AQP4 and transverse myelitis: As such, testing for serum AQP4 antibodies should be considered in the following settings: (a) patients presenting with longitudinally extensive transverse myelitis (involving more than three vertebral levels); (b) patients with atypical optic neuritis (i.e., bilateral optic nerve or chiasmal involvement, long lesion of the optic nerve, poor recovery, or inadequate response to steroids); (c) patients with diencephalic syndrome and nonspecific MRI findings; (d) patients with unexplained encephalopathy; and (e) patients with area postrema syndrome (APS) [14].