AQP4 and neuromyelitis optica: The typically long spinal cord segmental involvement, longitudinally extensive transverse myelitis (LETM) and neutrophilic pleocytosis in the cerebrospinal fluid (CSF) became incorporated into the first Wingerchuk diagnostic criteria for NMO in 1999.1 Subsequently, the discovery of anti AQP4-IgG2 led to the revised Wingerchuk criteria in 2006.3 The term “NMO spectrum disorders”4 (NMOSD) encompasses forms of NMO that do not satisfy the 2006 criteria.