Scanning laser polarimetry with variable corneal compensation: identification and correction for corneal birefringence in eyes with macular disease

PURPOSE: In scanning laser polarimetry with variable corneal compensation (SLP-VCC), the macula is used as an intraocular polarimeter to calculate and neutralize corneal birefringence based on an intact Henle's layer. The purpose of this investigation was to validate this strategy in eyes with macular structural disease. METHODS: A nerve fiber analyzer was modified to enable the measurement of corneal polarization axis and magnitude so that compensation for corneal birefringence was eye specific. Normal subjects and patients with a variety of pathologic macular conditions underwent complete ocular examination, SLP-VCC, and direct measurement of the corneal polarization axis (CPA), with a slit-lamp-mounted corneal polarimeter. Macular birefringence patterns were classified as well defined, weak, or indeterminate bow ties. A new "screen" method is described that determines the anterior segment birefringence without relying on the presence of macular bow-tie patterns. RESULTS: Forty-seven eyes (20 normal, 27 with maculopathy) of 47 patients (mean age, 59.0 +/- 19.0 years; range, 24-88) were enrolled. The correlation between CPA measured with corneal polarimetry (CPA by P(IV) [fourth Purkinje image]) and SLP-VCC was less in eyes with macular disease (R(2) = 0.22, P = 0.024) compared with normal eyes (R(2) = 0.72, P < 0.0001). Eyes with macular disease had significantly (P = 0.007) more indeterminate macular bow ties (8/27; 29%) than did normal eyes (0/20). The magnitude of difference between CPA by P(IV) and CPA by SLP-VCC was significantly (P = 0.0007) greater in eyes with indeterminate bow-tie patterns than in weak and well-defined patterns. Although no relationship was observed between CPA and 12 retardation parameters obtained with SLP-VCC in normal eyes (P > 0.05), eyes with macular disease showed a significant association between CPA and average thickness (R(2) = 0.27, P = 0.005), ellipse average (R(2) = 0.24, P = 0.0085), superior average (R(2) = 0.24, P = 0.009), inferior average (R(2) = 0.28, P = 0.004), and superior integral (R(2) = 0.37, P = 0.0008), suggesting incomplete corneal compensation. Greater correlation between CPA by P(IV) and CPA derived by SLP-VCC was found by using the screen method (R(2) = 0.83, P < 0.0001) compared with the bow-tie method (R(2) = 0.22, P = 0.024) in eyes with maculopathy. CONCLUSIONS: Macular strategies for neutralization of corneal birefringence using SLP-VCC can fail if Henle's layer is disrupted by macular disease. The screen method provides a more robust measure of the anterior segment birefringence in some eyes with macular disease.