AQP4 and optic neuritis: Of note, NMOSD encompasses a group of syndromes that share optic neuritis and/or acute myelitis as their manifestation, but the patients can be seropositive or seronegative for AQP4-IgG, supporting the heterogeneity of the conditions included in NMOSD (Papadopoulos and Verkman, 2012), and particularly in AQP4-IgG-negative NMOSD (Wingerchuk et al., 2015).