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Up Front | Jul 2002

The Importance of Pupil Size

When considering refractive surgery, patients with larger pupils should be advised that they are potentially at an increased risk for night glare.

In last month's issue of Cataract & Refractive Surgery Today, I participated in a point/counterpoint article about the controversial topic of pupil size. It became a subject of concern to me in the early 1980s with the introduction of radial keratotomy. With that procedure, which requires optical zones of 3.0 mm (or less for some surgeons), it was obvious that the scotopic pupils would almost always be larger than the central portion of the fine scars left by the incisions. The fact that night glare, or “starbursting,” would result from this procedure was accepted not as a complication, but as an almost certain side effect. I always mentioned this as a possibility in the informed consent, and softened it by stating that in most patients, starbursting would improve with time, although some may experience permanent difficulty. It remains surprising to me how infrequently this was a major problem for patients. Among the 407 patients in the PERK (Prospective Evaluation of Radial Keratotomy) study, in which adequate data were available, 37% stated they experienced glare, halos, radiating lines, or discomfort in bright light prior to their surgery. This increased to 52% at one year and the difference was statistically significant. However, only three patients felt this was severe enough to limit their night driving and all three refused surgery on their second eye.1 My associates and I were involved in the early VISX PRK study for low-to-moderate myopia, which started patient recruitment in 1990. Pupil measurements were not required in the preoperative workup, but we mentioned glare and halos as a possible complication in the informed consent. There were reports of significant night glare in the early days of PRK, when there was concern about the risk of haze being related to the depth of the ablation. Ablation diameters of 5.0 mm and less were selected for higher corrections in order to minimize ablation depth, and halos and glare at night were frequent complications. These early reports put us on alert about the relationship between ablation depth and glare and halos, but there were very few formal studies in print.

Patient No. 1
The following are the results of wavefront studies performed on four of my patients who have undergone conventional LASIK using the LADARVision system (Alcon Laboratories, Fort Worth, TX). Patient No. 1 was a 28-year-old white female who was having difficulty wearing a contact lens in her right eye, but was satisfied with a contact lens in the left eye. We enrolled her in the LADARVision FDA LASIK study in 1998. Her pupils measured 7.5 mm, and her refraction was -5.75 -1.75 X 180. The required treatment zone at that time was 5.5 mm, with a 1.0-mm blend zone, for a total ablation diameter of 7.5 mm. Since the time of her 1-month postoperative examination, the patient's UCVA has remained at 20/20. She was bothered by night glare and halos and elected not to undergo surgery on her other eye, which had a refractive error of -6.00 -2.25 X 02. Although the ablation is well centered (Figure 1) and well outside her photopic 4.5-mm pupil, it is apparent that if the pupil was 7.5 mm, as it would be in the dark, light striking the peripheral cornea would be outside the ablated area, likely causing glare and aberrations.

The LADARWave study of this patient's post-LASIK right eye showed RMS values of 0.32 for coma and 1.60 for spherical aberration (Figure 2). The refractive data found in these images are not accurate, as the readings reflect the power across the entire cornea for 7.5 mm, and thus include the steep area beyond the ablation. For accurate refractive data, the software can constrict the pupil to 3.0 mm. Although we do not have the preoperative wavefront image for that eye, it would probably be similar to that of the unoperated left eye (Figure 3), in which we see RMS values of 0.28 for coma and 0.71 for spherical aberration. The LASIK procedure more than doubled the patient's spherical aberration, which explains the night glare she describes in the right eye. This glare is markedly reduced when her pupil is constricted through the consensual light reflex. The LADARWave study of the left eye with the contact lens in place shows that the RMS values for both coma and spherical aberration have reduced dramatically to 0.14 and 0.11, respectively, explaining her satisfaction with a contact lens for her night vision (Figure 4).

Patient No. 2
The next patient was a 47-year-old white male who underwent LASIK with the LADARVision system 2 years ago. His preoperative measurements are as follows: OD -4.25 -1.50 X120=20/20, OS -5.75 -1.25 X 90=20/20. His PupilScan (Keeler Instruments, Broomall, PA) measurements were OD 6.5 mm and OS 6.4 mm. The ablation diameters were OD 6.6 mm with a 1.0-mm blend zone, for a total of 8.6 mm. Because the patient was experiencing some night glare, the left eye had an ablation of 7.0 mm with a 1.0-mm blend zone, for a total of 9.0 mm. One year postoperatively, his UCVA is 20/15 OD, 20/20 OS, refraction OD -0.25 sphere, OS +0.75 - 0.75 X 180. His photopic vision is excellent, but he has significant scotopic glare and halos including viewing television programming in dark, indoor rooms while at work. He has been able to control his symptoms by using Alphagan drops (Allergan, Inc., Irvine, CA) twice daily. His topography shows well-centered ablations, but LADARWave studies with his pupils at 6.5 mm show RMS values for both eyes of 0.75 for spherical aberration and 0.62 for coma. If this patient's pupils measure 5.0 mm, as they usually do after applying Alphagan, the RMS values reduce by over 60% to 0.23 for spherical aberration and to 0.25 for coma.

Patient No. 3
Another patient, a 32-year-old white male, was concerned about night glare because his pupils measured 6.5 mm. The refraction in his left eye was -2.25 -2.00 X 02. He had an ablation of 7.0 mm with a 1.0-mm blend zone, for a total ablation of 9.0 mm in diameter. The post-LASIK topography of his left eye is shown in Figure 5. His uncorrected vision is 20/15, and his refraction is plano. Although this patient was satisfied with the results of the procedure, he continues to experience night vision disturbances, and he waited more than 2 years before undergoing surgery on his right eye. With a 6.2-mm pupil, the RMS values for his left eye are 0.60 for spherical aberration and 0.31 for coma. With a pupil of 4.0 mm, the RMS values are again dramatically reduced to 0.02 for spherical aberration and 0.06 for coma.

In patient No.1, it is easy to understand her problems with night vision, because of the 7.5-mm pupil and an ablation diameter of 5.5 mm with a 1.0-mm blend zone; the wavefront readings give us objective confirmation of her symptoms. We can also see that in the second and third patients, simply enlarging the optical zone diameter did not completely eliminate the problems with scotopic vision. Although each had far less spherical aberration than did the first patient, both require Alphagan drops to minimize their problems with night vision. The wavefront measurements of their higher order aberrations are significantly reduced when their pupil size is diminished, giving us objective evidence of the important role of pupils. If both of these patients had a wavefront-guided ablation, rather than simply a larger ablation, their results would most likely have been even better.

Patient No. 4
To show that a larger diameter optical zone can at times be helpful, consider the next patient. This 31-year-old white female has 8.0 mm pupils. Her refractions were OD -5.25 D, OS -5.25 -1.00 X172. The patient underwent sequential LASIK, allowing 1 week between eyes to ensure that she was satisfied with her night vision. I created the flaps using the INTRALASE FS Laser (IntraLase Corp, Irvine, CA) and used the LADARVision 4000 to produce ablation diameters of 7.0 mm OD and 7.0 mm OS, with a 1.0-mm blend zone. Her uncorrected vision is currently 20/15 OD and 20/20 OS, and she is completely satisfied with her night vision. The LADARWave RMS measurements with pupils of 5.5 mm are between 0.23 and 0.28 for both spherical aberration and coma. These values would undoubtedly be higher if her pupils were larger, and we would like to repeat the study in the future with her pupils slightly dilated.

Surgeons who disagree with the correlation of pupil size and glare quickly cite two recent articles by Weldon Haw, MD, and Edward Manche, MD2 and Mihai Pop, MD3 each of which failed to find a correlation between pupils and glare. Both of these articles summarized their results in a relatively small series of patients and a correlation may have been found if more patients were treated.

There is no doubt that factors other than pupil size are important in refractive surgery, but it cannot be denied that large pupils certainly increase the risk for some patients. The challenge lies in trying to identify these patients. A simple office maneuver can confirm the importance of the pupil in reducing night glare. When a postoperative LASIK patient complains of night vision problems, I examine him or her in a completely dark, windowless refracting lane. I ask the patient to look at a single projected line of letters and tell him or her to concentrate not on the clarity of the letters, but on the glare and halos around the rectangular light. I determine which eye has the most glare and then hold an occluder in front of the other eye while shining a penlight directly into the pupil to consensually constrict the pupil in the eye that is observing the chart. Invariably, the glare significantly decreases, at times even disappearing completely. Applying a drop of Alphagan as first suggested on the Internet user's group, Keranet by Jay McDonald, MD, and retesting the patient 30 to 40 minutes later frequently minimizes night vision difficulties.

In summary, refractive surgeons should carefully measure the scotopic pupils as accurately as possible and properly advise patients with larger pupils, especially those requiring a large correction, that they are potentially at an increased risk for night glare. Wavefront testing is beginning to provide us with a method of objectively correlating the quality of night vision with measurements of higher-order aberrations. Ablation diameters as large as or larger than the scotopic pupil can reduce, but not eliminate night vision problems. Wavefront-guided ablations will hopefully minimize the increase in higher-order aberrations that frequently accompanies standard ablations, and thus improve the quality of vision for our patients.

James J. Salz, MD, practices at the American Eye Institute in Los Angeles, California. He is a consultant to Alcon and VISX. Dr. Salz may be reached at (323) 653-3800; jjsalzeye@aol.com

1. Waring, GO, Lynn MS, Gelender H, et al: Results of the prospective evaluation of radial keratotomy (PERK) study one year after surgery. Ophthalmology 92:177-198, 1985
2. Haw W, Manche E: Effect of preoperative pupil measurements on glare, halos, and visual function after photoastigmatic refractive keratectomy. J Cataract Refract Surg 27:907-916, 2001
3. Pop M: Fall ISRS Symposium, November 2000, Dallas, TX

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