AQP4 and neuromyelitis optica: Absolute criteria include presence of optic neuritis and acute myelitis, and supportive criteria negative brain magnetic resonance imaging (MRI) at disease onset, spinal cord MRI with contiguous T2-weighted signal abnormality based centrally in the cord and extending over 3 or more vertebral segments (LETM - longitudinally extensive transverse myelitis), and NMO-IgG (aquaporin-4 IgG) seropositive status [1,2].