Epidemiological studies of the uncommon disorder neuromyelitis optica spectrum disorder (NMOSD) may be difficult to interpret because of the evolving nature of diagnostic criteria, differences in the definition and accuracy of NMOSD diagnosis, the completeness of case ascertainment, and variability in assays for the disease‐specific biomarker aquaporin‐4 (AQP4)‐IgG. Here, AQP4 is linked to neuromyelitis optica.