greater than −0.50 diopters. A recent review found that 16.4% of Australians aged 40 or over have at least −1.00 diopters of myopia and 2.5% have at least −5.00 diopters.

Brazil

In Brazil
Brazil
Brazil , officially the Federative Republic of Brazil , is the largest country in South America. It is the world's fifth largest country, both by geographical area and by population with over 192 million people...

, a 2005 study estimated that 6.4% of Brazilians between the ages of 12 and 59 had −1.00 diopter of myopia or more, compared with 2.7% of the indigenous people in northwestern Brazil. Another found nearly 1 in 8 (13.3%) of the students in the city of Natal
Natal, Rio Grande do Norte
-History:The northeastern tip of South America, Cabo São Roque, to the north of Natal and the closest point to Europe from Latin America, was first visited by European navigators in 1501, in the 1501–1502 Portuguese expedition led by Amerigo Vespucci, who named the spot after the saint of the day...

 were myopic.

Ethnicity and race

The prevalence of myopia has been reported as high as 70–90% in some Asia
Asia
Asia is the world's largest and most populous continent, located primarily in the eastern and northern hemispheres. It covers 8.7% of the Earth's total surface area and with approximately 3.879 billion people, it hosts 60% of the world's current human population...

n countries, 30–40% in Europe and the United States, and 10–20% in Africa.

Myopia is less common in African people and associated diaspora. In Americans between the ages of 12 and 54, myopia has been found to affect African Americans less than Caucasians.

A study of 2523 children in grades 1 to 8 (age, 5-17 years) found that 9.2% had at least − 0.75 diopters (D) of myopia; 12.8% had at least +1.25 D hyperopia
Hyperopia
Hyperopia, also known as farsightedness, longsightedness or hypermetropia, is a defect of vision caused by an imperfection in the eye , causing difficulty focusing on near objects, and in extreme cases causing a sufferer to be unable to focus on objects at any distance...

 , and 28.4% had at least 1.00-D difference between the 2 principal meridians (cycloplegic autorefraction) astigmatism
Astigmatism (eye)
Astigmatism is an optical defect in which vision is blurred due to the inability of the optics of the eye to focus a point object into a sharp focused image on the retina. This may be due to an irregular or toric curvature of the cornea or lens. There are two types of astigmatism: regular and...

. For myopia, Asians had the highest prevalence (18.5%), followed by Hispanics (13.2%). Caucasians had the lowest prevalence of myopia (4.4%), which was not significantly different from African Americans (6.6%). For hyperopia, Caucasians had the highest prevalence (19.3%), followed by Hispanics (12.7%). Asians had the lowest prevalence of hyperopia (6.3%) and were not significantly different from African Americans (6.4%). For astigmatism, Asians and Hispanics had the highest prevalences (33.6% and 36.9%, respectively) and did not differ from each other (P
P-value
In statistical significance testing, the p-value is the probability of obtaining a test statistic at least as extreme as the one that was actually observed, assuming that the null hypothesis is true. One often "rejects the null hypothesis" when the p-value is less than the significance level α ,...

 = .17). African Americans had the lowest prevalence of astigmatism (20.0%), followed by Caucasians (26.4%).

Education and IQ

A number of studies have shown the incidence of myopia increases with level of education, and many studies have shown a correlation between myopia and a higher intelligence quotient
Intelligence quotient
An intelligence quotient, or IQ, is a score derived from one of several different standardized tests designed to assess intelligence. When modern IQ tests are constructed, the mean score within an age group is set to 100 and the standard deviation to 15...

 (IQ), possibly due to the confounding factor of formal education.

A 2008 literature review writes that studies in several nations have found a relationship between myopia and higher IQ and between myopia and school achievement. A common explanation for myopia is near-work. Regarding the relationship to IQ, several explanations have been proposed. One is that the myopic child is better adapted at reading, and reads and studies more, which increases intelligence. The reverse explanation is that the intelligent and studious child reads more, which causes myopia. Another is that myopic children have an advantage at IQ testing which is near-work because of less eye strain. Still another explanation is that pleiotropic gene(s) affect the size of both brain and eyes simultaneously. According to the two most recent studies, higher IQ may be associated with myopia in schoolchildren, independent of books read per week.

Other personal characteristics, such as value systems, school achievements, time spent in reading for pleasure, language abilities and time spent in sport activities correlated to the occurrence of myopia in studies.

Society and culture

The terms "myopia" and "myopic" (or the common terms short sightedness or 'short sighted) have been used metaphor
Metaphor
A metaphor is a literary figure of speech that uses an image, story or tangible thing to represent a less tangible thing or some intangible quality or idea; e.g., "Her eyes were glistening jewels." Metaphor may also be used for any rhetorical figures of speech that achieve their effects via...

ically to refer to cognitive thinking and decision making that is narrow in scope or lacking in concern for wider interests or 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
Hyperopia
Hyperopia, also known as farsightedness, longsightedness or hypermetropia, is a defect of vision caused by an imperfection in the eye , causing difficulty focusing on near objects, and in extreme cases causing a sufferer to be unable to focus on objects at any distance...

, the biological opposite of myopia, is also used as a metaphor for those who exhibit "far-sighted" behavior; that is, overprioritizing long-term interests at the expense of present enjoyment.

Many instances of myopic individuals have emerged in popular culture, though not always accurately. One such instance is in William Golding's Nobel Prize-winning novel Lord of the Flies
Lord of the Flies
Lord of the Flies is a novel by Nobel Prize-winning author William Golding about a group of British boys stuck on a deserted island who try to govern themselves, with disastrous results...

, which features a character named Piggy who is very nearsighted, and as a result, wears thick glasses. The children (who are marooned on an isolated island alone) use Piggy's glasses in the same manner as a magnifying glass might be used to start fires. However, if Piggy is truly myopic and not hyperopic, starting fires with his glasses would be impossible. Myopia is corrected through the use of diverging lenses to properly focus light on the retina. These lenses do not converge light in a single point—as would be required to start a fire—but rather scatter it. If Piggy were hyperopic, he would have convex, converging lenses, so they would theoretically be able to serve this purpose.

Research

Normally eye development is largely genetically controlled, but it has been shown that the visual environment is an important factor in determining ocular development.

Genetic basis for myopia

Genetically, linkage
Genetic linkage
Genetic linkage is the tendency of certain loci or alleles to be inherited together. Genetic loci that are physically close to one another on the same chromosome tend to stay together during meiosis, and are thus genetically linked.-Background:...

 studies have identified 18 possible loci on 15 different chromosomes that are associated with myopia, but none of these loci are 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.

Visual environment

To induce myopia in lower as well as higher vertebrates, translucent goggles can be sutured over the eye, either before or after natural eye opening. Form-deprived myopia induced with a diffuser, like the goggles mentioned, shows significant myopic shifts. Anatomically, the changes in axial length of the eye seem to be the major factor contributing to this type of myopia. Diurnal growth rhythms of the eye have also been shown to play a large part in form-deprived myopia. Chemically, daytime retinal dopamine levels drop about 30%.

Normal eyes grow during the day and shrink during the night, but occluded eyes are shown to grow both during the day and the night. Because of this, form-deprived myopia is a result of the lack of growth inhibition at night rather than the expected excessive growth during the day, when the actual light-deprivation occurred. Elevated levels of retinal dopamine transporter (which is directly involved in controlling retinal dopamine levels) in the RPE have been shown to be associated with FDM.

Dopamine

Dopamine
Dopamine
Dopamine is a catecholamine neurotransmitter present in a wide variety of animals, including both vertebrates and invertebrates. In the brain, this substituted phenethylamine functions as a neurotransmitter, activating the five known types of dopamine receptors—D1, D2, D3, D4, and D5—and their...

 is a major neurotransmitter
Neurotransmitter
Neurotransmitters are endogenous chemicals that transmit signals from a neuron to a target cell across a synapse. Neurotransmitters are packaged into synaptic vesicles clustered beneath the membrane on the presynaptic side of a synapse, and are released into the synaptic cleft, where they bind to...

 in the retina involved in signal transmission in the visual system. In the retinal inner nuclear layer, a dopaminergic neuronal network has been visualized in amacrine cells. Also, retinal dopamine is involved in the regulation of electrical coupling between horizontal cells and the retinomotor movement of photoreceptor cells.
Although FDM-related elongations in axial length and drops in dopamine levels are significant, after the diffuser is removed, a complete refraction recovery is seen within four days in some laboratory mice. Although significant, what is even more intriguing is that within just two days of diffuser removal, an early rise and eventual normalization of retinal dopamine levels in the eye are seen. This suggests dopamine participates in visually guided eye growth regulation, and these fluctuations are not just a response to the FDM.

L-Dopa has been shown to re-establish circadian rhythms in animals whose circadian rhythms have been abolished. Dopamine, a major metabolite of levodopa, releases in response to light, and helps establish circadian clocks that drive daily rhythms of protein phosphorylation in photoreceptor cells. Because retinal dopamine levels are controlled on a circadian pattern, intravitreal injection of L-dopa in animals that have lost dopamine and circadian rhythms has been shown to correct these patterns, especially in heart rate, temperature, and locomotor activity. The occluders block light completely for the animals, which does not allow them to establish correct circadian rhythms, which leads to dopamine depletion. This depletion can be rectified with injections of L-dopa and hopefully contribute to the recovery from FDM.
  • L-Dopa metabolism is important to consider due to its extensive presystemic metabolism, rapid absorption in the proximal small intestine and short plasma half-life. The major metabolites of L-dopa are dopamine, dihydroxyphenylacetic acid, homovanillic acid, and 3-O-methyldopa and 3-methoxytyramine. Levodopa can be converted into dopamine in the presence of aromatic L-amino acid decarboxylase (L-AAAD). L-AAAD activity in rat retinas is modulated by environmental light, and this modulation is associated with dopamine D1 receptors and alpha 2 adrenoceptors. Also, the synthesis and release of dopamine are light dependent, and light accelerates the formation of dopamine from exogenous L-Dopa.

  • Past treatments with dopamine has been used as the gold-standard drug in the treatment of Parkinson's disease
    Parkinson's disease
    Parkinson's disease is a degenerative disorder of the central nervous system...

     and low-dose administration of the drug has been the most effective treatment of Parkinson’s. Possible treatments involving dopamine in preventing a decrease in visual acuity have shown to be successful in the past. L-Dopa treatment in children with amblyopia showed an improvement in visual acuity. In rabbits, injections of dopamine prevented the myopic shift and vitreous chamber and axial elongation typically associated with FDM. In guinea pigs, systemic L-dopa has been shown to inhibit the myopic shift associated with FDM, and has compensated for the drop in retinal dopamine levels. These experiments show promise in treating myopia in humans.

  • Side effects of L-dopa have been experimentally determined. L-Dopa and some of its metabolites have been shown to have pro-oxidant properties, and oxidative stress has been shown to increase the pathogenesis of Parkinson's disease. This promotion of free-radical formation by L-dopa does seem to directly affect its possible future treatment of myopia because free-radicals could cause further damage to those proteins responsible for controlling structural proteins in the eye. Levodopa and some of its metabolites such as dopa/dopamine quinone have also been shown to be toxic for nigral neurons. This toxic effect must be analyzed before treatment with levodopa for myopia to prevent damaging effects to these neurons.

L-Dopa inhibits myopic shifts

In guinea pigs, intraperitoneal injections of L-dopa have shown to inhibit the myopic shift associated with FDM and have compensated to the drop in retinal dopamine levels. In this study specifically, 60 animals were used and the L-dopa treatments inhibited the myopic shift (from −3.62 ± 0.98 D to −1.50 ± 0.38 D; p < 0.001) due to goggles occluding and compensated retinal dopamine (from 0.65 ± 0.10 ng to 1.33 ± 0.23 ng; p < 0.001). Daily L-dopa (10 mg/kg) was shown to increase the dopamine content in striatum. The axial length and retinal dopamine changes were positively correlated in the normal control eyes, deprived eyes, and L-dopa-treated deprived eyes. The increase in retinal dopamine and subsequent retardation of myopia may be associated with the fact that exogenous L-dopa was converted into dopamine. This suggests retinal dopaminergic function in the development of form-deprivation myopia in guinea pigs. The inhibitory effect of L-dopa on FDM may be associated with the fact that retinal L-AAAD can convert it into dopamine to balance the deficiency in the retina of the deprived eyes.
  • Areas of future research include intraperitoneal injection of L-dopa; its use at 10 mg/kg could not completely suppress the development of form-deprivation myopia. Perhaps the dose may be too low to completely suppress myopia. Another possibility of the incomplete suppression of myopia may be because it is a complex process, and retinal dopamine content is only one factor. It is also unclear whether systemic application of L-dopa is able to suppress the development of form-deprivation myopia.

See also

  • Astigmatism (eye)
    Astigmatism (eye)
    Astigmatism is an optical defect in which vision is blurred due to the inability of the optics of the eye to focus a point object into a sharp focused image on the retina. This may be due to an irregular or toric curvature of the cornea or lens. There are two types of astigmatism: regular and...

  • Hyperopia (longsightedness)
    Hyperopia
    Hyperopia, also known as farsightedness, longsightedness or hypermetropia, is a defect of vision caused by an imperfection in the eye , causing difficulty focusing on near objects, and in extreme cases causing a sufferer to be unable to focus on objects at any distance...

  • Presbyopia
    Presbyopia
    Presbyopia is a condition where the eye exhibits a progressively diminished ability to focus on near objects with age. Presbyopia’s exact mechanisms are not known with certainty; the research evidence most strongly supports a loss of elasticity of the crystalline lens, although changes in the...

  • Optician
    Optician
    An optician is a person who is trained to fill prescriptions for eye correction in the field of medicine, also known as a dispensing optician or optician, dispensing...

  • Optometry
    Optometry
    Optometry is a health care profession concerned with eyes and related structures, as well as vision, visual systems, and vision information processing in humans. Optometrists, or Doctors of Optometry, are state licensed medical professionals trained to prescribe and fit lenses to improve vision,...

  • Eye care professional
    Eye care professional
    An eye care professional is an individual who provides a service related to the eyes or vision. It is a general term that can refer to any healthcare worker involved in eye care, from one with a small amount of post-secondary training to practitioners with a doctoral level of education.-Current...

  • Relaxed selection
  • Bates method
    Bates Method
    The Bates method is an alternative therapy aimed at improving eyesight. Eye-care physician William Horatio Bates attributed nearly all sight problems to habitual strain of the eyes, and felt that glasses were harmful and never necessary...


External links

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