Measurement of the magnitude and axis of corneal polarization with scanning laser polarimetry

Background: Scanning laser polarimetry uses a polarization compensator to isolate corneal birefringence from the birefringence of the retinal nerve fiber layer. This compensator assumes a fixed corneal polarization magnitude (CPM) of 60 nm and a fixed corneal polarization axis (CPA) of 15degrees in all subjects. Objectives: To measure the CPM and CPA with a scanning laser polarimeter and to determine if the assumed compensation values are representative of those observed in healthy and glaucomatous eyes. Methods: The CPM and CPA were measured in 51 healthy eyes and 55 glaucomatous eyes using a modified scanning laser polarimeter (GDx Nerve Fiber Analyzer; Laser Diagnostic Technologies Inc, San Diego, Calif) with an experimental variable CPM and CPA compensator. The CPM and CPA distributions in health), and glaucomatous eyes were compared, and the CPM and CPA relationships with age, corneal thickness, and corneal curvature were also investigated. Nasally upward CPA values (in degrees) were recorded as negative; nasally downward CPA values were recorded as positive. Results: The CPM and CPA measurements were normally distributed with many eyes having values different from those assumed by the GDx corneal compensator. For healthy and glaucomatous eyes combined, CPM measurements ranged from 7 nm to 91 nm (mean +/- SD, 40.0 +/- 15.7 nm). The CPA measurements ranged from -13degrees to 73degrees (mean +/- SD, 24.5degrees +/- 17.4degrees). A significant effect of age on CPA was observed when all eyes were combined (R-2 = 0.10; P<.001). There were no differences in CPM or CPA between healthy and glaucomatous eyes after adjusting for age. No effects of corneal thickness on CPM (R-2 = 0.04; P = .05) or CPA (R-2 = 0.01; P = .24) or of corneal curvature on CPM (R-2 = 0.002; P = .67) or CPA (R-2 = 0.009 P = .34) were observed. Conclusions: The range of CPM and CPA values observed in glaucomatous and healthy eyes suggests that the narrow-band corneal compensator used by the GDx scanning laser polarimeter is inappropriately compensating for anterior segment birefringence in many eyes.