The effect of stimulus rate upon common peroneal, posterior tibial, and sural nerve somatosensory evoked potentials

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We examined the effect of stimulus rate on somatosensory evoked potentials (SEPs) following stimulation of the common peroneal nerve (CPN) at the knee, and the posterior tibial nerve (PTN) and sural nerve (SN) at the ankle. We measured the amplitude of P40-N50 and N50-P60 in the PTN-SEP and corresponding amplitude of CPN-SEP and SN-SEP at the rate of 2.3, 3.4, 4.1, and 5.1 Hz. When the stimulation rate was increased from 2.3 to 5.1 Hz, the P40-N50 amplitude decreased by 50% for the CPN-SEP and 20% for the PTN-SEP. Also, the N50-P60 amplitude was reduced by 30% in the CPN-SEP and 20% in the PTN-SEP. In contrast, this change in stimulus rate produced no significant amplitude decline in the SN-SEP. Blocking the peroneal nerve with lidocaine just distal to the stimulating electrodes eliminated the descending peroneal nerve volley and abolished the amplitude attenuation observed with the faster stimulus rate. The findings suggest that at higher rates of stimulation, the afferent volleys induced by the movements that follow mixed nerve stimulation interfere with the SEP produced by electrical activation of the sensory afferents. The interference is greater when the more proximal site of the mixed nerve is stimulated.