Purpose: Whether neuromodulation using an implanted sacral nerve stimulator acts by its effects on pelvic afferent or efferent nerves remains to be determined. However, it has been observed that eliciting an "anal wink" is helpful in the optimal siting of the foraminal stimulating electrode. This observation has been interpreted as indicating that a direct effect on efferent pelvic innervation is an important functional component of the technique. We studied the latency of this motor response to determine whether it is consistent with neuromodulation working via a direct efferent mechanism.
Materials and methods: We studied 9 women with urinary retention undergoing the first stage of a stimulator implant (peripheral nerve evaluation). Stimulation was applied to an electrode placed in the S3 foramen and the latency of the response of the striated anal sphincter, a contraction which gives rise to the "anal wink," was measured using a concentric needle electrode placed in the striated part of the anal sphincter.
Results: Mean latency of response was 98 milliseconds (range 52 to 140), which is approximately 10 times longer than would be expected from that resulting from direct motor nerve stimulation.
Conclusions: Our results indicate that anal sphincter contraction observed during peripheral nerve evaluation is the result of an afferent mediated response.