A combination of the following search terms was used: “neuromyelitis optica,” “NMO,” “NMOSD,” “aquaporin 4,” “AQP4,” “myelin oligodendrocyte glycoprotein,” “MOG,” “optico-spinal multiple sclerosis,” “OSMS,” “idiopathic inflammatory demyelinating disease,” “IIDD,” “epidemiology,” “prevalence,” “population,” and “demographic.” The reference lists in published articles on NMOSD were also queried to identify further studies. This evidence concerns the gene OMG and neuromyelitis optica.