Correction for the erroneous compensation of anterior segment birefringence with the scanning laser polarimeter for glaucoma diagnosis.

PURPOSE To evaluate whether erroneous compensation for anterior segment retardation can be estimated and used to correct peripapillary (PP) retinal nerve fiber layer (RNFL) retardation measurements. METHODS Retardation measurements (for the 780-nm wavelength), given as RNFL thickness by the scanning laser polarimeter, were obtained at the macula and PP retina in 45 eyes of 45 normal subjects and 53 eyes of 53 patients with early glaucoma. The correlation of macula and PP retardation was assessed. The normal range for RNFL retardation was defined as 97.5th minus 2.5th percentile (normal subjects). This was calculated for uncorrected PP RNFL retardation and for PP RNFL retardation corrected by retardation measurements taken in the macula (analysis 1) and in the temporal aspect of the PP measurement annulus (analysis 2). Further ranges were defined at different percentile cutoffs, and normal and glaucomatous eyes were classified as abnormal if retardation measurements were below each cutoff. The accuracy of classification by uncorrected and corrected measurements was assessed by receiver operating characteristic curve analysis. Uncorrected and corrected RNFL retardation was correlated with visual field mean deviation (MD). RESULTS PP retardation correlated significantly with of macular retardation in normal (r(2) = 0.71, P < 0.000) and glaucomatous (r(2) = 0.41, P < 0.000) eyes. The normal range for uncorrected PP retardation was 25.4 degrees and for corrected retardation, 18.0 degrees (analysis 1) and 14.6 degrees (analysis 2), a reduction of 29% and 43%, respectively. For a specificity of 85%, the sensitivity to identify glaucomatous eyes of uncorrected and corrected (analyses 1 and 2) retardation was 26%, 55%, and 66%, respectively. Corrected PP retardation measurements correlated better with visual field MD (analysis 1: r(2) = 0.21; analysis 2: r(2) = 0.18) than did uncorrected measurements (r(2) = 0.05). CONCLUSIONS Erroneously corrected anterior segment birefringence significantly affects PP RNFL retardation measurements. Retardation arising from the cornea-corneal compensator interaction can be partially estimated from the macula and temporal aspect of the PP measurement annulus, allowing correction of PP RNFL retardation. This provides a narrower normal range and greater sensitivity for glaucoma diagnosis.