Neuromyelitis optica (NMO, Devic's disease) is an autoimmune, inflammatory, demyelinating central nervous system disorder in which a person's own immune system attacks the optic nerves and spinal cord and is characterized by concurrence of optic neuritis and transverse myelitis, typically associated with a lesion in the spinal cord extending over three or more vertebral segments. Although it is most commonly relapsing, it is distinct from multiple sclerosis in that it is more severe, tends to spare the brain, and is associated with a longitudinally extensive lesion on spinal cord MRI. Furthermore, NMO is associated with a highly specific serum autoantibody marker, NMO-IgG, which targets the water channel aquaporin-4. The disease follows a relapsing course in more than 90% of patients.