Neuromyelitis optica spectrum disorders (NMOSD) is an etiologically heterogeneous syndrome predominantly characterized by acute inflammation of the optic nerve (optic neuritis, ON) and the spinal cord (myelitis). Episodes of ON and myelitis can be simultaneous or successive. A relapsing disease course is common, especially in untreated patients. In more than 80% of cases, NMO is caused by immunoglobulin G autoantibodies to aquaporin 4 (anti-AQP4), the most abundant water channel protein in the central nervous system. A subset of anti-AQP4-negative cases is associated with antibodies to myelin oligodendrocyte glycoprotein (anti-MOG). Rarely, NMO may occur in the context of other autoimmune diseases (e.g. connective tissue disorders, paraneoplastic syndromes) or infectious diseases. In some cases, the etiology remains unknown (idiopathic NMO).
Multiple sclerosis (MS) and NMO can be similar in clinical and radiological presentation, and MS may very rarely present with an NMO-like phenotype (e.g. in patients with long-standing MS resulting in confluent spinal cord lesions mimicking the long spinal cord lesions typically seen in MS). In consequence, NMO was wrongly considered a clinical variant of MS in the past. However, NMO is not caused by MS in the vast majority of cases but differs from MS substantially in terms of pathogenesis, clinical presentation, magnetic resonance imaging, cerebrospinal fluid findings, disease course and prognosis.