Stability of Corneal Polarization axis measurements for scanning laser polarimetry

David S. Greenfield, M.D. and Robert W. Knighton

Bascom Palmer Eye Institute, University of Miami School of Medicine, Miami, USA

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.

Patients and Methods. We constructed a non-invasive slit lamp-mounted device incorporating two crossed linear polarizers and an optical retarder in order to measure the slow axis of corneal birefringence, or CPA. Subjects with normal corneas were enrolled; eyes with less than one year of follow-up from the initial CPA measurement were excluded.

Results. Seventy-one eyes of 40 patients (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 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). CPA measurement variability 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. CPA measurements have good short-term stability. These data suggest that CPA variability does not contribute to longitudinal changes in retinal nerve fiber layer thickness obtained with scanning laser polarimetry.