Specific evaluations (I to V) must be done to determine if LASIK is appropriate:
I. Tear film evaluation
It is normal for patients to experience temporary dryness after LASIK.
Patients who have inadequate tears before surgery are at higher risk of prolonged
dry-eye symptoms after LASIK and should have increased lubrication before surgery.
Some patients have the feeling of dryness while wearing contact lenses since
lenses absorb tear moisture. This does not mean that the eyes are dry when contact
lenses are not being worn.
There are different methods to evaluate tear function: Some involve evaluation
with a special "tearscope"; some place colored agents in the tears; another,
physically measures the tears themselves. Visual inspection of the tear film
with a microscope is another technique.
ASK YOURSELF: If the tear film is not assessed, how can the surgeon know
if a patient is at high risk of developing dry eyes after surgery?
II. Pupil size evaluation
Following LASIK, some patients experience glare and halos. Often this
is due to a combination of larger pupil sizes and higher amounts of nearsightedness
or farsightedness that was not recognized as a risk factor prior to surgery.
With large pupils, light rays from the peripheral cornea are more likely
to cause symptoms of glare and halos after LASIK.
These symptoms can be minimized by programming a larger optical zone of the
laser for patients who have a combination of larger pupils and higher amounts
of nearsightedness. If central optical zones are custom-programmed with lasers
that have an adjustable central optical zone from 6.0 mm to 8.0 mm, even patients
with large pupils and high prescriptions can do very well and avoid troubling
glare, halos, and night-driving difficulties. To accomplish this, the surgeon
must have a high level of understanding and experience with advanced laser and
corneal optics in order to know how to program the laser based on a given
individual patient's measurements.
How are pupils measured? There are four categories of hand-held tools for
1. Infrared. Infrared technology gives a digital readout of the pupil
size, so there is no need for the examiner to make estimates. Infrared technology
is the gold standard method to measure pupil size, albeit the most expensive.
2. Light amplification. An examiner looks through one end of the device
at the pupil. Pupil diameter is measured against a tiny ruler in the viewfinder.
3. Ruler card. An examiner uses a hand-held card with different sized
circles on it to match the diameter of the pupil to that of a circle on the
4. Visual estimation. Here the pupil size is "guestimated" by the examiner.
This is the least accurate method.
ASK YOURSELF: If the pupils are not measured, or the surgeon did not understand
how to use the measurement in programming the laser, how can the risk of glare
and halos be minimized?
III. Corneal Topography
Corneal topography shows the surgeon whether or not the cornea has normal astigmatism
or abnormal astigmatism. Some patients with abnormal astigmatism have a condition
called keratoconus, or "bulging of the cornea." These corneas
often are weaker than normal corneas and thus should not be treated by LASIK.
ASK YOURSELF: If the topography test is not performed, how will the surgeon
know if the patient has keratoconus?
Pachymetry is the medical term for "corneal thickness." As you
already know, the LASIK procedure involves creating a flap on the surface of
the cornea and using the laser to reshape the cornea by removing tissue. It
is possible that too much tissue can be removed by the laser. This can destabilize
the cornea and lead to corneal bulging. This condition is called ectasia and
results in distorted vision.
Before surgery, the surgeon should calculate how deep the laser will
penetrate and be sure it does not penetrate beyond the safe level, which is
why measuring corneal thickness is imperative.
ASK YOURSELF: If pachymetry is not performed, or even in conjunction with
the test, if the calculation is not done, how can the surgeon be sure that the
laser is not going too deep?
V. Epithelium evaluation
Attached to the surface of the cornea there is a thin clear layer of "skin"
or epithelium. In some patients, this layer of skin may not be firmly
attached. If so, while making the flap in the LASIK procedure, this skin layer
may be brushed off by the microkeratome, leading to a higher risk of complications.
It is important to identify beforehand with a microscopic examination of
the cornea if the "loose skin" condition exists.
ASK YOURSELF: If a careful exam is not performed, how can "loose skin" be identified?