The Most Common Eye Disorders
The four most common eye disorders, or "refractive errors", are nearsightedness,
farsightedness, astigmatism and presbyopia. They are not diseases, but simply minor
flaws in the construction of the eye.
The human eye attains its adult size and shape when a person reaches his or her twenties.
At that point, nearsightedness, astigmatism, and farsightedness usually stabilize.
Nearsightedness, known scientifically as myopia, means that a person sees nearby objects
more clearly than distant objects. It does not mean that a myopic person has excellent
near vision and poor distance vision. Extremely nearsighted people do not see anything
clearly. In the normal eye, parallel rays of light passing through the cornea bend slightly
and then bend again as they penetrate the lens to focus on a small, single spot on the
retina. In the nearsighted eye, a steeply curved cornea or an elongated eyeball, or both,
focus rays of light before they reach the retina. From this focal point the light rays begin
to diverge. By the time light reaches the retina, the image is blurred.
Farsightedness, known as hyperopia, is the opposite of myopia. It is not necessarily the
ability to see objects clearly at a distance and poorly close up, although this can be the
case. In the farsighted eye, rays of light streaming through the cornea and lens reach the
retina before they come into focus, creating a blurred image. In a sense, the eyeball is too
short for the refractive power of its own optical system. Sometimes, mildly farsighted
people do not discover this refractive error until middle age, when they begin to have
trouble seeing clearly at close distances. This explains the common need for reading
eyeglasses among our middle-aged population.
An astigmatic eye results from an imperfectly shaped cornea. The cornea of the
astigmatic eye might be poorly curved from top to bottom or from side to side. This error
of design gives the cornea different refractory powers across its length and width and
distorts sight. Some light rays focus in front of the retina and others, behind it. As a
result, the retina never receives a single, sharp image.
Presbyopia is an optical flaw that eventually affects almost everyone. As the eyes age,
their powers of accommodation decline. It becomes increasingly difficult for the eye to
bring nearby objects into focus. An individual with presbyopia can also be farsighted,
nearsighted, or astigmatic at the same time. If this is the case, bifocals can give the eyes
extra help with nearby objects, no matter what other refractive errors the eyes may have.
By M Adley
http://www.eyes-and-vision.com
If you have an eye disorder or specific vision problem, you can learn about ways to
correct it (including laser eye surgery, contact lenses and eyeglasses) at eyes-and-
vision.com, where M Adley is webmaster.