Myopia is a name used to describe the refractive disorder known as nearsightedness. With nearsightedness, light rays from nearby objects focus on the retina, but distant objects focus in front of the retina, causing those distant objects to be blurred.
Some scientists believe this disorder to be inherited. Babies are not usually born nearsighted. Generally it first occurs in kids 5-8 years old. When the child becomes a teenager and grows rapidly, the myopia often worsens. Fortuantely for most, this usually levels off when they reach their 20's.
This debate over whether myopia is inherited or
acquired has been going on for some time now. Experiments have shown a
trend in which environmental conditions, such as people who attended
school as opposed to those who didn't, appear to have an impact on
the development of myopia in a person. One explanation supporting the
acquired theory goes like this:
The ciliary muscle (the ring-shaped muscle behind the iris) is relaxed when you rest your eyes or focus on something far away. The ciliary contracts when focusing on things up close, making the lens more convex. For some people, when the ciliary is tens ed like this for a long time, for example during long periods of reading, the muscle spasms. It gets more difficult for it to relax as needed to focus on things at a distance. The spasming increases the pressure of the liquid which keeps the eyeball in shape. This increase in pressure in turn pushes teh front portion of the eye outward. Since the distance from lens to retina has increased, the focus of the light rays falls short, resulting in a blurred image.
Another explanation for the development of myopia in
children suggests a progressive myopia concept as follows:
Prescribed distance glasses for light myopic kids, intended to be used when viewing things at distances, are also used by these kids when reading when not necessary. This aggravates the problem, since the eye must work even harder to focus on things up close, forcing the eye to become more myopic. This is because glasses for nearsightedness spread out (diverges) light rays rather than focus them. When viewing things far away, this is good since a myopic eye's focusing power is too strong. However, a light myopic person may be able to see things up close fine without the glasses. With glasses though, the eye must work harder to focus those diverging rays. Essentially, wearing the right glasses at the wrong time is bad for your eyes.
Such explanations do not take into account other environmental factors, such as a bad diet or poor lighting. In any case, it appears that myopia occurs because of both genetic (inherited) and enviornmental factors.