Neuromyelitis optica spectrum disorders (NMOSD), including neuromyelitis optica (NMO), are autoimmune diseases 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 against 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 in the past wrongly considered a clinical variant of MS. However, NMO is not caused by MS in the vast majority of cases and differs from MS substantially in terms of pathogenesis, clinical presentation, magnetic resonance imaging, cerebrospinal fluid findings, disease course, and prognosis. Signs and symptoms The signs and symptoms of NMOSD depend on the neurologic structures the disease affects, and, to some extent, the antibodies involved. Signs and symptoms usually follow a relapsing and remitting course, but occasionally can be progressive (monophasic). Deficits can be temporary or permanent, the latter especially in the absence of treatment. The most common initial manifestation of the disease is inflammation of the spinal cord (myelitis). Myelitis causes spinal cord dysfunction, which can result in muscle weakness, paralysis in the limbs, lost or reduced sensation, spasms, loss of bladder and bowel control, or erectile dysfunction. The myelitis can be transverse, affecting an entire cross-section of the spinal cord, and showing bilateral symptoms. The second most common initial manifestation of the disease is inflammation of the optic nerve and/or optic chiasm (optic neuritis, ON). ON may lead to varying degrees of visual impairment with decreased visual acuity, although visual field defects, or loss of color vision, may occur in isolation or prior to formal loss of visual acuity. Compared to idiopathic ON and ON due to multiple sclerosis (MS), ON due to NMOSD more often results