AQP4 and optic neuritis: The serum identification of AQP4-IgG expanded the clinical spectrum of the disease to include its limited forms (single or recurrent longitudinally extensive transverse myelitis [LETM], defined by MRI as a lesion extending for three or more vertebral segments, or recurrent isolated optic neuritis) [33], along with a wide variety of brainstem, diencephalic, and cerebral manifestations [34,35].