AQP4 and neuromyelitis optica: In addition, among their 21 NMO patients seropositive for AQP4 autoantibodies by cell-based IIFA, patients seropositive for NMO-IgG by tissue-based IIFA (15 patients) had a higher frequency of longitudinally extensive spinal cord lesions on MRI than patients seronegative for NMO-IgG by tissue-based IIFA (6 patients), 100% versus 50% (p = 0.015) [34]; this was not observed in our patients.