Distribution of brainstem somatosensory evoked potentials following upper and lower limb stimulation

Objective: Because somatosensory evoked potentials (SEP) to lower limb stimulation have not been recorded from the brainstem to the extent that upper limb SEPs have been studied, we compared brainstem recordings in response to both median nerve (MN) and posterior tibial nerve (PTN)stimulation. Methods: SEPs were recorded directly from the dorsal surface of the brainstem in four patients with fourth ventricle tumors. Results: Following MN stimulation, medullary SEPs were characterized by a major negativity (N1) preceded by a small positivity (P1) and followed by a large positivity (P2). In the pons, triphasic waves with predominant negativity were obtained. With PTN stimulation, similar medullary SEPs with a P1′-N1′-P2′ configuration and pontine SEPs with a triphasic waveform were obtained. Conclusions: Since the distribution of PTN SEP was identical to that of MN SEP, PTN SEPs are thought to be generated by mechanisms similar to those for MN SEP. Thus, the P1′ and N1′ of medullary SEP would be generated by the dorsal column fibers that terminate in the nucleus, with P2′ possibly arising postsynaptically in the nucleus. The triphasic PTN SEP from the pons reflects an axonal potential generated in the medial lemniscal pathway.