The Fixed and Variable Corneal Compensator in Scanning Laser Polarimetry: A Comparison of Normative Data Distribution and Correlation With Visual Function

Abstract

Abstract: : Purpose: The current scanning laser polarimeter (SLP) has a fixed corneal compensator (FCC) that only allows the neutralization of corneal birefringence in eyes with birefringence that matches the population mode. A variable corneal compensator (VCC) has been developed that permits the neutralization of corneal birefringence in individual eyes. The aims of the study were i) to assess the age dependency of nerve fiber layer thickness (RNFLT) measurements, ii) to quantify the normal range of retinal nerve fiber layer thickness (RNFLT) measurements, iii) to evaluate the relative ability of the VCC and FCC SLP to discriminate between normal and glaucomatous eyes, and iv) to evaluate the correlation of RNFLT with visual function. Methods: Peripapillary (PP) images were taken of 56 eyes of 56 normal subjects and 55 glaucomatous eyes of 55 patients (average mean defect -7.2dB) with FCC and VCC settings. RNFL measurements were made in 4 quadrants in ellipses set at 1.75 the optic disc diameter. Correlation of RNFLT with age in normal subjects was sought by linear regression analysis and measurements were corrected for age where a relationship was found. The normal range for age-corrected RNFLT was defined as 97.5_th to the 2.5_th percentile. An eye was classified as "outside normal limits" if a quadrant RNFLT fell below the 2.5_th percentile. Correlation of RNFLT (measured RNFLT/age-corrected average RNFLT) with visual function (visual field measured sensitivity/age-corrected average sensitivity) was performed by linear regression analysis. Results: Superior and inferior quadrant RNFLT was related to subject age with VCC, but not FCC, measurement. The mean quadrant normal range was 27.4 µm for the VCC and 67.4 µm for the FCC. The specificity and sensitivity to discriminate between normal and glaucomatous eyes for the FCC was 89% and 42%, and for the VCC 88% and 78%. The correlation RNLFT with visual function was R₂ = 0.03 (FCC) and R₂ = 0.43 (VCC). Conclusion: Correction of corneal birefringence in individual eyes with the VCC gives rise to narrower normal ranges, greater sensitivity to identify glaucoma, and greatly improved correlation with visual function.

Keywords: 432 imaging methods (CT, FA, ICG, MRI, OCT, RTA, SLO, ultrasound) • 484 nerve fiber layer • 624 visual fields