AQP4 and neuromyelitis optica: The discovery of NMO-IgG or AQP4-Ab has prompted revisions of the diagnostic criteria for NMO [7] and facilitated an appreciation for the wide spectrum of this disorder (NMO spectrum disorders (NMOSDs)), which includes a proportion of patients with recurrent, isolated, longitudinally extensive myelitis (LEM) or ON as well as patients with LEM or ON associated with systemic autoimmune disease or with brain lesions typical of NMO [8] (Table 1).