American Medical Association Family Medical Guide - American Medical Association [725]
• Visual field test To check your peripheral (side) vision, the ophthalmologist performs a visual field test, in which one eye is covered and you look straight ahead with the other while he or she moves an object (such as a pen) into and out of different areas of your field of vision. The doctor instructs you to respond as soon as you see the object, and he or she records your responses. In a computerized version of this test, you sit facing a screen (with your chin on a chinrest) looking straight ahead. The doctor instructs you to press a button each time you see a tiny flash of light. A computer records your responses and produces a printout for the doctor to evaluate.
• Eye pressure test An ophthalmologist uses a test called applanation tonometry to measure the pressure inside your eyes. First, the doctor places a drop of a local anesthetic on each cornea to numb the eye, and then places a drop of an orange fluid called fluorescein in each eye. He or she then places the tip of an instrument called a tonometer against the cornea to measure the pressure inside the eyeball. You will not feel the tonometer against your eyeball. During the test, which is painless and takes only a few seconds, you will see a bright blue circle of light moving toward your eye. Sometimes a doctor performs a similar test called air tonometry, which uses a puff of air to measure the pressure inside the eyes, but this test is less accurate than applanation tonometry.
• Inner eye examination The ophthalmologist dilates (widens) the pupils with eyedrops and looks through an ophthalmoscope (a viewing instrument that projects a bright light onto the back of the eye) to examine the structures of the inner eye, including the retina and the optic nerve.
If the doctor detects a focusing disorder, he or she may prescribe glasses or contact lenses. If you already wear glasses or contacts, he or she will probably change your prescription. If the doctor detects a more serious eye disease, he or she may order additional testing or may recommend treatment with medication or surgery.
Slit lamp
A slit lamp is a powerful microscope with a light attached that enables a doctor to examine the structures at the front of the eye—the eyelid, cornea, sclera, conjunctiva, iris, and lens—under high magnification.
Focusing Disorders
In the eye, the cornea and lens work together to refract (bend) light rays from viewed objects and focus them on the retina, thereby producing an image. In normal vision, the light focuses directly on the retina, producing a sharp, clear image. However, if a person has a focusing disorder, the light focuses either behind or in front of the retina, producing a blurred image. The four most common focusing disorders are nearsightedness, farsightedness, astigmatism, and presbyopia.
Normal vision
The cornea and lens of the eye act as convex (curved outward) lenses to refract (bend) light rays from a viewed object and focus them on the retina. The retina and the brain work together to transform these light rays into the image you see when you look at an object.
Nearsightedness
In nearsightedness (also called myopia), either the eyeball is too long from front to back, or the focusing power of the cornea and lens is too great. As a result, images of distant objects are focused in front of the retina and are blurred. Images of nearby objects are focused on the retina and are usually sharp and clear.
About one in every five Americans is nearsighted. The disorder usually begins to develop at about age 12 and may gradually worsen until about age 30. Nearsightedness tends to run in families.
Symptoms
The main symptom of nearsightedness is blurred vision when viewing distant objects. Constant straining to see faraway objects can produce headaches and eyestrain (aching in the eyes). If you have any problems with your vision, see an ophthalmologist (eye MD) for a thorough eye examination (see previous pages).
Nearsightedness
In nearsightedness,