Giant somatosensory evoked potentials in a patient with the anterior spinal artery syndrome.

We studied a previously healthy 25-year-old woman with the anterior spinal artery syndrome, a rare thoracocervical myelopathy with multiple potential etiologies. Quantitative and clinical sensory examination showed dissociated loss of pin-prick and temperature discrimination below the level of the lesion, with normal light touch, vibratory, and position sense. Magnetic resonance imaging was consistent with cervical spinal cord infarction. Median SEPs showed normal Erb's potential with absent spinal N13- and normal scalp N20- latency. Tibial SEPs showed normal lumbosacral responses and normal scalp P30- latency. Both median and tibial nerve stimulation produced cortical responses of unusually large amplitude (median 38 microV, tibial 17 microV). We hypothesize that large SEP amplitudes in this patient resulted from loss of anterolateral inhibitory influences on the dorsal column-medial lemniscal system.