Myopia

Myopia (, muōpia, "nearsightedness"), is a refractive defect of the eye in which collimated light produces image focus in front of the retina when accommodation is relaxed.

For those with myopia, far away objects appear blurred and near objects appear clearly. With myopia, the eyeball is too long, or the cornea is too steep, so images are focused in the vitreous inside the eye rather than on the retina at the back of the eye. The opposite defect of myopia is hyperopia or "farsightedness" or "long-sightedness"—-this is where the cornea is too flat or the eye is too small.

Eye care professionals most commonly correct myopia through the use of corrective lenses, such as glasses or contact lenses. It may also be corrected by refractive surgery, but this does have many risks and side effects. The corrective lenses have a negative optical power (i.e. are concave) which compensates for the excessive positive diopters of the myopic eye.

Alternative ideas and methods of treatment exist, most notably the claim that myopia is caused by excessive near sight work.

Classification

Myopia has been classified in various manners.

By cause

Borish and Duke-Elder classified myopia by cause: Refractive myopia is attributed to the condition of the refractive elements of the eye. :*Index myopia is attributed to variation in the index of refraction of one or more of the ocular media. Simple myopia is more common than other types of myopia and is characterized by an eye that is too long for its optical power (which is determined by the cornea and crystalline lens) 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. Nocturnal myopia, also known as night myopia 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 is the blurring of distance vision brought about by spasm of the ciliary muscle. Induced myopia, also known as acquired myopia, results from exposure to various pharmaceuticals, increases in glucose levels, nuclear sclerosis, oxygen toxicity (e.g., from diving or from oxygen and hyperbaric therapy) or other anomalous conditions. :*Index myopia is attributed to variation in the index of refraction of one or more of the ocular media. :*Form deprivation myopia is a type of myopia that 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. 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 of a corrective lens that focuses distant images on the retina, has also been classified by degree or severity: Those with moderate amounts of myopia are more likely to have pigment dispersion syndrome or pigmentary glaucoma. High myopia usually describes myopia of −6.00 or more. and primary open angle glaucoma. They are also more likely to experience floaters, shadow-like shapes which appear singly or in clusters in the field of vision. Roughly 30% of myopes have high myopia.

Age at onset

Myopia is sometimes classified by the age at onset: This form of myopia is attributed to the use of the eyes for close work during the school years.

Among mainstream researchers and eye care professionals, myopia is now thought to be a combination of genetic and environmental factors.

There are currently two basic mechanisms believed to cause myopia: form deprivation (also known as pattern deprivation) and optical defocus. Form deprivation occurs when the image quality on the retina is reduced; optical defocus occurs when light focuses in front of or behind the retina. Numerous experiments with animals have shown that myopia can be artificially generated by inducing either of these conditions. In animal models wearing negative spectacle lenses, axial myopia has been shown to occur as the eye elongates to compensate for optical defocus. It has been suggested that accommodative lag leads to blur (i.e. optical defocus) which in turn stimulates axial elongation and myopia.

Theories

Combination of genetic and environmental factors—In China, myopia is more common in those with higher education background and some studies suggest that near work may exacerbate a genetic predisposition to develop myopia. Other studies have shown that near work (reading, computer games) may not be associated with myopic progression, however. A "genetic susceptibility" to environmental factors has been postulated as one explanation for the varying degrees of myopia among individuals or populations, but there exists some difference of opinion as to whether it exists. High heritability simply means that most of the variation in a particular population at a particular time is due to genetic differences. If the environment changes—as, for example, it has by the introduction of televisions and computers—the incidence of myopia can change as a result, even though heritability remains high. From a slightly different point of view it could be concluded that—determined by heritage—some people are at a higher risk to develop myopia when exposed to modern environmental conditions with a lot of extensive near work like reading. In other words, it is often not the myopia itself which is inherited, but the reaction to specific environmental conditions—and this reaction can be the onset and the progression of myopia. Medina showed that myopia is a feedback process where genetic and environmental factors can coexist.

*Genetic factors—The wide variability of the prevalence of myopia in different ethnic groups has been reported as additional evidence supporting the role of genetics in the development of myopia. Measures of the heritability of myopia have yielded figures as high as 89%, and recent research has identified genes that may be responsible: defective versions of the PAX6 gene seem to be associated with myopia in twin studies. Under this theory, the eye is slightly elongated front to back as a result of faults during development, causing images to be focused in front of the retina rather than directly on it. It is usually discovered during the pre-teen years between eight and twelve years of age. It most often worsens gradually as the eye grows during adolescence and then levels off as a person reaches adulthood. Genetic factors can work in various biochemical ways to cause myopia, a weak or degraded connective tissue is a very essential one. Genetic factors include an inherited, increased susceptibility for environmental influences like excessive near work, and the fact that some people do not develop myopia in spite of very adverse conditions is a clear indication that heredity is involved somehow in any case.

*Environmental factors—It has been suggested that a genetic susceptibility to myopia does not exist.

:Near work and nightlight exposure in childhood have been hypothesized as environmental risk factors for myopia. Although one initial study indicated a strong association between myopia and nightlight exposure, recent research has found none. :*Near work. Near work has been implicated as a contributing factor to myopia in some studies, Near work in certain cultures (e.g. Vanuatu) does not result in greater myopia It has been hypothesized that this outcome may be a result of genetics or environmental factors such as diet or over-illumination, changes which seem to occur in Asian, Vanuatu and Inuit cultures acclimating to intensive early studies. :*Diet and nutrition – One 2002 article suggested that myopia may be caused by over-consumption of bread in childhood, or in general by diets too rich in carbohydrates, which can lead to chronic hyperinsulinemia. Various other components of the diet, however, were made responsible for contributing to myopia as well, as summarized in a documentation. :*Stress has been postulated as a factor in the development of myopia. :*The periods of eyelid closure during excess sleeping is another possible cause of myopia.

Research

A Turkish study found that accommodative convergence, rather than accommodation, may be a factor in the onset and progression of myopia in adults. A recent Polish study revealed that "with-the-rule astigmatism" may lead to the creation of myopia.

Benefits

Many people with myopia are able to read comfortably without eyeglasses even in advanced age. Myopes considering refractive surgery are advised that this may be a disadvantage after the age of 40 when the eyes become presbyopic and lose their ability to accommodate or change focus.

Diagnosis

A diagnosis of myopia is typically confirmed during an eye examination 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 is used to subjectively refine the patient's eyeglass prescription.

Prevention

There is no universally accepted method of preventing myopia. 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 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. By reducing the focusing effort needed (accommodation), reading glasses or convex lenses essentially relax the focusing ciliary muscles and may consequently reduce chances of developing myopia. Inexpensive non prescription 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 suggested that 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, lack of adequate control group, failure to mask examiners from knowledge of treatments used, etc.

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

Daylight

Daylight may prevent myopia. Australian researchers had concluded that exposure to daylight appeared to play a critical role in restricting the growth of the eyeball, which is responsible for myopia or short-sightedness. They compared children from other developed countries such as Singapore and Australian children spent about 2–3 hours a day outdoors which could increased dopamine in the eyes that restrict distorted shaping of the eyes.

Management

Eyeglasses, contact lenses, and refractive surgery are the primary options to treat the visual symptoms of those with myopia. Orthokeratology is the practice of using special rigid contact lenses to flatten the cornea to reduce myopia. Occasionally, pinhole glasses 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. 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 often recommend eye exercises and relaxation techniques such as the Bates method. 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, there was a flurry of interest in biofeedback as a possible treatment for myopia. A 1997 review of this biofeedback research concluded that "controlled studies to validate such methods ... have been rare and contradictory." It was found in one study 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 that 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. 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 have not shown significant differences in altering the progression of myopia. and Centre Distance bifocal contact lenses may arrest myopic development.

Epidemiology

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.

In some areas, such as China, India and Malaysia, up to 41% of the adult population is myopic to −1dpt, up to 80% to −0.5dpt.

A recent study involving first-year undergraduate students in the United Kingdom found that 50% of British whites and 53.4% of British Asians were myopic.

In Australia, 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.

In Brazil, 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 were myopic.

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

In India, the prevalence of myopia in the general population has been reported to be only 6.9%.

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. Approximately 25% of Americans between the ages of 12 and 54 have the condition. A recent review found that 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.

Myopia in China

China has the highest myopia rate the world: 400 million people are myopic out of its 1.3 billion people. The prevalence of myopia in highschool in China is 77.3%, and in college is more than 80%.

Ethnicity and race

The prevalence of myopia has been reported as high as 70–90% in some Asian countries, 30–40% in Europe and the United States, and 10–20% in Africa.

Education and myopia

A number of studies have shown that the incidence of myopia increases with level of education have shown a correlation between myopia and IQ, likely due to the confounding factor of formal education.

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 also been used metaphorically to refer to cognitive thinking and decision making that is narrow sighted 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 (see pragmatism, which tends to be myopic). Some antonyms of short sightedness are foreseeing, "forward thinking" and prophecy. Hyperopia, the biological opposite of myopia, is also used as a metaphor for those who exhibit "far-sighted" behavior; that is, over-prioritizing long-term interests at the expense of present enjoyment.

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 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.

The Visual Environment and Myopia

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 that is 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. It has also been shown that an elevated level of retinal dopamine transporter (which is directly involved in controlling retinal dopamine levels) in the RPE is associated with FDM.

The Role of Dopamine

Dopamine is a major neurotransmitter 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 4 days in some laboratory mice. Although this is significant, what is even more intriguing is that within just 2 days of diffuser removal, an early rise and eventual normalization of retinal dopamine levels in the eye are seen. This suggests that 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. 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 adrenoceptor. 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: L-Dopa has been used as the gold-standard drug in the treatment of Parkinson's disease 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 shown to inhibit the myopic shift associated with FDM and has compensated to the drop in retinal dopamine levels. These experiments show promise in treating myopia in humans. Possible Side Effects of Dopamine Treatment: Unfortunately, several 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 effect its possible future treatment of myopia due to the fact that free-radicals could further cause damage to those proteins responsible for controlling structural proteins in the eye. It has also been shown that levodopa and some of its metabolites such as dopa/dopamine quinone are toxic for nigral neurons. This toxic effect must be analyzed before treatment of 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 L-DOPA into dopamine to balance the deficiency in the retina of the deprived eyes.

Areas of Future Research: Intraperitoneal injection of L-DOPA (10 mg/kg) could not completely suppress the development of form-deprivation myopia. Perhaps the dose of L-DOPA may be too low to completely suppress myopia. Another possibility of the incomplete suppression of myopia may lie in the fact that myopia is a complex process of which 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)

Hyperopia (longsightedness)

Presbyopia

Optician

Optometry

Eye care professional

Relaxed selection

References

External links

Degenerative Myopia aka "Myopic Macular Degeneration"

pupilEyes – Learn how Myopia happens

Myopia Manual — an impartial documentation of all the reasons, therapies and recommendations—summary of scientific publications, status February 2010, printed version ISBN 158961271X (2004)

VisionSimulations.com |What the world looks like to people with various diseases and conditions of the eye

The Wildsoet Lab – Myopia research at the University of California, Berkeley

Scottish Sensory Centre – Medical Info on Myopia

Generation specs: Stopping the short-sight epidemic Article reviewing latest research in New Scientist

Category:Disorders of ocular muscles, binocular movement, accommodation and refraction Category:Vision