A young woman presented with a 2-month history of progressive severe spastic paraparesis. An MR scan of spine showed a longitudinally-extensive myelopathy (figure 1). MR scan of the brain was normal. Cerebrospinal fluid showed elevated protein at 1.33 g/L, a normal white cell count, no malignant cells and normal glucose. An exhaustive biological blood screening was negative except for detection of antibodies targeting the M23 isoform on aquaporin-4 transfected cells.