Short-latency somatosensory evoked potentials following median nerve stimulation in four patients with medial medullary infarction.

Somatosensory evoked potentials (SEPs) to median nerve stimulation were studied with noncephalic reference in 4 patients with unilateral or bilateral infarctions in the upper medulla to decide the contribution of scalp-recorded P14, N18 in detecting medial medullary lesions. The P9-P14 interpeak latencies did not change, while the P14-N18 and P14-N20 intervals tended to be slightly prolonged on the severely affected side. The amplitude ratios of P14 and N20, evaluated by dividing the amplitude on the severely affected side by that on the unaffected or less severely affected side, were significantly smaller than that of the cervical N13/P13 component. The amplitude ratios of N18 and N13/P13 did not differ significantly, and a preserved N18 with a depressed P14 in 2 cases indicates separate generators. A characteristic change of P14 in medial medullary infarction is that its amplitude is reduced without delay. Reduction of N18 indicates the presence of lesions in the lower brainstem; however, it is not necessarily associated with medial medullary lesions.