Retinal Imaging by laser polarimetry and optical coherence tomography evidence of axonal degeneration in multiple sclerosis

Background: Optical coherence tomography ( OCT) and scanning laser polarimetry with variable corneal compensation ( GDx) are similar yet provide information on different aspects of retinal nerve fiber layer ( RNFL) structure ( thickness values similar to histology for OCT vs birefringence of microtubules for GDx). Objectives: To compare the ability of OCT and GDx to distinguish eyes of patients with multiple sclerosis ( MS) from eyes of disease- free controls and thus identify RNFL abnormalities. We also sought to examine the capacity of these techniques to distinguish MS eyes from those without a history of optic neuritis and to correlate with visual function. Design: Cross- sectional study. Setting: Academic tertiary care MS center. Participants: Eighty patients with MS ( 155 eyes) and 43 disease- free controls ( 85 eyes) underwent both OCT and GDx imaging using protocols that measure RNFL thickness. Main Outcome Measures: Areas under the curve ( AUC), adjusted for within-patient, intereye correlations, were used to compare the abilities of OCT and GDx temporal-superior-nasal-inferior-temporal average RNFL thicknesses to discriminate between MS and control eyes and to distinguish MS eyes with a history of optic neuritis. Visual function was evaluated using low-contrast letter acuity and high-contrast visual acuity. Results: Average peripapillary RNFL thickness ( 360 around the optic disc) was reduced in patients with MS compared with controls for both methods. Age- adjusted AUC did not differ between OCT ( 0.80; 95% confidence interval [ CI], 0.72- 0.88) and GDx ( 0.78; 95% CI, 0.680.86; P=.38). Optical coherence tomography - measured RNFL thickness was somewhat better at distinguishingMS eyes with a history of optic neuritis from those without ( OCT: AUC, 0.73; 95% CI, 0.64- 0.82; GDx: AUC, 0.66; 95% CI, 0.57- 0.66; P=. 17). Linear correlations of RNFL thickness for OCT vs GDx were significant yet moderate ( r= 0.67, P <. 001); RNFL thickness measures correlated moderately and significantly with low- contrast acuity ( OCT: r= 0.54, P <. 001; GDx: r= 0.55, P <. 001) and correlated less with high-contrast visual acuity ( OCT: r= 0.44, P <. 001; GDx: r= 0.32, P <. 001). Conclusions: Scanning laser polarimetry with variable corneal compensation measurements of RNFL thickness corroborates OCT evidence of visual pathway axonal loss in MS and provides new insight into structural aspects of axonal loss that relate to RNFL birefringence ( microtubule integrity). These results support validity for RNFL thickness as a marker for axonal degeneration and support use of these techniques in clinical trials that examine neuroprotective and other disease-modifying therapies.