Myopia

Myopia
Classification and external resources
ICD-10 H52.1
ICD-9 367.1
DiseasesDB 8729
MeSH D009216

Myopia (Greek: μυωπία, muōpia, "nearsightedness"),[1] is a refractive defect of the eye in which collimated light produces image focus in front of the retina when accommodation is relaxed.

Those with myopia see near objects clearly but far away objects appear blurred. 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.

Contents

Classification

Myopia has been classified in various manners.[2][3][4]

By cause

Borish and Duke-Elder classified myopia by cause:[3][4]

  • Curvature myopia is attributed to excessive, or increased, curvature of one or more of the refractive surfaces of the eye, especially the cornea.[5] In those with Cohen syndrome, myopia appears to result from high corneal and lenticular power.[6]
  • Index myopia is attributed to variation in the index of refraction of one or more of the ocular media.[5]

Elevation of blood-glucose levels can also cause edema (swelling) of the crystalline lens (hyperphacosorbitomyopicosis) as a result of sorbitol (sugar alcohol) 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:[4][7]

  • Index myopia is attributed to variation in the index of refraction of one or more of the ocular media.[5] Cataracts may lead to index myopia.[14]
  • Form deprivation myopia is a type of myopia that occurs when the eyesight is deprived by limited illumination and vision range,[15] or the eye is modified with artificial lenses[16] or deprived of clear form vision.[17][18] In lower vertebrates this kind of myopia seems to be reversible within short periods of time.[18] Myopia is often induced this way in various animal models to study the pathogenesis and mechanism of myopia development.[18]

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:[2]

Age at onset

Myopia is sometimes classified by the age at onset:[2]

  • School myopia appears during childhood, particularly the school-age years.[26] This form of myopia is attributed to the use of the eyes for close work during the school years.[5]
  • Early adult onset myopia occurs between ages 20 and 40.[8]
  • Late adult onset myopia occurs after age 40.[8]

Signs and symptoms

Near-sighted vision (left) Normal vision (right)

Myopia presents with blurry distance vision but good near vision.

Cause

Because in the most common, "simple" myopia, the eye length is too long, any etiologic explanation must account for such axial elongation. To date, no single theory has been able to satisfactorily explain this elongation.

In the mid-1900s, mainstream ophthalmologists and optometrists believed myopia to be primarily hereditary; the influence of near work in its development seemed "incidental" and the increased prevalence of the condition with increasing age was viewed as a "statistical curiosity".[3][4][27]

Among mainstream researchers and eye care professionals, myopia is now thought to be a combination of genetic and environmental factors.[8][26][28]

There are currently two basic mechanisms believed to cause myopia: form deprivation (also known as pattern deprivation[29]) and optical defocus.[30] 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.[30] The exact physiological mechanism of this image-controlled elongation of the eye is still unknown, but the mechanism has been described quantitatively with mathematical precision.[28][31][32] It has been suggested that accommodative lag leads to blur (i.e. optical defocus) which in turn stimulates axial elongation and myopia.[33]

Theories

One Austrian study confirmed that the axial length of the eye does mildly increase while reading, but attributed this elongation due to contraction of the ciliary muscle during accommodation (the process by which the eye increases optical power to maintain a clear image focus), not "squeezing" of the extraocular muscles.[41]
Near work and nightlight exposure in childhood have been hypothesized as environmental risk factors for myopia.[42] Although one initial study indicated a strong association between myopia and nightlight exposure,[43] recent research has found none.[42][44][45][46]
  • Near work. Near work has been implicated as a contributing factor to myopia in some studies, but refuted in others.[36] One recent study suggested that students exposed to extensive "near work" may be at a higher risk of developing myopia, whereas extended breaks from near work during summer or winter vacations may retard myopic progression.[47] Near work in certain cultures (e.g. Vanuatu) does not result in greater myopia[48][49][50] 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.[50]
  • Time spent indoors – A number of studies have shown that children who spend more time outdoors have lower rates of myopia, possibly explaining the observed increase in myopia. It is theorized that the higher brightness or the larger distances outdoors play a role.[51]
  • 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.[52]
  • The periods of eyelid closure during excess sleeping is another possible cause of myopia.[53]

Research

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.[56] 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.[8] Commonly attempted preventative methods include wearing reading glasses, eye drops and participating in more outdoor activities are 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.[8][57] 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.[31] For people with Presbyopia, whose eye's lens 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.[58] 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.[59] 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.[60] A recent Malaysian study reported in New Scientist[61] suggested that undercorrection of myopia caused more rapid progression of myopia.[62] However, the reliability of these data has been called into question.[63] 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.[64]

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.[65] 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.[66][67][68]

Management

Glasses are commonly used to address near-sightedness.
Compensating for myopia using a corrective lens.

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

Prismatic color distortion shown with a camera set for nearsighted focus, and using −9.5 diopter eyeglasses to correct the camera's myopia. (left) Close-up of color shifting through corner of eyeglasses. The light and dark borders visible between color swatches do not exist. (right)

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.[69] 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.[70]

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."[71] 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.[72] 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.[73] Finally, in an evaluation of a training system designed to improve acuity, "no significant difference was found between the control and experimental subjects".[74]

Myopia control

Various methods have been employed in an attempt to decrease the progression of myopia.[30] 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.[75][76] 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".[8] In some studies, bifocal and progressive lenses have not shown significant differences in altering the progression of myopia.[30] More recently robust studies on children have shown that Orthokeratology[77] and Centre Distance bifocal contact lenses[78] may arrest myopic development.

Epidemiology

The global prevalence of refractive errors has been estimated from 800 million to 2.3 billion.[79] The incidence of myopia within sampled population often varies with age, country, sex, race, ethnicity, occupation, environment, and other factors.[25][80] Variability in testing and data collection methods makes comparisons of prevalence and progression difficult.[81]

In some areas, such as China, India and Malaysia, up to 41% of the adult population is myopic to −1dpt,[82] up to 80% to −0.5dpt.[83]

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.[84]

In Australia, the overall prevalence of myopia (worse than −0.50 diopters) has been estimated to be 17%.[85] 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.[86] 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.[87]

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.[88] Another found nearly 1 in 8 (13.3%) of the students in the city of Natal were myopic.[89]

In Greece, the prevalence of myopia among 15 to 18 year old students was found to be 36.8%.[90]

In India, the prevalence of myopia in the general population has been reported to be only 6.9%.[90][91]

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.[87]

In the United States, the prevalence of myopia has been estimated at 20%.[25] Nearly 1 in 10 (9.2%) American children between the ages of 5 and 17 have myopia.[92] Approximately 25% of Americans between the ages of 12 and 54 have the condition.[93] 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.[87]

A study of Jordanian adults aged 17 to 40 found that over half (53.7%) were myopic.[94]

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.[80]

Myopia is less common in African people and associated diaspora.[25] In Americans between the ages of 12 and 54, myopia has been found to affect African Americans less than Caucasians.[93] 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 = .17). Blacks had the lowest prevalence of astigmatism (20.0%), followed by whites (26.4%).[95]

Education and myopia

A number of studies have shown that the incidence of myopia increases with level of education[90][93] and many studies[96] 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.[97][98]

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.[99] 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.[100]

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.[101]

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.[102] Form deprived myopia that is induced with a diffuser, like the goggles mentioned, shows significant myopic shifts.[103] Anatomically, the changes in axial length of the eye seem to be the major factor contributing to this type of myopia.[104] 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%.[105] 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.[106] 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.[107]

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.[108] 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.[109] 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.[105] 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 Inhibits Myopic Shifts[117]

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.

See also

References

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