Axon Content of the Circumpapillary Nerve Fiber Layer in Experimental Glaucoma

To determine axonal content from optical coherence tomography (OCT)-derived circumpapillary retinal nerve fiber layer (RNFL) measures using histologically informed models in a non-human primate experimental glaucoma model.

Unilateral experimental glaucoma was induced in 22 animals (mean age, 6.1 ± 1.9 years; 10 females) who were monitored with OCT. At varying stages of neuropathy, animals were euthanized and perfusion fixed. For RNFL histology, circumpapillary tissue (∼0.75 × 0.75 mm) from six equally spaced regions was obtained and processed for serial block face scanning electron microscopy in five animals. A single high-resolution image of the block face was used to quantify axon size (cross-sectional area) and non-axonal tissue area within the RNFL. Using the 95% limits of the histological data as a starting point, models for estimating axon count from OCT RNFL area were optimized and compared to total retrobulbar optic nerve axon counts in 22 animals.

The mean circumpapillary RNFL thickness of control eyes was 112 ± 6 µm, and it was 70 ± 24 µm in eyes with experimental glaucoma. Axon size and non-axonal area varied across the circumpapillary regions (F5, 31 = 9.06 and F5, 36 = 3.94, respectively; both P < 0.01), but did not differ between the two eyes (F1, 22 = 0.59, P = 0.45 and F1, 15 = 2.10, P = 0.17). The optimized model for estimating axon content from the circumpapillary RNFL profile had good agreement with the retrobulbar axon count (mean difference = 25; 95% limits of agreement, -334,239 to 334,289).

The axon size distribution and non-axonal area in the circumpapillary RNFL are similar between healthy and stable glaucomatous eyes. Building on this, a location-specific model is proposed that estimates axon content from the circumpapillary RNFL profile after accounting for regional variability in axonal and non-axonal components.