AQP4 and neuromyelitis optica: Neuromyelitis Optica Spectrum Disorder (NMOSD) is an autoimmune disease that causes blindness and paralysis by attacking the aquaporin-4 (AQP4) water channel of astrocytes in the optic nerves and spinal cord.1 With an estimated prevalence of 0.5 to 10 cases per 100,000 people NMOSD is an uncommon condition.2 The immune system's induced secondary demyelination and astrocyte destruction cause acute episodes, which can cause paralysis, transverse myelitis, other neuropathological diseases, or even death.