AQP4 and optic neuritis: Neuromyelitis optica (NMO) is an inflammatory disease associated with recurrent episodes of optic neuritis (ON) and longitudinally extensive transverse myelitis (LETM), leading to severe loss of visual and motor function.1 A specific feature of NMO is the presence of an autoantibody against aquaporin 4 (AQP4), which is densely expressed in the astrocytic foot processes; recent studies have demonstrated that anti‐AQP4 antibodies have a pathogenic role.2