Increases in voltage may produce false-negatives when using transcranial motor evoked potentials to detect an isolated nerve root injury

Abstract

Transcranial Motor Evoked Potentials (TcMEPs) are sometimes used during lumbar spine surgery in order to detect and prevent an intraoperative nerve root injury. Typically, a fixed stimulus is applied, and one monitors for changes in response amplitude from several myotomes. Increased stimulating voltage may or may not alter the response after an acute injury. We suture ligated the dominant root innervating the tibialis anterior (TA) muscle in 7 experiments in swine monitored with TcMEPs. Injury to the root was confirmed by an increase in threshold current needed to evoke an EMG response in the TA (from 0.32 ± 0.10 to 2.3 ± 0.9 mA, P < 0.01). We recorded TcMEPs at baseline, after injury, and with incremental 25 V increases in stimulation up to 100 V. After ligation, mean TcMEP amplitude in the TA decreased by 56% from baseline (P < 0.01). Adding voltage progressively restored mean amplitude to within 17% of baseline, but with wide variability in the response. In 1 experiment, there was no augmentation; 3 studies showed partial improvement toward baseline; and in 3 studies, the amplitude was augmented to levels above baseline. An acute nerve root injury may be detected by TcMEP monitoring. However, if the stimulating voltage is increased after injury, the response may or may not be affected. In complex spine procedures, adjustments to TcMEP stimulating parameters are often needed to maintain reproducible responses. However, if these changes are made during a period where injury might occur, this could mask the changes and lead to a false-negative interpretation.