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Posts Tagged ‘snellen eye chart’

Measuring Visual Acuity

You probably remember reading the big E on your doctor’s Snellen eye chart at your last appointment. Many people confuse their cheap contact lenses prescription, which is measured in diopters, with their eye chart readings, which assess visual acuity (VA). By ascertaining the smallest line of figures that you can distinguish at a specified distance, your physician can determine your VA. If you can read the 20/20 line at 20 feet (about 6 meters), your visual acuity equals 20/20 (or 6/6 in meters), which is normal. If you read the 20/40 line at 20 feet, you see at 20 feet what the normally-sighted person sees at 40 feet.

Your VA, as measured by the Snellen eye chart, cannot be accurately converted to diopters. Nonetheless, assuming that your eyes are healthy, and you have no refractive error affecting distance vision, you should be able to read the 20/20 line or better. If your myopic refraction is -1 diopter, you should be able to read the 20/40 or 20/50 line of the eye chart. If you have significant astigmatism, however, you probably wouldn’t have 20/20 visual acuity. In addition, if your eye has some underlying pathology – even though light is perfectly focused on your retina – you may be unable to read the 20/20 tine.

Now, after years of research, ophthalmologists finally have a “patient friendly” procedure that, in exquisitely skilled hands, delivers excellent, relatively stable results in a more elegant fashion than the older refractive operations. Known as LASIK eye surgery (an acronym for laser in situ keratomileusis), this new excimer laser outpatient procedure is one of the first operations in the history of medicine to use a computer-driven light scalpel to reshape part of the human body. The Greek word keratomileusis is literally translated as “carving of the cornea.” (Kerato means “cornea” and mileusis means “carving.”)

During LASIK laser eye treatment, an ophthalmologist uses an electromechanically controlled surgical blade to cut a round “hinged” flap from the surface of the anesthetized cornea, the eye’s curved window. Leaving one uncut edge of the protective flap attached to the eye, the doctor carefully folds back this thin tissue, exposing the delicate inner corneal layer called the stroma.

To correct myopia, he then aims the cool, ultraviolet laser directly over the light-gathering pupil to remove microscopic layers of corneal tissue. For most nearsighted patients, the laser beam is concentrated on the eye only ten or twenty seconds, although larger refractive errors require more treatment. After the cornea’s curvature is thus remodeled, the physician gently puts the living flap back in place. No stitches are necessary.

The results depend on the patient’s eyes and healing response, the doctor’s skill, and the laser. One study involving thousands of patients using the broad beam Summit Apex Plus and the VISX Star showed good results at three months after LASIK surgery. With the Summit, about 50 percent of mildly and moderately near- sighted eyes up to -7 diopters achieved 20/20 on the Snellen eye chart or better without glasses.

Approximately 93 percent of nearsighted eyes with corrections between -8 and -14 diopters achieved 20/40 or better. With the VISX Star, over 54 percent of patients with corrections up to -7 diopters achieved 20/20 or better. Almost 95 percent of patients between -8 and -14 diopters achieved 20/40 or better.