"Quadripolar" Transcranial Electrical Stimulation for Motor Evoked Potentials.

PURPOSE: To determine if transcranial electrical stimulation (TES)-induced motor evoked potentials (MEPs) are of higher amplitude when using two electrodes as anodes and two as cathodes, known as "quadripolar stimulation."

METHODS

Patients who underwent TES MEP monitoring in which control, bipolar stimulation and four variations of quadripolar stimulation were used were evaluated. The bipolar stimulation montage was C3-C4 (C3 was used as anode for stimulation first, then the polarity was switched to stimulate the contralateral side). Four quadripolar montages were used: C3/C1-C4/C2 (step 1), M3/M1-M4/M2 (step 2), C3/M1-C4/M2 (step 3), and M3/C1-M4/C2 (step 4). The area under the curve for the right foot TES MEP was compared for the various montages using descriptive statistics and Fisher exact test for proportions.

RESULTS

Sixteen patients were retrospectively evaluated. The mean age as 51.6 years, range 4 to 80 years; 11 were female. The transcranial electrical stimulation MEP area under the curve for the right foot MEP was highest in the bipolar montage in 1 of 16 patients (6.3%). Meanwhile, it was highest in step 4 (M3/C1-M4/C2) in 9 of 16 patients (56.3%; P = 0.027). The highest right foot MEP area under the curve with one of the quadripolar montages was seen in 15 of 16 patients (93.8%; P = 0.0001).

CONCLUSIONS

Quadripolar stimulation resulted in higher area under the curve for right foot MEP compared with conventional bipolar stimulation.