Somatosensory evoked potential from S1 nerve root stimulation

Abstract

The objective of this study was to detect cerebral potentials elicited by proximal stimulation of the first sacral (S1) nerve root at the S1 dorsal foramen and to investigate latency and amplitude of the first cerebral potential. Tibial nerve SEP and S1 nerve root SEP were obtained from 20 healthy subjects and 5 patients with unilateral sciatic nerve or tibial nerve injury. Stimulation of the S1 nerve root was performed by a needle electrode via the S1 dorsal foramen. Cerebral potentials were recorded twice to document reproducibility. Latencies and amplitudes of the first cerebral potentials were recorded. Reproducible cerebral evoked potentials were recorded and P20s were identified in 36 of 40 limbs in the healthy subjects. The mean latency of P20 was 19.8 ± 1.6 ms. The mean amplitude of P20-N30 was 1.2 ± 0.9 lV. In the five patients, P40 of tibial nerve SEP was absent, while well-defined cerebral potentials of S1 nerve root SEP were recorded and P20 was identified from the involved side. This method may be useful in detecting S1 nerve root lesion and other disorders affecting the proximal portions of somatosensory pathway. Combined with tibial nerve SEP, it may provide useful information for diagnosis of lesions affecting the peripheral nerve versus the central portion of somatosensory pathway.

An Initial Passive Phase That Limits the Time to Recover and Emphasizes the Role of Proprioceptive Information

“…Later, the flow of proprioceptive inputs gradually involved more proximal segments (e.g., pelvis, lumbar column, neck). A delay of 40 ms was required for afferent signals to reach the cortex ( 61 ) and 30 more milliseconds were needed to trigger EMG activities in lower-leg muscles ( 62 ). Since there was an additional 11 ms for electro-mechanical coupling ( 63 ), it would take at least 87 ms to generate forces at the ankle joint, based on proprioceptive information, which is close to the 90 ms latency of the first muscle activities observed here and in previous studies on recoverable falls ( 64 – 67 ).…”

H‐reflex and M‐wave recordings: effect of pressure application to the stimulation electrode on the assessment of evoked potentials and subject's discomfort

“…Second, the distance between the stimulation electrode and the posterior tibial nerve was also reduced with pressure. Because the current varies with the inverse of the square of the distance (Coulomb's law), a lower electrical stimulation intensity is required to evoke a similar potential (Wu et al, 2011) in conditions with pressure compared with NP condition. This finding is in line with previous studies in which the increased distance between the stimulation site and the nerve or muscle was associated with reduced stimulation efficacy.…”