Lower cervical origin of the P13-like potential in median SSEPS.

The authors studied the origin of the scalp P13-like potential in median somatosensory evoked potentials, which have been reported to be preserved in patients with cervicomedullary lesions or in brain death. There were five patients with high to middle cervical lesions (C2/3 or C3/4 level). Small P13-like potentials after P11 were identified for all patients with a noncephalic reference but not with an ear reference. Their onset latencies were slightly earlier than the expected latency of the true P13/14 onset. In two patients, delayed true P13/14s followed by N18s were identified with both noncephalic and ear references. The authors argue that the P13-like potential observed in these patients is a different entity from scalp P13 in normal subjects. Because the C3/4 vertebral level corresponds to the C5 cord level, the origin of the P13-like potential must be below C5, contradicting the previous opinion that it is generated at the cervicomedullary junction or at the high cervical dorsal column. The authors named this potential lower cervical P13 (or lcP13), and present an opinion that it is generated by the beginning of the second spinal ascending volley, which has been described by direct-recording studies in humans.