AQP4 and neuromyelitis optica: These concepts were reflected by the evolution of the diagnostic criteria in 2006 (5) and 2015 (6), the latter emphasizing the importance of AQP4-IgG serostatus, and the adoption of the new nomenclature of “neuromyelitis optica spectrum disorder” (NMOSD) including both AQP4-IgG seropositive and seronegative cases.