Does loss of vasculature contribute to early stage glaucomatous thinning of the retinal nerve fiber layer (RNFL)?

Abstract

Purpose : To test the hypothesis that loss of vasculature contributes to early stage glaucomatous RNFL thinning.

Methods : OCT-Angiography scans (OCTA, Spectralis OCT2, Heidelberg Engineering, GmbH) were obtained longitudinally in 18 eyes of 9 rhesus monkeys at baseline and though an early stage of unilateral experimental glaucoma (EG). Each scan contained 768x768 A-lines over a 15°x15° area manually centered on the optic nerve head and set to 5x averaging. All follow-up scans were registered to baseline during acquisition. OCT/A data were exported to custom software for segmentation and measurements. Circumpapillary (cp) B-scans were then created by sampling 1536 A-lines along a 6° radius from the centroid of the manually delineated Bruch’s membrane opening. RNFL segmentation was done semiautomatically within each structural OCT cp B-scan, used to measure average RNFL thickness (RNFLT), then applied to the corresponding OCTA cp B-scan for assessment of RNFL vascular content (RNFLV), calculated in two ways: 1) average thickness with all pixels weighted by their flow probability signal (0-1); 2) average thickness for all pixels above a threshold flow probability of 0.3 (binarized).

Results : For all eyes and time points, the Pearson correlation between the two methods of measuring RNFLV was 0.998, so we report here only results of the threshold method. From the first observation after induction of EG, RNFLT declined from 113.5 ± 14.2 to 89.5 ± 11.8 µm, a relative loss of 20.2 ± 13.1% (p=0.001). RNFLV declined from 17.5 ± 4.0 to 13.0 ± 2.8 µm, a relative loss of 22.5 ± 21.7% (p=0.01). The non-vascular portion of the RNFL (RNFLT – RNFLV) declined from 96.1 ± 11.2 to 76.5 ± 9.3 µm, a relative loss of -19.5 ± 12.3% (p=0.001). The vascular proportion of RNFL tissue declined from 15.3 ± 2.3% to 14.4 ± 1.7% (not significant, p=0.19). RNFLT declined at an average rate of -8.9 µm/month and was significantly faster than zero in 7/9 EG eyes. The non-vascular RNFL declined at an average rate of -6.9 µm/month, significant in the same 7/9 EG eyes. RNFLV declined at -2.0 µm/month on average (significant in 5/9 EG eyes). The proportion of RNFL that was vascular declined at a significant rate in only 1 EG eye.

Conclusions : Loss of vasculature occurs in proportion to overall and non-vascular RNFL tissue loss but accounts for only a small absolute change in RNFLT during early stages of experimental glaucoma.

This is a 2020 ARVO Annual Meeting abstract.