Spectrally dependent contrast of retinal nerve fiber layer reflectance in retinas with optic nerve crush

Abstract

Purpose : The reflectance of the retinal nerve fiber layer (RNFL) changes non-uniformly across wavelength in retinas with optic neuropathy. This study developed a method for in vivo measurements of RNFL reflectance at multi-wavelengths and studied spectrally dependent contrast of RNFL reflectance in retinas with optic nerve injury.

Methods : Wistar rats and a model of optic nerve crush (ONC) were used. Fundus images were taken with Heidelberg Retinal Angiography (HRA). The tomography mode provides a series of 64 images scanning through the retina (Fig. 1A). Areas were defined on nerve fiber bundles and nearby gaps (inset in Fig. 1A). Intensity profiles of bundle area (P_total) and gaps (P_gap) were derived (Fig. 1B). An intensity profile of a bundle only was estimated as P_bdl = P_total - P_gap by assuming that the reflectance of tissues underlying a bundle is similar to nearby gaps_. P_bdl was fitted with a Gaussian function (Fig. 1C). The area under the Gaussian was used as an estimate of RNFL reflectance (R_bdl). Gap reflectance (R_gap) was estimated from P_gap at the location corresponding to the peak of P_bdl. Spatial contrast of bundle reflectance was calculated as C = (R_bdl – R_gap) / R_gap. Fundus images were taken at 488 nm and 820 nm. The ratio, CC = C₄₈₈/C₈₂₀, was used to study spectrally dependent changes of RNFL contrast over time.

Results : Ten retinas from untreated, sham-operated or ONC eyes were imaged weekly for 4 – 7 weeks (Fig. 2). CC of normal retinas was not affected by the orientation of bundles in the retina and approximately unchanged over time. Some eyes with a sham operation had decreased corneal clarity; measured CC, however, did not show significant change over time. In contrast, CC of all ONC retinas decreased significantly.

Conclusions : The developed method for measuring the spectrally dependent contrast of RNFL reflectance was not affected by measurement conditions (e.g. bundle orientation and cornea clarity). Change of CC in ONC injured retinas suggests wavelength-dependent change of RNFL reflectance. Measurements of CC may provide a new means for optical assessment of RNFL damage in a clinical setting.

This is a 2020 ARVO Annual Meeting abstract.

 

Fig. 1 (A) Image series taken with HRA. (B-C) Intensity profiles of bundle and gap areas in A.

 

Fig. 2 Spatial contrast ratios of the RNFL over time in normal, sham operated and ONC retinas.