Acute myelitis is a neurological entity, often posing the problem of etiology. The two main causes are infectious or autoimmune. French Guyana is a region of the world where infectious etiologies are multiple, in particular human immunodeficiency virus infection, and autoimmune diseases appear to be emerging. The aim of our study was firstly to determine the proportion of each etiology of acute myelitis in French Guyana, and secondly to describe the clinical, paraclinical and epidemiological characteristics of autoimmune myelitis in French Guyana, with particular reference to neuromyelitis optica spectrum disorder (NMOSD).

This retrospective, observational study included all patients who presented with acute myelitis between January 2015 and August 2023 at Cayenne Hospital Center. Each patient's chart was reviewed and patients were classified according to etiology. Demographic and clinical data were collected, as well as blood, lumbar puncture, and cerebral and spinal cord magnetic resonance imaging results.

Of the 40 patients included, immune etiology was found in 74%, including 49% with NMOSD (37% with positive anti-aquaporin-4 antibodies) compared with three patients with infectious etiology. There was no statistically significant difference in complementary examinations between immune and infectious etiologies. The prevalence of NMOSD in French Guyana was estimated at 8/100,000 (6/100,000 for patients with positive anti-aquaporin-4 antibodies). No significant difference in the geographic distribution of patients with NMOSD in French Guyana was demonstrated.

Our results show a high proportion of autoimmune etiology of acute myelitis in French Guyana, and more particularly of NMOSD. There is a high prevalence of NMOSD, the second highest in the world after the French West Indies. Given this high proportion of autoimmune myelitis, several hypotheses can be put forward, with genetic and environmental factors in the foreground. For patients with acute myelitis in French Guyana, an immune cause is the most likely. It is therefore important to think about this and look for NMOSD in particular, without ignoring an infectious etiology.