Cortical localization of external urethral sphincter activation by transcranial electrical stimulation

Repetitive transcranial electrical stimulation (rTES) was used to activate descending output to the external urethral sphincter muscle.

Motor evoked potentials (MEPs) were recorded from external urethral sphincter (EUS), and anterior tibial (TA) muscles following high voltage rTES in 9 consecutive patients undergoing spine surgery. Anesthesia was achieved by continuous propofol/narcotic infusion without paralytic agents. Anodal cortical stimulation was delivered at C4/C3, C2/C1, and Cz/Fz locations in each patient. Latency and amplitude of the MEP was measured and compared for each bipolar stimulation montage.

The mean latency was 20.24 +/- 1.3 msec. for Cz/Fz; 20.19 +/- 1.1 msec. for C4/C3 and 20.19 +/- 1.1 msec. for C2/C1. Statistical analysis showed no significant difference in latency between the three sites (F(2,15) = 0.004; p > 0.05). The mean amplitude was 37.14 +/- 24.3 microV for Cz/Fz; 113.33 +/- 100.6 microV for C4/C3; and 85 +/- 73.9 microV for C2/C1. A significant difference between the amplitudes at three sites was observed (F(2,8) = 5.2; p < 0.05). The amplitude at C4/C3 was significantly greater than amplitude at CzlFz (t (8) = 3.08; p < 0.05), but data did not give enough evidence to believe that difference between amplitudes for site C4/C3 & C2/C1 was significant (p > 0.05).

This study shows that the intraoperative MEP monitoring of the EUS is a feasible method. Furthermore, activation of descending axonal outflow to the EUS muscle is best achieved by cortical stimulation directed from C4 to C3 or C2 to C1 points.