Myopia

Myopia (μυωπία, muōpia, "nearsightedness" (AmE), "shortsightedness" (BrE)) is a refractive defect of the eye in which collimated light produces image focus in front of the retina when accommodation is relaxed. 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 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, though there are cases of associated 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. Myopia is partly hereditary.

Classification
Myopia has been classified in various manners.

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, 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) 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:
 * Simple myopia, more common than other types of myopia, 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.
 * Degenerative myopia, also known as malignant, pathological, or progressive myopia, is characterized by marked fundus changes, such as posterior staphyloma, 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.
 * 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 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. 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 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 or pigmentary glaucoma.
 * High myopia usually describes myopia of −6.00 or more. People with high myopia are more likely to have retinal detachments 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:
 * 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 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
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 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, 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), 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 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, 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-controlled, double-blind, prospective-controlled study.

Orthokeratology ("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 cililary 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. The 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 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 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, biofeedback 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 have not shown significant differences in altering the progression of myopia. More recently robust studies on children have shown that Orthokeratology 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, myopia is the norm.

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

China 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 in China is 77.3%, and in college is more than 80%.

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 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 whites and 53.4% of British Asians were myopic.

In Greece, 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, 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, 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, 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.

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.

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, and 28.4% had at least 1.00-D difference between the 2 principal meridians (cycloplegic autorefraction) astigmatism. 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 = .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 (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 metaphorically 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, 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, 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 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 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 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 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 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.