LETTERS

Time to next relapse as a primary endpoint in neuromyelitis optica clinical trials Neuromyelitis optica (NMO) and its spec-trum disorder (NMOSD) is a severe auto-immune inflammatory disorder of the central nervous system, with recurrent attacks mainly focused on the optic nerves and spinal cord. Untreated, it has a high morbidity and mortality. Serum aquaporin-4 (AQP4) antibodies are present in the majority of patients, are recognised as pathogenic and are a pre-dictor of further relapses.