There are many resources available to explain the LASIK procedure; however, very few detail the testing necessary leading up to the procedure. These tests can be seemingly overwhelming and technical. Some of the testing could have been done at an initial consultation, though, some tests may need to be repeated. Each test has a specific purpose to assist the surgeon in obtaining the best results possible.
The first step is to determine the uncorrected visual acuity, which is vision without glasses or corrective lenses. This raises the question of “What does 20/20 mean?” In the U.S. vision is recorded on a chart with standardized letters calibrated to be read at 20 feet away. The fraction that describes vision, for instance, 20/20, 20/40, compares the smallest letters that a person with perfect vision can see at 20 feet to your vision at 20 feet. If you are only capable of seeing larger letters on the chart that can be seen by a normal person standing back 40 feet from it, then your vision is 20/40. 20/400, means at twenty feet from the chart you can only distinguish the giant E on top, which could be seen by a normal person from 400 feet away.
Some visual acuity levels bear special legal significance. For example, you must be able to see the 20/40 line of letters, or smaller, in order to drive without glasses in the U.S. For this reason, statistics for laser vision correction are often quoted in relation to the percent of patients receiving 20/40 vision or better. If the best vision a person can see with corrective lenses is 20/200 in both eyes, that person is legally blind.
Next, best-corrected visual acuity must be determined. Refractive error or acuity is measured one eye at a time, covering the other eye with an occluder. Refractive error is the state of the eye for which a prescription is normally given for glasses or contacts. Generally two refractions are completed on a person considering LASIK surgery. The first refraction is called a manifest refraction. A determination of the refractive error is the “dry” or undilated state. The second is a cycloplegic refraction. This is similar to a manifest refraction except it is completed while your eyes are dilated. The reason for the cycloplegic refraction is to insure that the manifest is correct. When your eyes are dilated, you are unable to accommodate (focus up close) during the test.
Once your visual acuity has been determined, the results will be compared to your previous prescription to assure stability in refractive error. There should not be more than a half a diopter change in the past two years.
The next eye examination is performed with an instrument known as a slit lamp microscope that allows a binocular, high magnification view of the eye. This examination is used to rule out the presence of disease.
Keratometry is used to measure corneal curvature and is completed with a keratometer. This measures the shape of your cornea. Many surgeons have sophisticated topography equipment that obtains these measurements.
An Opthalmoscope is used to examine the back of the eye or “retina” to exclude any tears, detachment or predisposing conditions. The optic nerve that carries vision from the retina to the brain is also observed for signs of glaucoma.