Pupillometry using videokeratography in eyes with dark brown irides

Abstract

In their article, Wachler and Krueger1 compare the accuracy and repeatability of the pupil-measuring modules of 3 Placido-based topography systems and an infrared pupillometer. They highlight the underestimation of mesopic pupil size by the computerized videokeratoscopes and caution against using such measurements to preoperatively determine halo-related safety in patients having excimer corneal refractive surgery. The pupil depicted in topographic maps is measured in ambient illumination, enhanced by the luminance of the Placido rings, and represents the patient's pupil size for most of his or her functional activities. Since Placido-based videokeratoscopic images are centered on the corneal apex while refractive surgical procedures are centered on the line of sight, knowledge of the pupillary position is critical to interpret the significance of corneal topographic maps in patients having corneal refractive surgery. Placido-based topography devices use the contrast between the dark pupil and lighter iris to detect and display pupillary position. In the Indian population, dark brown irides often cause insufficient contrast difference at the pupil, resulting in the instrument's not mapping the pupil. In the past 8 years, we have faced this problem in many patients while using the TMS-1 and TMS-2 corneal topography devices (Computed Anatomy). We describe a simple modification that allows the pupil to be imaged in a reproducible manner in such eyes. A 3-volt battery-operated torchlight is held 12 inches away, temporal to the eye being imaged. The light is projected tangential to the iris surface, while the videokeratoscope image is captured. This technique results in the pupil being imaged consistently in eyes in which the standard technique fails. Concerned with the possibility of reflections from the torchlight interfering with the ability of the instrument to detect and map the Placido rings, we conducted a study in 20 patients. The instrument was unable to detect the pupil in these patients using the standard technique. After obtaining topographic maps without the pupillary outline, we repeated the measurements using the torchlight, as described. Parameters from the resulting topographic maps, which depicted the pupil outline, were compared with those of the earlier maps. There were no significant differences between the 2 maps. We now routinely use this technique to image the pupil in our patients. The article by Wachler and Krueger1 indicates that the pupil size depicted by the 3 topography devices tested were different. Although physiologic changes in the pupil may partly explain these differences, they indicate also that difference in luminance of the Placido rings in the different devices contributed to the observed differences in pupil size. We are therefore not sure whether our method of using the torchlight results in an underestimation of the pupil size in ambient illumination. However, since we do not use the topographic maps to preoperatively determine halo-related safety, we continue to use our modification during corneal topography. Rajesh Fogla DNB, FRCS Srinivas K. Rao DO aChennai, India