You’ve done your research. LASIK is the answer to your dreams. But are you sure you’re a candidate? While most people fit the characteristics of a ‘good LASIK candidate,’ others simply do not fit the bill. So how do you know if you are one of them? To maximize the chance of a successful result, it is imperative that your refractive surgeon performs certain examinations as part of the preoperative evaluation.

Specifically, there are five crucial tests that need to be performed during the preoperative evaluation: Tear Film Evaluation, Pupil Size Evaluation, Corneal Topography, Pachymetry, and the Epithelial Evaluation. In this article I outline each of these steps and provide you with some important questions to ask yourself at each stage.

I. Tear film evaluation

It is normal for patients to experience temporary dryness after LASIK. However, patients who have inadequate tears before surgery are at higher risk of prolonged dry-eye symptoms after LASIK and should undergo increased lubrication before surgery. Some patients feel a dry sensation while wearing contact lenses since lenses absorb tear moisture. This does not necessarily mean that the eyes are dry when contact lenses are not being worn.

There are different methods to evaluate tear function: Some tests involve evaluation with a special “tearscope”; some place colored agents in the tears; finally, one test physically measures the tears themselves.


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. Oftentimes this is due to a combination of large pupils and a higher-than-satisfactory degree 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.

Glare and halos can be minimized by programming the laser to cover a larger optical zone for those patients with a combination of larger pupils and higher amounts of nearsightedness. Some lasers allow for larger optical zones than others. 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 resultant night-driving difficulties. In order to correctly program the laser based on a given individual patient’s measurements, the surgeon must have a high level of understanding and experience with advanced laser and corneal optics.


How are pupils measured? There are four categories of hand-held tools for evaluation:

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 card.

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 does not understand how to use the measurement in programming the laser, how can the risk of glare and halos be minimized?


III. Corneal Topography

To perform corneal topography, the doctor seats the patient in front of a special machine and an image is snapped. A computer then converts the image into a topographic map of the eye with different colors representing different levels of steepness. Corneal topography shows the surgeon whether or not the cornea has a form of abnormal astigmatism called keratoconus, or “bulging of the cornea.” Corneas with this abnormality are often 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?


IV. Pachymetry

Pachymetry is the name given for measurement of “corneal thickness.” As you may 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. While the cutting of the corneal flap usually goes smoothly, it is possible to remove too much tissue with the laser. This can destabilize the cornea and lead to corneal bulging. This condition is called ectasia and results in distorted vision.

To avoid postoperative complications, measuring corneal thickness is imperative. Before surgery, the surgeon should calculate how deep the laser will penetrate and be sure it does not penetrate beyond the safe level. This is done through pachymetry, which measures corneal thickness with a small ultrasound probe.


ASK YOURSELF: If pachymetry is not performed, how can the surgeon be sure that the laser is not going too deep?


V. Epithelium evaluation

Attached to the surface of the cornea is a thin, clear layer of skin: the epithelium. In some patients, this layer of skin may not be firmly attached, creating a “loose skin” effect. If so, during the LASIK procedure this skin layer may be brushed off by the microkeratome (the special scalpel used to create the corneal flap), leading to a higher risk of complications. It is important for the refractive surgeon to perform a microscopic examination of the cornea beforehand, in order to determine whether the “loose skin” condition exists.


ASK YOURSELF: If a careful exam is not performed, how can “loose skin” be identified?




Brian S. Boxer Wachler, M.D., is director of the UCLA Laser Refractive Center and Assistant Professor in the Cornea - External Ocular Disease Division of the UCLA Department of Ophthalmology. In addition, Dr. Boxer Wachler is one of the board of directors for the LASIK Institute - a nonprofit educational organization dedicated to education about the LASIK procedure for refractive surgery. He has also published the LASIK Consumer Report online. For more information about the author, visit the website of LASIK Consumer Report at www.lasikconsumerreport.com




Copyright 2001. Brian S. Boxer Wachler, M.D. All Rights Reserved.