In vivo structural and functional assessment of optic nerve damage in neuromyelitis optica spectrum disorders and multiple sclerosis.

Early detection of neuromyelitis optica spectrum disorders (NMOSD), especially after optic neuritis, a presenting manifestation commonly observed also in multiple sclerosis (MS), is crucial for timely treatment and prognosis. Integrated visual pathway assessment with optical coherence tomography (OCT) and visual evoked potentials (VEP) may help in this task, showing in vivo different pathophysiological backgrounds. We evaluated combined VEP and OCT in a cross-sectional, single-centre study assessing 50 consecutive NMOSD patients, 57 MS patients and 52 healthy controls. After optic neuritis, VEP were more frequently absent in NMOSD compared to MS; most NMOSD eyes with recordable VEP showed prolonged latency, but extreme latency delays were less common than in MS. OCT showed predominantly axonal involvement in NMOSD, with 88% eyes (95% CI: 69-97%) displaying retinal nerve fibre layer thickness <60 µm even after first optic neuritis episode. Accuracy of OCT was further enhanced by combination with VEP into a new Z-score derived OCT-VEP index, measuring prevalence of axonal damage or demyelination. Our results suggest that integrated optic nerve assessment may elucidate differences in optic neuritis pathophysiology; conduction slowing with relatively preserved nerve fibre layer suggests MS, while severe neuroaxonal loss after optic neuritis, often hindering VEP response, characterizes NMOSD.