BACKGROUND AND OBJECTIVE: To evaluate the distribution of retinal nerve fiber layer thickness measurements in normal eyes corrected for corneal polarization axis (CPA). METHODS: Complete ocular examination, standard automated perimetry, peripapillary and macular scanning laser polarimetry imaging, and CPA measurements were performed in normal eyes. A noninvasive device mounted on a slit lamp that incorporated two crossed linear polarizers and an optical retarder to measure the slow axis of corneal birefringence was constructed. One eye per patient was enrolled. Exclusion criteria consisted of visual acuity of less than 20/40, a refractive error exceeding ± 5.0 diopter sphere, 2.0 diopter cylinder, or both, previous intraocular surgery, or ocular disease except cataract. According to scanning laser polarimetry, peripapillary retardation parameters within 90% normal limits were recorded as "normal," parameters outside 95% normal limits were recorded as "abnormal," and parameters between 90% and 95% limits were recorded as "borderline." RESULTS: Fifty-one eyes of 51 subjects (14 men, 37 women) were enrolled (mean age, 51 ± 17 years). Prediction limits (± 2 standard deviation outside of regression line) were calculated for 14 peripapillary retardation parameters. The prediction limits of the average thickness of the retinal nerve fiber layer (μm) were reduced 35% by correction of the CPA; for ellipse average (pm), the limits were reduced 33%. Sixteen of 51 normal eyes (31%) had at least one abnormal retardation parameter, and among these 16 eyes there was a bimodal distribution of CPA values (0° to 20° nasally downward, 60° to 80° nasally downward). CONCLUSIONS: Correction for CPA reduces the variance of normal retardation measurements. Normal eyes with a CPA of 0° to 20° nasally downward and 60° to 80° nasally downward were characterized by "abnormal" summary retardation parameters as interpreted by scanning laser polarimetry.
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