The Importance of Pupil Measurement
The following abstract was created from the article entitled Inaccurate Pupillometry Very Costly. The author would like to disclose his financial interest in the subject: he is the developer of the Holladay Pupil Gauge from ASICO.
As you may or may not know, people with very large pupils are generally bad candidates for LASIK and other refractive procedures. As a result, accurate measurement of a patient's pupils (pupillometry) as a part of the evaluation for refractive surgery is essential. With reports of halos and glare following refractive surgery on many of the prime time news shows, pupillometry has become one of the preoperative tests expected by the patient. It is very clear from the published and anecdotal reports of nighttime glare and halos that large pupils are the predominant factor resulting in these problems.
Avoiding Halos and Glare:
To understand why nighttime glare and halos occur, it is first important to understand two central factors: the effective optical zone and scotopic pupil size. In LASIK, the effective optical zone refers to the portion of the cornea that the refractive surgeon removes or ablates with the laser in order to reshape the cornea. Try and imagine the area of the eye covered by a contact lens. The effective optical zone is very similar.
Scotopic pupil size is the size a patient's pupils dilate to in a dark room. Postoperative halos are caused by disparity between scotopic pupil size and the effective optical zone.
Halos are literally a ring of glare seen through the outer portion of the pupil at night by some patients. This occurs when the diameter of the central area of the cornea ablated by the surgeon (the effective optical zone) is not quite as large as the patient's dilated eye at night (the scotopic pupil size). Ideally, the two should be the same size, and the effective optical zone should be perfectly centered with the pupil.
Requiring the effective optical zone size to be equal to the scotopic pupil is the strictest possible criterion a refractive surgeon can follow, but it is the safest approach. I have treated patients in years past without following this criterion and most of them were very happy with the results. However, our understanding of postoperative night vision problems and the standard of care have increased, and I would be much less likely to treat these patients the same way today.
In my opinion, today's surgeons should also use a blend zone (literally a ‘blended' zone of ablation between the effective optical zone and the outer edge of the cornea) of 1 or 2 mm to avoid a sharp transition from the treated to untreated area that can cause unwanted images in the peripheral vision. This requires the use of a laser capable of creating a blend zone.
There are three basic types of commercial pupillometers, the instrument used by refractive surgeons to measure scotopic pupil size. They are: 1) objective infrared video camera with pupil detecting system (Procyon), 2) infrared tubes with a reticule or display (Colvard & Pupilscan II), and 3) gauges (Holladay, Rosenbaum card, etc.).
If you are interested in how each of these instruments works, your refractive surgeon should be able to explain it to you. However, no matter what instrument is chosen, the information it reveals should be documented in the patient's chart and the measurement should be repeated if there is any question on the part of the technician as to the exact pupil size.
What is considered ‘normal' pupil size varies with age and gender. The pupil gets smaller with age, and men usually have smaller pupils than women of the same age. Studies show that 67% of refractive surgery patients' pupils are 5 to 7 millimeters in diameter. Since most lasers are capable of ablating out to 7 millimeters, this means that a majority of refractive surgery patients should have no problem.
As I stated at the outset, an accurate scotopic pupil measurement as a part of the preoperative evaluation is just as important as accurately measuring refraction and thickness of the cornea. This measurement, along with determining the effective optical zone size, is the best way to avoid disabling nighttime glare and halos.
Jack Holladay, MD, MSEE, FACS, is Clinical Professor of Ophthalmology at Baylor College of Medicine. He has devoted his Houston practice, Holladay Lasik Institute, exclusively to Excimer Laser Vision Correction. Dr. Holladay is also the developer of the Holladay Pupil Gauge from ASICO.
For more information on the author, please visit www.docholladay.com
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