Murat V. Kalayoglu, M.D., Ph.D.
Contributing Editor
Pupillometry has long been used in a research setting to study the autonomic nervous system, drug metabolism, pain responses, psychology, fatigue and sleep disorders. However, advances in computerized pupillometry have enabled its use in the clinical realm.
Dynamic, accurate measurement of pupil size is an important test for several medical and ophthalmic sub-specialties. Refractive surgeons are interested in measuring pupil size because large pupils measured preoperatively correlate with increased post-LASIK visual disturbances during scotopic conditions. Neuro-ophthalmologists are interested in measuring pupil size to distinguish physiological anisocoria from Horner syndrome, quantify afferent papillary defects, and even plot visual fields through measuring graded pupil constriction to focal light stimuli. Other medical specialties also use clinical pupillometers for diagnosis; critical care and emergency physicians use pupillometry to quickly and accurately measure pupillary responses in their injured patients.
Pupil size and movement can be measured by either infrared videography or computerized pupillometry. Infrared videography is used in order to detect magnified movement of both pupils at the same time. The technique allows the pupils to be visualized in the dark, and therefore is used to confirm difficult-to-test afferent papillary defects. Such cases may occur when an iris is heavily pigmented. Infrared videography takes advantage of such dark pigmentation, since melanin actually reflects the infrared light shone on to the iris. Therefore, pigmented irises appear light on the screen, and the black pupils stand out in contrast to the surrounding light-appearing iris. Such melanin-reflection of infrared light also can be used to detect transillumination defects, such as in pigment dispersion syndrome. Furthermore, videography can be very useful in detecting a subtle dilation lag in Horner’s syndrome.
In contrast to infrared videography, computerized pupillometry can record pupil size and movement in both the light and dark. These instruments can digitally record, then analyze with sophisticated software algorithms, various types of information gathered by the testing. Digital pupillometers enable rapid and accurate assessment of pupil size. The typical instrument captures several frames per second over several seconds, and then averages the measurements. Such averaging compensates for the highly variable pupillary response that changes second to second. These fluctuations in pupillary response can be averaged out only by capturing multiple frames over a period of time. Regardless, a small amount of afferent asymmetry exists and fluctuates with time, even when recorded by digital pupillography. Therefore, clinical measurements of relative afferent defects usually have an error factor, which is reported after the measurement values.
Computerized pupillography has been used successfully to determine how color, movement, visual acuity and other types of stimuli all affect the pupillary light reflex in different ways. These instruments are now used more and more frequently in the clinical setting, especially by refractive surgeons, neuro-ophthalmologists and even retinologists to determine clinical deficits due to retinal and optic nerve diseases. Instruments may feature such functions as the capacity to alter illumination at various settings, vertex distance correction for more accuracy in measuring the pupil size, and variable distance and near accommodation functions. Most instruments have built-in algorithms to average the pupil size over multiple frames.
An important observation has validated the use of computerized pupillometry in patients undergoing refractive surgery. In this study, Helgesen et al. (Acta Ophthalmol Scand. 2004 Aug; 82 (4): 454-60) determined if preoperative evaluation of pupil size could be useful in predicting the risk of night vision disturbances post- bilateral LASIK in myopic patients. The authors used infrared pupillometry to measure pupil sizes in 46 patients undergoing bilateral LASIK. Patients then were asked to complete a questionnaire containing questions on their night vision both before and after surgery. The authors found a significant correlation between large scotopic pupil sizes and their impression of worsened night vision, especially in men. The authors recommended preoperative evaluation of pupil size in all patients prior to LASIK surgery. In fact, the Eye Surgery Education Council (ESEC), established by the American Society for Cataract and Refractive Surgery (ASCRS), recommends evaluation of pupil size in room and dim light in all patients prior to LASIK surgery. Additional scientific studies supporting the role of pupillometry in ophthalmic diagnostics, combined with advances in pupillometry technology, will help ophthalmologists obtain more useful information applicable to many diseases.