AQP4 and neuromyelitis optica: Of course, we might have missed a few patients with NMO-SD, even though a highly sensitive and specific AQP4-ab immunoassay [10] was used, but this may be due to the fact (1) that AQP4-ab titres were below the detection threshold or (2) that AQP4-ab were presented as isoforms, which escaped detection, or (3) that the diagnosis of NMO/NMO-SD has yet not been established in few patients.