Intraoperative monitoring of the bulbocavernosus reflex: the method and its problems

This study evaluates the method of intraoperative neurophysiological monitoring of the bulbocavemosus reflex (BCR) with reference to stimulation and detection.

The study was performed on a group of 65 patients, 53 men and 12 women, who underwent surgery for lower thoracic and lumbar spinal trauma, with no neurological deficit prior to or following surgery. Stimulating electrodes were placed on the dorsum of the penis or the clitoris. Single and double stimuli were used, as well as trains of 3, 4 and 5 stimuli. Detecting wire electrodes were introduced into the perianal region.

A single stimulus elicited the BCR in 50%, pairs in 75%, trains of 3 in 95%, and trains of 4 and 5 in 100% of patients. With placement of the detecting electrodes 2.5 cm deep to the skin, the BCR was detected in 13% of hemisphincters in women, and in 81% of hemisphincters in men. With controlled intramuscular placement of detecting electrodes, the BCR was detected in 97% of hemisphincters in men. With bifocal detection the BCR amplitudes were 30-312 mV (median 90), and interside amplitude ratio was 0.05-1 (median 0.66); with monofocal detection, amplitudes and interside amplitude ratio were 30-560 mV (median 200) and 0.15-1 (median 0.86), respectively.

A train of 4 electrical stimuli is optimal in eliciting the BCR in anesthetized patients. The low rate of elicitability in women was most probably due to inefficient stimulation. Detection was improved by controlled intramuscular placement of electrodes. Monofocal detection yielded higher BCR amplitudes. Interside difference and interindividual variability of the BCR amplitude were considerable.