AQP4 and optic neuritis: The discovery of autoantibodies directed against the aquaporin-4 (AQP4) astrocyte water channels led to the concept of Neuromyelitis Optica Spectrum Disorders (NMOSD) for seropositive patients, in whom typical NMO, isolated optic neuritis (ON), or inaugural longitudinally extensive transverse myelitis (LETM) could coexist with or even be preceded by other clinical presentations (3).