Motor evoked potentials from the striated urethral sphincter and puborectal muscle: normative values

Transcranial magnetic stimulation enables the study of corticospinal motor pathways through evoked potential testing. By separate stimulation of cortical centers and spinal roots, it is possible to test motor conduction times segmentally. This method enables the localization of central or peripheral demyelinating neuropathies. As lower urinary tract dysfunction often accompanies these diseases, we wanted to present normative data for the motor innervation of the external urethral sphincter (EUS) and the puborectal muscle (PR).

Thirty healthy women without evidence of neurologic or lower urinary diseases were studied. They had a mean age of 52 years. Transcutaneous magnetic stimulations were applied to the vertex, the upper lumbar spine, and the sacrum. Evoked potentials were recorded with concentric needle electrodes placed in the EUS and the PR by a transvaginal route. Stimulations were performed with relaxed pelvic floor and with tonic contraction (facilitated responses). At least three responses were recorded for each modality. Stepwise multiple regression analysis was performed to test for the effect of age, height, body mass index, vaginal parity, and menopause.

In some subjects, it was only possible to record cortical latencies with facilitation. Technical difficulties were encountered with spinal stimulations. Reference values are presented as means with 95% confidence intervals. Regression analysis revealed height as a dependent factor for sacral latencies.

Our data can serve as reference values for future studies of the fast conducting fibers in patients with sphincter dysfunction. The height of the patient should be taken into account. The clinical applicability of the method is questionable.