Myopia

Overview


Myopia (from Greek: μυωπία myopia "near-sightedness" ), also called near- or short-sightedness, 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 nearby objects clearly but distant 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 short.

Mainstream ophthalmologists and optometrists most commonly correct myopia through the use of corrective lenses, such as glasses or contact lenses. It may also be corrected by refractive surgery, such as LASIK. The corrective lenses have a negative optical power (i.e. are concave) which compensates for the excessive positive diopters of the myopic eye. In some cases, pinhole glasses are used by patients with low-level myopia. These work by reducing the blur circle formed on the retina.

Classification
Myopia has been classified in various manners.

Etiology
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.

Clinical entity
Various forms of myopia have been described by their clinical appearance:
 * 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.
 * 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. Myopia with degenerative changes has been described as being very common in certain races and cultures, such as Chinese, Japanese, Arab, and Jewish people.
 * 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, 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 is a type of myopia that occurs when the eye is deprived of clear form vision. Myopia is often induced this way in various animal models to study the pathogenesis and mechanism of myopia development.

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 classfied by degree or severity:
 * Low myopia usually describes myopia of −3.00 diopters or less.
 * 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 of onset
Myopia is sometimes classified by the age of 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.

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 Japan, Singapore and Taiwan, up to 44% of the adult population is myopic.

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 or 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 one city 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.

In the United States, the prevalence of myopia has been estimated at 20%. Nearly 1 in 10 (9.2%) American children between the ages of 5 and 17 have myopia. 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.

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

Ethnicity and race
The prevalence of myopia in 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 black, Nubians, and Sudanese people. In Americans between the ages of 12 and 54, myopia has been found to affect whites less than blacks. . Asians had the highest prevalence (18.5%), followed by Hispanics (13.2%). Whites had the lowest prevalence of myopia (4.4%), which was not significantly different from African Americans (6.6%). For hyperopia, whites 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 whites (26.4%).

Education, intelligence, and IQ
A number of studies have shown that the prevalence of myopia increases with level of education and many studies have shown a relationship between myopia and IQ. However, care must be taken in interpreting these results as correlation does not imply causation.

According to Arthur Jensen, myopes average 7-8 IQ points higher than non-myopes. The relationship also holds within families, and siblings with a higher degree of refraction error average higher IQs than siblings with less refraction error. Jensen believes that this indicates myopia and IQ are pleiotropically related as they are caused or influenced by the same genes. The mechanism that has caused a relationship between myopia and IQ is not yet known with certainty.

Etiology and pathogenesis
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 early 1900s, William Bates controversially asserted that myopia, as with all refractive errors, resulted from a particular type of "eyestrain" that was itself a result of "mental strain". He stated that the shape of the eyeball responded instantaneously to the action of the extraocular muscles upon it and that myopia was produced due to contraction of the inferior oblique and superior oblique muscles which lengthened the eye. According to Bates, myopia was associated with a "strain" to see distance objects rather than near work. Bates theories were rejected by mainstream ophthalmologists of his time and remain so today.

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".

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 focus 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. The exact mechanism of this image-controlled elongation of the eye is still unknown. 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.


 * 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. A high heritability of myopia (as for any other condition) does not mean that environmental factors and lifestyle have no effect on the development of the condition. Some recommend a variety of eye exercises to strengthen the ciliary muscle. Other theories suggest that the eyes become strained by the constant extra work involved in "nearwork" and get stuck in the near position, and eye exercises can help loosen the muscles up thereby freeing it for far vision. These primarily mechanical models appear to be in contrast to research results, which show that the myopic elongation of the eye can be caused by the image quality, with biochemical processes as the actuator. Common to both views is, however, that extensive near work and corresponding accommodation can be essential for the onset and the progression of myopia.


 * 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.


 * 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, but refuted in others . 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 . Near work in certain cultures (e.g. Vanuatu) does not result in greater myopia. It has been hypothesized that this outcome may be a results of genetics or environmental factors such as diet or over-illumination, changes in which seem to occur in Asian, Vanuatu and Inuit cultures acclamating 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.

Relevant 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.

Presbyopia and the 'payoff' for the nearsighted
Many people with myopia are able to read comfortably without eyeglasses. 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 or an optometrist. 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.

Treatment, management, and prevention
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.

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 eighties and nineties, 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"

Prevention
There is no universally accepted method of preventing myopia. Some clinicians and researchers recommend plus power (convex) lenses in the form of single vision reading lenses or bifocals. A recent Malaysian study reported in New Scientist suggested that undercorrection of myopia caused more rapid progression of myopia, However, the reliability of this 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.

Myopia control
Various methods have been employed in an attempt to decrease the progression of myopia. Altering the use of eyeglasses between full-time, part-time, and not at all does not appear to alter myopia progression. Bifocal and progressive lenses have not shown significant differences in altering the progression of myopia.

Myopia as metaphor
The terms myopia and myopic have also been used metaphorically to refer to cognitive thinking and decision making that is narrow sighted or lacking in concern for long-term consequence.