AQP4 and optic neuritis: Neuromyelitis optica spectrum disorder (NMOSD) is an autoimmune disease of the central nervous system that is associated with serum aquaporin-4 antibodies (AQP4-IgG) directly against the AQP4 channels on the foot processes of astrocytes (1), clinically with serious optic neuritis (ON), longitudinally extensive transverse myelitis (LETM), and intractable nausea, vomiting, and hiccups induced by area postrema syndrome (APS) as the main manifestations (2).