Corneal Refractive Eye Surgery, Part 2

The Complete Book of Laser Eye Surgery
by Stephen G. Slade, M.D., Richard N. Baker, O.D., Dorothy Kay Brockman

(Page 2 of 2)

What Are "Diopters"?

To write your glasses or contact lens prescription, your doctor must determine your exact refractive error. Your correction is expressed in diopters that measure how much a corrective lens must bend light to focus it on your retina to normalize your vision. A lens that can bend parallel light rays to a focal point of 1 meter is said to have a power of 1 diopter (1.00D). A 2-diopter lens can focus light rays at a point 0.5 meters away from itself.

If you are blessed with normal eyesight, your eye doctor will write in your chart that your "sphere" is 0.0D, or "plano" (pl). This means, of course, that you have no refractive error. Department store sunglasses without power are plano lenses. Only curved surfaces bend light; flat surfaces do not.

If you have a myopic correction of -1 diopter, you are focused at 1 meter, or approximately the length of your arm. Without glasses, objects farther away than your hand will be slightly blurred and will become progressively more indistinct the further in the distance you look. With this refractive error, and no other visual defects, you should be able to read most street signs in good light. If your refraction is -2 diopters, you are focused at 0.5 meters, and you can only see clearly halfway down your arm. With this refractive error, you dare not drive without glasses. Nonetheless, you can see better in the distance than a person who is more nearsighted - someone, for example, whose correction is -3 diopters.

If you are farsighted, your amount of correction is expressed in positive numbers (+1 diopter of hyperopia, e.g.).

With nearsightedness, a refractive or focusing error less than -5 diopters is considered mild "myopia." Myopia is the medical term for nearsightedness and is corrected with a "minus" lens. Five to 7 or 8 diopters is in the moderate range, and greater than 7 or 8 is called severe. About 90 percent of nearsighted people have a refractive error of less than -6 diopters. Only about one nearsighted person in ten has severe myopia. Nevertheless, even if you have only -4 diopters of nearsightedness, you may feel that your correction is severe because you are almost totally dependent on your glasses or contact lenses. With astigmatism, less than 1 diopter is considered mild. One to 2 diopters is moderate. Two to 3 diopters is severe, and greater than 3 diopters is extreme. With farsightedness, greater than +5 diopters is called severe "hyperopia," the medical term for farsightedness. This refractive error is corrected with a "plus" lens. See insert 3, Reading Your Eyeglasses Prescription, on page 25.

Measuring Visual Acuity: What is 20/20, or "Normal," Eyesight?

You probably remember reading the big E on your doctor's Snellen eye chart at your last appointment. Many people confuse their eyeglasses 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 line.

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 (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, 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 (see fig. 11). 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 (see fig. 3). 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. (See chapter 2 for a detailed description of this operation.)

Reading Your Eyeglasses Prescription

When your eye doctor hands you a prescription for glasses or contact lenses to correct nearsightedness, farsightedness, astigmatism, or age-related presbyopia, how do you read the esoteric notation? As you know, your prescription, which is measured in diopters, is the power that must be put in each corrective lens to counteract your refractive error in the respective eye. Your prescription is expressed with the following formula:

Sphere (D) + or - Cylinder Power (D) at Cylinder Axis (in degrees)

This simply means that if you have -3 diopters of nearsightedness in your right eye and -3.5 diopters of myopia in your left eye - and no astigmatism - your glasses prescription might read:

O.D. (right eye) -3.00(D) and O.S. (left eye) -3.50(D)

If you also have 0.25 diopters of myopic astigmatism in your left eye, your prescription may read:

O.S. -3.50(D) -0.25(D) at 180 degrees axis.

This means that to correct your left eye (O.S.) you need a minus (concave) -3.5 spherical lens combined with a -0.25 cylindrical lens at an axis of 180 degrees. Since the axis is at 180 degrees, the astigmatism is horizontal. By looking at this glasses prescription, your doctor knows that both meridians of your cornea bend light too much: the flattest meridian refracts light short of the retina by 3.5 diopters, and the steepest one by 3.75 diopters. Put another way, with this prescription, you would have -3.5 diopters of myopia combined with -0.25 cylinder astigmatism. If you also need bifocal correction for age-related presbyopia, your doctor might write:

Add = +1.50(D)

This means that you need +1.5 diopters more plus power in the lower portion of your glasses to help you focus to read. Added algebraically to the -3.5 diopter prescription of your left lens, the power in your bifocal prescription in that eye would be:

-2.00D (-3.50D plus +1.50D equals -2.00D).

Copyright © 2001 by Stephen G. Slade, M.D., Richard N. Baker, O.D., and Dorothy Kay Brockman