Myopia

Myopia is a refractive defect of the eye in which light focuses in front of the retina. Those with myopia are often described as nearsighted or short-sighted in that they typically can see nearby objects clearly but distant objects appear blurred because the lens cannot flatten enough. The opposite of myopia is hyperopia or "farsightedness". more...

Myopia is the most common eyesight problem in the world. About one quarter of the adult population in the United States has myopia. In places like Japan, Singapore and Taiwan, as many as 44% of the adult population is myopic.

Myopia is measured in diopters; specifically, the strength of the corrective lens that must be used to enable the eye to focus distant images correctly on the retina. Myopia of 6.00 diopters or greater is considered high, or severe, myopia. People with high myopia are at greater risk of more acute eye problems such as retinal detachment or glaucoma. They are also more likely to experience floaters.

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 dioptric value (i.e. are concave) which compensates for the excessive positive diopters of the myopic eye.

Prevalence

A recent Australian study found that less than 1 in 10 (8.4%) children between the ages of 4 and 12 were myopic . According to an American study published in Archives of Ophthalmology, nearly 1 in 10 children between the ages of 5 and 17 have myopia , and a recent Brazilian study found that nearly 1 in 8 (13.3%) of the students in one city were 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.

The prevalence of myopia in adults in the United States has been estimated to be approximately 25%, however, a study of Jordanian adults aged 17 to 40 found that over half (53.7%) were myopic .

Myopia is more common in Asians and Jews than in Whites, and more common in Whites than in Blacks (Jensen, 1998).

Pathogenesis

Theories

• Genetic Factors - The most widely held theory of the cause of myopia is that it is mainly hereditary. 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 - Another theory is that myopia is caused by a weakening of the ciliary muscle which controls the eye's lens. The weak muscle is unable to adjust the lens enough to see far distances, causing far-off things to be blurred. This theory states that the muscle's weakness is usually caused by doing lots of "nearwork", like reading books or using a computer screen. Since the eye rarely has to focus on far distances, the muscle is rarely used and, as a result, becomes weak. Since corrective lenses do the ciliary muscle's work for it, proponents of this theory suggest that they make it even weaker, increasing the problem. Instead, they recommend a variety of eye exercises to strengthen the muscle. A problem with this theory is that mainstream ophthalmology and medicine hold that the ciliary muscle is used when focussing at close distances, and is relaxed when accommodating for distant vision. 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. A variation of this theory was touted by William Bates in the early 1900s. Bates claimed that with nearwork and other "stresses", the extraocular muscles would squeeze the eyeball causing it to elongate.

• Near work. Near work has been implicated as a contributing factor to myopia in many studies. New research from NSU College of Optometry shows that students exposed to extensive "near work" are at a higher risk of developing myopia, whereas taking summer or winter vacations (which amount to extended breaks from near work) will either reduce or stop myopic progression .

• Combination of Genetic and Environmental Factors - Regardless of the accuracy of the ciliary muscle theory, 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. 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 little bit 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. In China, myopia is more common in those with higher education background ; some studies suggesting that nearwork may exacerbate a genetic predisposition to develop myopia .
• 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.

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