Scanning laser polarimetry uses an anterior segment compensating device that assumes
a fixed axis of corneal birefringence, which we call the corneal polarization axis.
The purpose of this investigation was to establish the distribution of corneal polarization
axes among a population of normal eyes and to evaluate the relationship between corneal
polarization axis and posterior segment retardation.
We constructed a noninvasive slit lamp-mounted device incorporating two crossed linear
polarizers and an optical retarder in order to measure the slow axis of corneal birefringence.
Normal subjects underwent corneal polarization axis measurement. A subset of eyes
underwent scanning laser polarimetry of the peripapillary retinal nerve fiber layer
(n = 32) and macula (n = 29), and retardation measurements were evaluated in each
group.
One hundred eighteen eyes of 63 normal subjects (35 female, 28 male) underwent corneal
polarization axis measurement (mean age, 45.5 +/- 17.1 years). Six eyes (5.1%) demonstrated
unmeasurable corneal polarization. In the remaining 112 eyes, the mode of the corneal
polarization axis distribution was 10 to 20 degrees nasally downward (range, 90 degrees
nasally downward to 54 degrees nasally upward). A significant (P <.0001) correlation
was observed between fellow eyes (R(2) =.52), with a mean difference of 11.2 +/- 10.5
degrees (range, 0-52 degrees). Corneal polarization axis was significantly associated
(R(2) =.52-.84) with retinal nerve fiber layer and macula summary retardation parameters
(average thickness, ellipse average, superior and inferior average, superior and total
integral; P <.0001 for all groups).
The mean corneal polarization axis among normal corneas is nasally downward; however,
considerable intraindividual and interindividual variability exists. The linear relationship
between corneal polarization axis and posterior segment retardation parameters is
responsible, in part, for the wide distribution of retinal nerve fiber layer thickness
data generated by scanning laser polarimetry.