Stability of corneal polarization axis measurements for scanning laser polarimetry

OBJECTIVE: Corneal polarization axis (CPA) has been reported to affect retardation measurements obtained with scanning laser polarimetry. The purpose of this investigation was to evaluate the longitudinal stability of CPA measurements. DESIGN: Prospective, noncomparative case series. PARTICIPANTS: Persons with normal corneas were enrolled; eyes with less than 1 year of follow-up from the initial CPA measurement were excluded. INTERVENTION: We constructed a noninvasive slit-lamp-mounted device incorporating two crossed linear polarizers and an optical retarder to measure the slow axis of corneal birefringence, or CPA. MAIN OUTCOME MEASURES: Corneal polarization axis measurements. RESULTS: Seventy-one eyes of 40 individuals (23 female, 17 male) were enrolled in this investigation (mean age, 42.9 +/- 13.6 years; range, 22-85 years). Initial CPA measurements (mean, 24.0 +/- 18.0 degrees nasally downward; range, 67 degrees downward to 13 degrees nasally upward) were strongly associated (R2 = 0.88; P < 0.0001) with repeat CPA measurements (mean, 20.9 +/- 14.6 degrees nasally downward; range, 59 degrees nasally downward to 14 degrees nasally upward). The mean change in CPA was 4.1 +/- 3.2 degrees (range, 0-13 degrees ). Corneal polarization axis stability was statistically associated with the mean (initial and repeat) CPA (R2 = 0.1; P = 0.009), but not associated with age (R2 = 0.0003; P = 0.9) or gender (R2 = 0.03; P = 0.2). CONCLUSIONS: Corneal polarization axis measurements have good 1-year stability. These data suggest that CPA should not contribute significantly to longitudinal measurements of retinal nerve fiber layer thickness obtained with scanning laser polarimetry.