Neuromyelitis Optica Spectrum Disorder (NMOSD) is a rare relapsing autoimmune disorder that preferentially causes inflammation in the optic nerve and spinal cord. It is typically characterized by longitudinally extensive transverse myelitis (LETM, myelitis which is 3 vertebral segments in length or greater), which can leave one quite debilitated at presentation, and unilateral or bilateral optic neuritis. Individuals can present with short lesions as well. It was once thought of as a variant of Multiple Sclerosis (MS), and is still often times misdiagnosed as MS. However, several factors can differentiate it from MS: 1) it typically involves different parts of the brain than in MS, 2) the severity of attacks can be more robust as compared to MS, and 3) the pathophysiology differs from MS – whereas MS is thought to largely be a T-cell mediated disease, NMOSD is mediated by anti-aquaporin 4 antibodies. Blood testing includes an anti-aquaporin-4 antibody (NMO-IgG) test, which is highly specific (>99%) and its sensitivity ranges from 48-72%, depending on the assay used. Antibodies to Myelin Oligodendrocyte Glycoprotein or anti-MOG have been found in individuals diagnosed with NMOSD. Those with anti-MOG NMOSD tend to have attacks most often in the optic nerve, or optic neuritis (ON). Treatment for this disorder involves acute management with therapies, including IV methylprednisolone and plasma exchange (PLEX), and prevention of future attacks with immunosuppressants, including mycophenolate mofetil or rituximab, and aggressive rehabilitation.
Previously the disorder was split into two categories: Neuromyelitis Optica and Neuromyelitis Optica Spectrum Disorder, but The International Panel for NMO Diagnosis (IPND) released an updated set of guidelines in 2015 for diagnosing Neuromyelitis Optica (NMO) and Neuromyelitis Optica Spectrum Disorder (NMOSD), and these disorders are now simply called Neuromyelitis Optica Spectrum Disorder. This is because those with NMO and NMOSD generally do not have differing clinical behavior and should receive the same treatment.