Myopia Cause & 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.

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 of onset

Myopia is sometimes classified by the age of 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, but refuted in others. 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.
  • Excess sleeping may cause myopia because of eyelids.

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.

Further Reading


This article is licensed under the Creative Commons Attribution-ShareAlike License. It uses material from the Wikipedia article on "Myopia" All material adapted used from Wikipedia is available under the terms of the Creative Commons Attribution-ShareAlike License. Wikipedia® itself is a registered trademark of the Wikimedia Foundation, Inc.

Last Updated: Feb 1, 2011

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