AQP4 and neuromyelitis optica: A considerable advancement in the understanding of those disorders was the identification of pathogenic autoantibodies against aquaporin‐4 (anti‐AQP4‐IgG) in patients with NMO, which allowed for the establishment of NMO as a distinct nosological entity (Fujihara & Misu, 2015; Jurynczyk et al., 2017; Prain et al., 2019).