AQP4 and neuromyelitis optica: Overall, AQP4-IgG mediated complement activation was observed exclusively in 27 patients positive for AQP4-IgG with a median titer of 1:1,280 (ranging from 1:160 to 1:20,480; Table 3 and additional file 3), consisting of NMO, HR-NMO and one CIS patient.