The effect of sacral nerve modulation on cerebral evoked potential latency in fecal incontinence and constipation.

Objective: Influence of sacral nerve modulation (SNM) on cerebral somatosensory evoked potentials (SEP) was determined in patients with incontinence and constipation.

Background: Selection of patients with incontinence and constipation for SNM could be improved.

Methods: The latency (ms) of SEP induced by pudendal nerve stimulation was compared before (T0) and at 1 month during peripheral nerve evaluation (PNE) of SNM at frequencies of 21 Hz (T1) and 40 Hz (T2). The results were correlated with clinical outcome at 6 months.

Results: In 16 of 23 incontinent patients with clinical "success" from SNM (Wexner incontinence score ≤7), there was a significant difference between P40 latency at T0 and T2 (38.81 vs. 37.49 ms, P = 0.049). In the 7 with "failure," there was no change between T0 and T2. In 12 of 19 constipated patients with "success" (Wexner constipation score ≤15), there was no difference between T0 and T2 P40 latency (39.28 vs. 38.25 ms, P = 0.374). In the 7 with "failure," there was a significant fall in P40 latency (41.20 vs. 39.30 ms, P = 0.047) but not to the normal range. The T0 P40 latency in incontinent patients having "success" was significantly higher than in the normal range (P = 0.044). In constipated patients it was significantly higher than in the normal range in both those with "success" (P = 0.001) and "failure" (P = 0.022).

Conclusions: Measurement of P40 latency of SEP at baseline and at 1 month of SNM at a frequency of 40 Hz may help to predict the outcome of SNM and thus influence the decision for permanent implantation for patients with incontinence and constipation.