Neural generators of N18 and P14 far-field somatosensory evoked potentials studied in patients with lesion of thalamus or thalamo-cortical radiations

Abstract

Somatosensory evoked potentials (SEPs) to electrical stimulation of the right or left median nerve were studied in 4 patients with hemianesthesia and a severe thalamic or suprathalamic vascular lesion on one side. The SEPs were recorded with a non-cephalic reference. The normal side of each patient served as his or her own control. The lesion consistently abolished the parietal N20-P27-P45 and the prerolandic P22-N30 SEP components. It did not significantly affect the P9-P11-P14 positive far fields, nor the widespread bilateral N18 SEP component. This allowed N18 features to be studied without interference from cortical components. It is proposed that N18 reflects several deeply located generators in brain stem and/or thalamus whereas N20 represents the earliest cortical response of the contralateral post-central receiving areas.

Abnormalities of somatosensory and motor evoked potentials in adrenomyeloneuropathy: Comparison with magnetic resonance imaging and clinical findings

“…On the other hand, the mean P14-N20 interval was significantly greater in patients than in normals. Since abnormalities of this interval have been correlated with various types of intracranial somatosensory lesion, 19 this finding suggests that the intracranial tracts of the somatosensory pathways were functionally affected in the group of patients, although this abnormality did not reach diagnostic significance in individuals. This finding could be explained by dying-back degeneration, which probably occurs later and less severely in intracranial fibers.…”

N18 in median somatosensory evoked potentials: a new indicator of medullary function useful for the diagnosis of brain death

“…The N18 potential was first described by Desmedt and Cheron11 as a long lasting negativity widely distributed over the scalp, and its generator has long been attributed to brain stem nuclei receiving collaterals from the medial lemniscus, especially the tectal nuclei at the midbrain 2526 However, we have shown that N18 was preserved in normal size and shape for patients with medial lemniscal lesions at pontine and medullary levels12-15 whereas it was lost in patients with C1–2 dorsal column lesions 14…”

Brain Death

“…Desmedt and Cheron 458 suggested that N18 probably reflected subcortical activities such as delayed potentials in the thalamus itself, or action potentials in the lower part of thalamocortical axons. For Mauguière et al, 459 N18 is not a unitary phenomenon, but reflects several deeply located generators on the brainstem or thalamus. Urasaki et al 460 suggested that N18 comes from brainstem activity between the upper pons and the midbrain rather than from the thalamus, and Sonoo et al 455,456 concluded that at least a significant part of the N18 potential is generated caudal to the pontine level (probably in the medulla oblongata), or at higher levels via extralemniscal ways.…”