It is generally accepted that a precise measurement of pupil size under low illumination levels is indispensable prior to refractive surgery because the induced optical zone should ideally exceed the pupil diameter under any level of ambient illumination to minimize the risk for glare and halos, ghost images, and blurred vision. Unfortunately, no “gold standard” for pupil size measurements in refractive surgery candidates has been established.
Devices (eg, Procyon pupillometer P2000D and 3000, Neuroptics pupillometer, Colvard pupillometer, Zywave aberrometer, Wasca aberrometer, Orbscan II topography system) differing in illumination level, measurement duration, and number of recorded frames are currently used in clinical practice to measure pupil diameters under low illumination levels prior to refractive surgery. Monocular devices, in contrast to binocular devices, generally have the inherent potential for consensual pupil accommodation (light perceived by fellow eye) to compromise the results. Moreover, often inappropriate or inconsistent dark adaptation protocols are used before measurement. The data comparing the currently available pupillometers are confusing: A small comparison study by Schallenberg et al.1 found smaller pupil sizes at scotopic conditions assessed with the Procyon P3000 pupillometer than with the Neuroptics pupillometer, but a study by Michel et al.2 showed good agreement of measurements with these devices.
I agree that it is nearly impossible to control for accommodative miosis during pupillometry in daily practice, and spontaneous pupillary unrest may be overestimated.
Refractive surgeons and patients should be aware that even meticulous pupillometry with the currently available pupillometers might underestimate pupil size under real-life conditions. Further pupillometry research is needed to improve and standardize the measurement of pupil size under low illumination levels prior to refractive surgery.
1. Schallenberg M, Bangre V, Steuhl K-P, Kremer S, Selbach JM. Comparison of the Colvard, Procyon, and Neuroptics pupillometers for measuring pupil diameter under low ambient illumination. In press, J Refract Surg 2010
2. Michel AW, Kronberg BP, Narváez J, Zimmerman G. Comparison of 2 multiple-measurement infrared pupillometers to determine scotopic pupil diameter. J Cataract Refract Surg. 2006;32:1926-1931.