AQP4 and neuromyelitis optica: Autoantibody-mediated autoimmune diseases comprise a broad spectrum that is rapidly evolving.1 Immunoglobulin G (IgG) antibodies against aquaporin-4 (AQP4-IgG) are found in most patients with neuromyelitis optica spectrum disorder (NMOSD), whereas antibodies against myelin oligodendrocyte glycoprotein (MOG-IgG) are associated with MOG-IgG–associated disease (MOGAD).2, -, 5 Both conditions are rare demyelinating disorders of the CNS, which have distinct clinical, neuroradiologic, neuropathologic, and pathophysiologic features than multiple sclerosis (MS).