Myopia or Near Sightedness
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| Normal vision for a achromatopsic colour-blind person. Courtesy NIH National Eye Institute |
The same view when achromatopsic and myopic. |
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. The opposite of myopia is hyperopia or "farsightedness".
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 one in three or one in two of the adult population is myopic. It is thought that this is due to the focus on academics in these countries.
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.
Pathogenesis of myopia
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.
- 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.
- Evolutionary Advantage - Most likely a neoteny
characteristic that survived as a link to intelligence(infants
normal focal range is the mother's face). Gene(s) first survived
in females (hormonal link; women often become more nearsighted
after giving birth) but is also present in men in races with
relaxed hunting pressures normally requiring distant vision and
expanded work opportunities with closer vision type crafts.
Relevant research
- One Austrian study ("Eye elongation during accommodation in
humans: differences between emmetropes and myopes" by Drexler et
al) suggests that there is eye elongation during accommodation,
but that it is caused by "accommodation-induced contraction of
the ciliary muscle", not "squeezing" of the extraocular muscles.
- Numerous experiments with animals showed that myopia can be
artificially generated either by reducing the image quality on
the retina, or by applying minus glasses. The exact mechanism of
this image-controlled elongation of the eye is still unknown.