AQP4 and neuromyelitis optica: In 2004, an antibody against aquaporin-4 immunoglobulin G (AQP4-IgG) was identified, culminating in the description of a new entity called AQP4-IgG-positive neuromyelitis optica spectrum disorder (AQP4-IgG NMOSD).1This antigen, related to water homeostasis, has a peculiar distribution within the CNS, is located in the astrocyte endfeet, and is abundant along the ependymal surface and in the circumventricular organs2and the blood‒brain barrier.3