AQP4 and neuromyelitis optica: Neuromyelitis optica spectrum disorder (Devic’s disease) is an uncommon autoimmune disease of the eye and central nervous system.16 In 2004, antibody against AQP4, an astrocyte water channel, was identified as the main pathogenic mechanism.17 Approximately one-quarter of cases remain antibody-negative despite repeated testing, implicating other as-of-yet unidentified antigens or pathogenic mechanisms.