You Know You Are A Candidate For LASIK - Now What?
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
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
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
LASIK surgery; nevertheless, it is always best to consult with an
ophthalmologist with regards to specific circumstances.
By Shawn Powell
Shawn Powell is the Director of Marketing and Advertising for the Howerton
Eye & Laser Center. Ernest E. Howerton, M.D. has been serving the Austin
community for over 20 years. Having the procedure performed on him, Dr. Howerton
understands the importance of the surgeon seeing the patient at every visit. He
believes this practice is paramount to patient care and recovery. The Howerton
Eye & Laser Center performs LASIK on site, with state-of-the-art equipment and
an affordable, all-inclusive fee structure.
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