I believe that it is unnecessary to pharmacologically dilate the pupil in order to measure the eye's aberrations, and I prefer to treat an undilated pupil when performing customized LASIK surgery. Aberrations are most significant for patients at night, when their pupil is the largest. Ideally, laser correction should address the aberrations patients will experience at night. I believe that we want to correct patients' vision so that it functions optimally with their natural nighttime pupil size. It is true that more aberrations are apparent with a pharmacologically dilated pupil, but our patients are not walking and driving at night pharmacologically dilated. The dark-adapted, naturally dilated pupil more readily reveals the aberrations that are important to treat in order to maximize night vision. These aberrations are best measured by wavefront analysis with a natural pupil in a dark room. Then, the surgeon should perform laser treatment based on those aberrations.
THE BENEFITS OF A SMALL PUPIL DURING LASIK TREATMENT
Although the measurement of aberrations should take place on a naturally dilated pupil, the surgeon can treat aberrations on any pupil size, because the laser ablates the cornea based on a preprogrammed, customized treatment. However, there are real benefits to treating a small pupil. First, determining the center of the pupil is easier when the pupil is not dilated. A dilated pupil forces the surgeon to try to register a preoperative image of an undilated pupil with an operative image of a dilated pupil. This requirement is an additional, manual step in the treatment process that will decenter the treatment if not performed precisely. The second and more significant problem of performing LASIK with an enlarged pupil is that it hinders the patient's ability to fixate during the treatment. There are two reasons for this difficulty: (1) The blur circle of the fixation light becomes huge, so it is difficult to fixate accurately; and (2) the microscope illumination is much more disturbing to the patient and may cause a Bell's phenomenon. Even eye trackers cannot compensate for a patient's loss of fixation, the result of which is a decentered ablation. Properly aligning the patient's eye during customized ablations is even more critical; just a small drift in fixation can misalign the aberration correction. For these reasons, I believe that the customized treatment of aberrations should be performed with an undilated pupil.
Generally, I favor larger treatment zones. On virtually every patient, I use a 6.5-mm optical zone with an 8.0-mm blend zone. I have had wonderful results using the Wavescan System (VISX, Inc., Santa Clara, CA) to measure aberrations and the STAR S4 Excimer Laser System (VISX, Inc.) to treat them. I am performing wavefront-guided LASIK with the VISX system in the manner I have described here: measuring the pupil without pharmacological dilation in a very dim room and then treating with a naturally constricted pupil. More than 95% of the low and moderately myopic eyes have 20/20 visual acuity at the 3-month visit, and amazingly, fewer patients complained of nighttime vision problems after surgery than they did preoperatively.
THE ISSUE OF STANDARDIZATION
Although some physicians are concerned with standardizing dilations for wavefront measurement, I believe that our goal is not to standardize, but to customize. We measure the wavefront for each patient's pupils under conditions of dim illumination because the wavefront map depends on pupil dilation. Dilation produces different maps for each patient, so dilating each to the same parameters would risk treating an aberration structure that misrepresented what the patient sees at night. To explain it another way, if a patient's pupil dilates to only 6 mm at night, measuring his eye with a 6-mm pupil will produce one particular aberration structure or set of aberrations. Pharmacologically dilating that same pupil to 8 mm will produce a completely different aberration structure.
A Shack-Hartmann aberrometer features a circle of lenslets that must align concentrically with the pupil in order to optimally measure aberrations. Pharmacologic dilation enlarges the pupil beyond the circumference of the array of lenslets, which are 6 or 7 mm in diameter. This discrepancy makes it more difficult to center the aberrometer and can affect the accuracy of the aberration measurement.
SYSTEM-SPECIFIC BENEFITS
With the VISX STAR S4 System, one advantage to not dilating the pupil is that it allows the surgeon to use the PreVue lens, a plastic lens used to simulate the wavefront correction. The surgeon preoperatively measures the patient's pupil, cuts a PreVue lens to simulate the proposed wavefront correction, and positions the lens in front of the patient's eye to let him see how his vision may improve postoperatively. The PreVue lens can assure the patient and the surgeon that the proposed wavefront correction is indeed optimal for the patient.
Robert K. Maloney, MD, is Director of the Maloney Vision Institute in Los Angeles. He is a consultant to VISX. Dr. Maloney may be reached at (310) 206-7692; drmaloney@maloneyvision.com.
