Adult Equine Optic Nerve Head Measurements Using Spectral Domain Optical Coherence Tomography

PURPOSE: To compare equine optic nerve head (ONH) measurements using spectral domain optical coherence tomography (SD-OCT) to detect clinically relevant ONH cupping. METHODS: Forty-five eyes from 34 horses were categorized as CONTROL or CUPPED based on indirect ophthalmoscopy and were measured using SD-OCT: Bruch's membrane opening (BMO), optic cup width (OC), anterior laminar depth (ALD), prelaminar thickness (PLT), and cup-to-disk ratio (OC: BMO) at superior (sup.), central (ctr.), and inferior (inf.) ONH locations. RESULTS: The OC:BMO was 0.61 ± 0.09 (sup.), 0.64 ± 0.09 (ctr.), and 0.55 ± 0.10 (inf.) (CONTROL), and 0.60 ± 0.09 (sup.), 0.62 ± 0.08 (ctr.) and 0.54 ± 0.10 (inf.) (CUPPED). The ALD was 0.48 ± 0.20 mm (sup.), 0.50 ± 0.20 mm (ctr.), and 0.42 ± 0.18 mm (inf.) (CONTROL), and 0.49 ± 0.16 mm (sup.), 0.57 ± 0.20 mm (ctr.), and 0.46 ± 0.16 mm (inf.) (CUPPED). The PLT was 0.22 ± 0.08 mm (sup.), 0.17 ± 0.08 mm (ctr.), and 0.17 ± 0.05 mm (inf.) (CONTROL), and 0.21 ± 0.05 mm (sup.), 0.18 ± 0.04 mm (ctr.), and 0.18 ± 0.05 mm (inf.) (CUPPED). The ctr. BMO was significantly increased (mean ± SD of 4.19 ± 0.52 mm) compared to CONTROL (ctr. p = 0.007) eyes. CONCLUSIONS: Equine ONHs suggestive of clinical optic nerve head cupping had similar degrees of ONH depression to CONTROL eyes using SD-OCT. Clinical signs such as a prominent lamina cribrosa, ONH pallor, and retinal vascular attenuation may not be reliable predictors of equine ONH cupping. Further research is necessary to determine accurate clinical signs associated with ONH cupping.