Howerton Eye Clinic - Austin Texas
You know you are a Lasik candidate.
by Shawn Powell
You understand the risk.
Now what? ……getting ready
for LASIK surgery.
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.
Corneal Topography
is used to demonstrate astigmatism
and other abnormalities such as keratoconus.
Topography is a map of the eye showing
the hills and valleys in color. Today’s
more progressive equipment also measures
pachymetry and keratometry during
the course of the test.
Pachymetry is a
measurement of the thickness of the
center of the cornea. An ultrasound
probe touching the surface of the
cornea after an eye drop is administered
for anesthesia constitutes Pachymetry.
The thickness of the cornea is essential
to determine the amount of correction
that can be obtained. When LASIK is
performed the laser ablates or removes
some of the corneal tissue. The more
correction that is needed, the more
tissue is removed. This test along
with the refraction is used to make
sure there will be enough tissue to
maintain a stable healthy cornea.
Determination of
the dominant eye is obtained for those
patients who are either having one
eye completed at a time or they have
chosen to have monovision. If a person
is having one eye completed at a time,
most surgeons begin with the dominant
eye.
The final test is
the measurement of pupil size. This
is important for those people who
have very large pupils in dim light.
This information can assist the surgeon
in determining the size of the ablation
area to avoid an extreme amount of
glare and starburst post surgically
in night driving.
All of these tests
are necessary to have the most accurate
results possible with LASIK surgery;
nevertheless, it is always best to
consult with an ophthalmologist with
regards to specific circumstances.
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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