Reflectance Spectrum and Birefringence of Retinal Nerve Fiber Layer in Hypertensive Retinas

Abstract

 

Purpose

 

Glaucoma damages the ultrastructure of the retinal nerve fiber layer (RNFL), hence changes the optical properties of RNFL. This study investigated the reflectance spectrum and birefringence of the RNFL in retinas with ocular hypertension.

 

Methods

 

A rat model of glaucoma was used. RNFL reflectance of isolated retinas was measured at wavelengths of 400 - 830 nm and RNFL birefringence was measured at a wavelength of 500 nm. Reflectance spectrum and birefringence of the same nerve fiber bundles were calculated at distances of 300 - 700 µm from the ONH center. Cytoskeleton of bundles was evaluated with confocal imaging.

 

Results

 

Twenty retinal regions in five normal retinas and 24 regions in seven treated retinas were studied. RNFL reflectance of normal bundles was higher at short wavelengths and decreased with increasing wavelength. Reflectance spectra were similar along bundles (Fig. A). RNFL birefringence was approximately constant along bundles (Fig. E). For bundles with mild cytoskeletal distortion, however, the reflectance spectra were different along bundles: the spectrum of the peripheral region was similar to the control, while the spectrum near the ONH decreased at short wavelengths (Fig. B). Birefringence of these bundles was either approximately constant along bundles, similar to the normal, or decreased near the ONH. The mean birefringence along bundles, however, was within the normal range (Fig. F). For bundles with moderate cytoskeletal distortion, reflectance spectra became flatter at short wavelengths (Fig. C) and birefringence profiles were lower than normal (Fig. G). For bundles with severe cytoskeletal distortion, reflectance spectra either decreased significantly at short wavelengths (Fig. D) or were approximately constant across wavelengths (not shown). Birefringence profiles, however, were within the normal range (Fig. H).

 

Conclusions

 

Ocular hypertension caused change of RNFL reflectance spectrum and birefringence. RNFL reflectance spectra and birefringence profiles along bundles differed in bundles with different degrees of damage.  

 

Fig. Reflectance spectra and birefringence profiles of a normal bundle and bundles with different degrees of damage. A - D: spectra measured along bundles (radius in µm shown in A); E - H: birefringence profiles of the same bundles in A - D. Gray bar: birefringence range of normal retinas.