Dissociation between electrical and mechanical bulbocavernosus reflexes

To record reflex motor responses, elicited by mechanical stimulation of the penis or clitoris, in each bulbocavernosus muscles and to compare left and right reflex pathways in normal subjects, then to compare electrical and mechanical responses in various neurological diseases.

Two groups of patients were studied: 22 patients without neurological disease considered as normal subjects; and 25 patients with neurological disease (three multiple sclerosis, six spina bifida, nine conus medullaris syndrome, three peripheral neuropathies, two lumbosacral lesions, one multisystem atrophy, and one syringomyelia). Electrical bulbocavernosus reflex (EBCR) was evoked by orthodromic stimulation of the dorsal nerve of the penis at the penile base or the clitoris. Mechanical bulbocavernosus reflex (MBCR) was elicited with an electromechanical hammer, tapping directly on the clitoris zone or on the ventral part of glans penis. For EBCR and MBCR, bulbocavernosus muscle contractions were successively recorded in the left and in the right side with a needle inserted under visual guidance.

Mean left mechanical latency was 31.7 msec (SD = 4.5) and right one 31.6 msec (SD = 3.8). The reproducibility of the responses was excellent (P < 0.0001). ). The mean difference between left and right latencies was 2 msec (SD = 1.2). In the neurological group, 22 EBCR (six right, nine left, seven bilateral) and 19 MBCR (eight right, nine left, two bilateral) were considered abnormal. The mean reflex latencies in patients with neurological lesions (lower motor neuron lesions) were statistically longer (P < 0.0001) than in normal subjects. Exact concordance (side of lesion) between MBCR and EBCR was observed in 15/25 cases (60%), poor concordance (presence of a sacral reflex alteration) in 3/25 (12%) cases, and in 7/25 (28%) cases there was a significant difference between the two techniques.

MBCR may provide a good alternative for electrical stimulation and can be used to evaluate urinary disorders when a neurological etiology is suspected. However, the presence of false negatives with MBCR suggest that it may be more useful as a screening test.