Mixed-muscle electrode placement ("jumping" muscles) may produce false-negative results when using transcranial motor evoked potentials to detect an isolated nerve root injury in a porcine model.

INTRODUCTION: Placing EMG electrode pairs that span several muscles is sometimes used to enhance the efficacy of electromyographic recordings. This technique, often referred to as "jumping," has not been studied when using Motor Evoked Potentials (TcMEP) for detecting isolated spinal nerve root injury during spine surgery.
METHODS: TcMEPs were obtained in seven pigs under general anesthesia. One pair of recording electrodes was placed entirely within the tibialis anterior (TA) muscle; a second pair had one lead in the TA and the other in the gastrocnemius muscle (TA-GAS). The dominant root innervating the TA was determined using evoked EMG. MEP amplitudes recorded by the TA and TA-GAS electrodes were compared before and after suture ligation of this root in 12 separate experiments.
RESULTS: Mean baseline TcMEP amplitude was not significantly different for the TA vs. TA-GAS. After root ligation, mean amplitude dropped from baseline by 72 +/- 13% in the TA vs. 50 +/- 29% in the TA-GAS (p < 0.01). All amplitudes decreased by >50% in the TA group; half of the TA-GAS group had <50% decrease in amplitude. DISCUSSION: Mixed-myotomal recording electrodes did not consistently increase baseline TcMEP amplitude. The decrease in amplitude after ligation was both smaller and more variable in the "jumped" TA-GAS electrodes. Thus, this technique may allow someone relying on TcMEP monitoring to miss an otherwise detectable isolated nerve root injury (i.e., have a false-negative result).