Relationship Between Optic Nerve Head Area and Peripapillary Retinal Nerve Fiber Layer Thickness in Laboratory Beagles.

Objective: To determine if the area of the canine optic nerve head (ONH) is related to the peripapillary retinal nerve fiber layer (RNFL) thickness.

Animals studied: A total of 44 eyes of 22 adult normal laboratory Beagles.

Procedures: In this retrospective study, confocal scanning laser ophthalmoscopy (cSLO) and spectral domain optical coherence tomography (SD-OCT) images were evaluated. The sum of eight RNFL thickness measurements along the 12° circular OCT b-scan around the ONH was used to represent the total number of retinal ganglion cell (RGC) axons. The area of the ONH was measured by tracing the outer edges of the ONH in the cSLO images and adjusted for axial globe length. Descriptive and Pearson correlation statistics were used to describe the distribution of the ONH sizes and the relationship between total RNFL thickness and ONH area.

Results: The ONH area distribution for all eyes combined was approximately symmetric with an overall mean (± standard deviation) of 129 ± 35 degrees². The ONH areas between right and left eyes showed a strong, significant linear correlation (r = 0.83, p = 0.00). The mean sum of the eight RNFL thicknesses measured along the 12° circular OCT b-scan around the ONH was 448 ± 50 μm. There was a moderate, significant positive linear correlation between peripapillary RNFL thickness and ONH area (r = 0.43, p = 0.004).

Conclusions: In laboratory Beagles, the ONH area is partly affected by the peripapillary RNFL thickness, representing the total RGC axon number forming the optic nerve.