Abstract:
Birefringence, i.e. retardation per unit thickness, of the retinal nerve fiber layer (RNFL) has been reported to vary with position around the optic nerve head (ONH) in normal eyes (Huang et al, Invest Ophthalmol Vis Sci. 2004;45:3073–3080). We investigated the RNFL birefringence around the ONH in healthy and glaucomatous eyes, to replicate the reported results and also to assess any RNFL birefringence differences in glaucoma.
We measured both eyes of eight healthy subjects and eight glaucoma patients with scanning laser polarimetry (SLP) with a variable cornea compensator and a bias retarder (GDx ECC, Laser Diagnostic Technologies, San Diego, CA, USA), as well as with optical coherence tomography (Stratus OCT, Carl Zeiss Meditec, Jena, Germany). Retardation (by SLP) and RNFL thickness (by Stratus OCT) were determined along a peripapillary circle with a radius of 1.81 mm. The data of these two imaging modalities were registered (based on blood vessels). Birefringence was calculated as the slope in linear regression analysis of corresponding data points, thus adjusting for any offset in the measuring instruments.
In healthy eyes, birefringence varied with position around the ONH, being higher superotemporally, inferonasally and inferotemporally. Birefringence in glaucomatous eyes did not show such a pattern. The average RNFL birefringence was also significantly lower in glaucomatous eyes than in normal eyes along the peripapillary circle (p=0.01) and most notably in the superotemporal sector (p<0.01) (table).
Peripapillary RNFL birefringence varies with position in normal eyes. In glaucomatous eyes, birefringence is significantly smaller. These results suggest that the RNFL not only thins in glaucoma, but that its physical properties (i.e., birefringence) change. It is therefore possible that SLP detects glaucomatous change without measurable (by Stratus OCT) RNFL thinning.
Keywords: imaging methods (CT, FA, ICG, MRI, OCT, RTA, SLO, ultrasound) • nerve fiber layer • optical properties