Assessment of the retinal nerve fiber layer of the normal and glaucomatous monkey with scanning laser polarimetry.

PURPOSE To describe and test a method for assessment of the monkey retinal nerve fiber layer (RNFL) with scanning laser polarimetry. METHODS A scanning laser polarimeter was modified to accommodate a variable corneal polarization compensator. Corneal polarization magnitude (CPM) and corneal polarization axis (CPA) of the anterior segment birefringence of normal and glaucomatous cynomolgus monkey eyes were determined from a polarimetry image of the Henle fiber layer. Next, the variable compensator was adjusted to minimize the anterior segment birefringence. RNFL measurements were then obtained. All images were compared with simultaneous optic disc stereoscopic photographs. RESULTS CPM was small in each of the eyes examined, ranging from 5.7 nm to 9.0 nm. CPA ranged from -62 degrees to 79 degrees. (Nasally upward CPA values were recorded as negative; nasally downward CPA values were recorded as positive.) When eye-specific compensation was used, RNFL retardation profiles mimicked the expected appearance of the RNFL in all eyes. We also observed a substantial decrease in retardation in eyes with experimental glaucoma compared with healthy fellow eyes. CONCLUSIONS Individualized anterior segment compensation can be achieved in the monkey eye so that the measured birefringence appears to largely reflect the birefringence of the RNFL. Observed differences in retardation between healthy eyes and eyes with experimental glaucoma suggest that scanning laser polarimetry may be useful for detecting and monitoring RNFL loss in experimental primate glaucoma.