Purpose: Corneal polarization axis (CPA) has been reported to affect retardation measurements obtained with scanning laser polarimetry (SLP). The purpose of this investigation was to prospectively determine whether correction for CPA improves the discriminating power of SLP for detection of mild-to-moderate glaucoma.
Design: Cross-sectional analysis of normal and glaucomatous eyes.
Methods: We constructed a noninvasive slit-lamp-mounted device incorporating two crossed linear polarizers and an optical retarder to measure the slow axis of corneal polarization. Complete ocular examination, standard automated perimetry, SLP imaging, and CPA measurements were performed on normal and glaucomatous eyes. One eye/subject was enrolled; if both eyes of a patient were eligible for the study, the right eye was selected. For each of the 13 SLP parameters, logistic regression was used to determine if including CPA in the model influenced the ability to discriminate between normal and glaucomatous eyes.
Results: Forty-three normal eyes (average visual field mean defect, -0.53 +/- 1.4 dB) and 33 glaucomatous eyes (average visual field mean defect, -5.93 +/- 6.5 dB) were enrolled. CPA was significantly correlated with summary retardation parameters (average thickness and integral values) in normal (r = 0.72-0.83, P <.001 for all values) and glaucomatous eyes (r = 0.43-0.62, P =.013 to <.001). Including CPA in the model improved the ability to discriminate between normal and glaucomatous eyes for five retardation parameters quantifying retinal nerve fiber layer (RNFL) thickness (range of P values: 0.045-0.001). For inferior average thickness, area under the receiver operating characteristic (ROC) curve increased significantly (P =.002) from 0.70 to 0.78 after accounting for CPA; with a sensitivity set at 80% specificity improved from 33% to 72%. Correlations between visual field corrected pattern standard deviation and average thickness, ellipse average, superior average, and inferior average significantly increased (range of P values,.018-.001) after adjustment for CPA (r = -0.35 and -0.45, -0.38 and -0.47, -0.46 and -0.57, and -0.42 and -0.49, respectively).
Conclusions: Correction for CPA significantly increases the correlation between retinal nerve fiber layer structural damage and visual function and significantly improves the discriminating power of SLP for detection of mild-to-moderate glaucoma.