title: Stability of corneal polarization axis measurements for scanning laser polarimetry.
abstract: 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.
  
  
  METHODS
  Prospective, noncomparative case series.
  
  
  METHODS
  Persons with normal corneas were enrolled; eyes with less than 1 year of follow-up from the initial CPA measurement were excluded.
  
  
  METHODS
  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.
  
  
  METHODS
  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.
authors.affiliation: Department of Ophthalmology, University of Miami School of Medicine, Bascom Palmer Eye Institute, Miami, Florida 33418, USA. dgreenfield@bpei.med.miami.edu
authors.forename: D S
authors.initials: DS
authors.lastname: Greenfield
authors.forename: R W
authors.initials: RW
authors.lastname: Knighton
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D003315:Cornea / Q000033:anatomy & histology*; D003941:Diagnostic Techniques, Ophthalmological; D005260:Female; D006801:Humans; D007834:Lasers; D008297:Male; D008875:Middle Aged; D009412:Nerve Fibers; D011446:Prospective Studies; D012165:Retinal Ganglion Cells / Q000166:cytology*