We aimed to evaluate the frequency of brain magnetic resonance imaging (MRI) lesions in acute optic neuritis (AON) associated with neuromyelitis optica spectrum disorder (NMOSD) at disease onset in clinical practice in a LATAM population.

Few studies regarding brain MRI lesions in AON associated with NMOSD have been published and no studies from Latin American (LATAM) cohorts were researched.

We retrospectively reviewed the medical records and brain MRIs (within 30 days of symptom) of patients with a first episode of AON associated with NMOSD in accordance with the 2015 diagnostic criteria. Patients from Argentina (=47), Brazil (n=22) and Venezuela (n=10) we included, and they were divided into two groups according to either presence (P-MRI) or absence (A-MRI) of optic nerve MRI lesions (high T2 signal and/or positive gadolinium). Clinical, paraclinical, imaging and prognosis data were compared.

Of 79 patients with AON associated with NMOSD, 54 (68.3%) were P-MRI at disease onset. Positive aquaporin-4 antibodies, measured by cell-based assay and indirect immunofluorescence, were found in 67%. This cohort had 43% of non-Caucasian population. There were no statistically significant differences in age, gender, mean time of follow-up, clinical course, ethnicity, frequency of aquaporin-4 and antinuclear antibodies, oligoclonal bands, disability, spinal cord MRI and type of treatment use in both groups. However, time between AON onset and immunosuppressant start was lower in P-MRI than in A-MRI (6.2 vs. 25.3 months, p=0.02). Brain MRI showed unilateral (53.7%), bilateral (44.4%) and chiasm (27.7%) lesions in P-MRI.

This study showed a lower frequency of P-MRI in patients with AON associated with NMOSD at disease onset. These findings confirm the importance of making a clinical diagnosis of AON without the need of confirmation by MRI and therefore to avoid treatment delay.