AQP4 and neuromyelitis optica: Immunohistochemical analyses of archival brain, spinal cord, and optic nerve tissues obtained from patients with NMO have demonstrated a novel NMO lesion phenotype in the medullary floor of the fourth ventricle (including the area postrema), which exhibits loss of AQP4 and contains inflammatory cells, but lacks demyelination or necrosis [12, 13]. Binding of NMO-IgG to AQP4 in the peri-third/fourth ventricle areas may be less efficient at activating the complement system [11].