Estimation of Retinal Ganglion Cell Loss in Glaucomatous Eyes With a Relative Afferent Pupillary Defect

Abstract

To estimate retinal ganglion cell (RGC) losses associated with a relative afferent pupillary defect (RAPD) in glaucoma. A cross-sectional study was conducted including both eyes of 103 participants from the Diagnostic Innovations in Glaucoma Study. A total of 77 subjects had glaucoma in at least one eye and 26 were healthy. Pupil responses were assessed using an automated pupillometer that records the magnitude of RAPD as an "RAPD score." Standard automated perimetry (SAP) and optical coherence tomography (OCT) also were performed. Retinal ganglion cell counts were estimated using empirical formulas that combine estimates from SAP and OCT. The estimated percentage RGC loss was calculated using the combined structure function index (CSFI). There was good correlation between RAPD magnitude and intereye differences in estimated RGCs (R(2) = 0.492, P < 0.001), mean deviation (R(2) = 0.546, P < 0.001), retinal nerve fiber layer thickness (R(2) = 0.362, P < 0.001), and CSFI (R(2) = 0.484, P < 0.001). Therefore, a high RAPD score is likely to indicate large asymmetric RGC losses. The relationship between intereye difference in RGC counts and RAPD score was described best by the formula; RGC difference = 21,896 + 353,272 * RAPD score. No healthy subjects had an absolute RAPD score > 0.3, which was associated with asymmetry of 105,982 cells (or 12%). Good correlation between the magnitude of RAPD and intereye differences in mean deviation and estimated RGC counts suggests pupillometry may be useful for quantifying asymmetric damage in glaucoma. (ClinicalTrials.gov number, NCT00221897.).