Mechanisms for closure of the human urethra during intravaginal electrical stimulation.

The effect of intravaginal electrical stimulation (IVS) on the urethral pressure profile (UPP) before and during succinylcholine blockade or spinal anaesthesia was studied in patients with stress incontinence and in patients with clinically normal urethral function. During succinylcholine blockade, the UPP was lowered to 74% and during spinal anaesthesia to 39% of the original maximal UPP. IVS could not influence the UPP during succinylcholine blockade, while the UPP could be restored during spinal anaesthesia when the stimulation strength was increased 3 to 4 times. The following conclusions were arrived at: 1) The urethral effect of IVS is due to activation of somatic nerves and not to activation of nerves supplying smooth muscles or direct activation of striated and smooth muscles. 2) The more pronounced depression of the UPP during spinal anaesthesia compared to succinylcholine blockade should be caused by the loss of nervous activity of the smooth muscles. 3) The fact that the UPP could be restored during spinal anaesthesia indicates that activation of somatic efferents can compensate for the loss of striated and smooth muscle activity. Therefore, IVS can control incontinence in partial lower motor neuron lesions, provided a sufficient number of efferent neurons are preserved.