Abstract: Six crossover trials and two parallel group trials were included. Six trials assessed the effects of SNS for FI. In the parallel group trial conducted by Tjandra, 53 participants with severe FI in the SNS group experienced fewer episodes of faecal incontinence compared to the control group who received optimal medical therapy (mean difference (MD) −5.20, 95% confidence interval (CI) −9.15 to −1.25 at 3 months; MD −6.30, 95% CI −10.34 to −2.26 at 12 months). Adverse events were reported in a proportion of participants: pain at implant site (6%), seroma (2%) and excessive tingling in the vaginal region (9%). In the parallel group trial carried out by Thin, 15 participants with FI in the SNS group experienced fewer episodes of FI compared with the percutaneous tibial nerve stimulation (PTNS) group (MD −3.00, 95% CI −6.61 to 0.61 at 3 months; MD −3.20, 95% CI −7.14 to 0.74 at 12 months). Adverse events were reported in three participants: mild ipsilateral leg pain during temporary testing (n = 1); and stimulator-site pain following insertion of neurostimulator (n = 2). In the crossover trial by Leroi 7 of 34 recruited participants were excluded from the crossover due mainly to complications or immediate device failure. Twenty-four of the remaining 27 participants while still blinded chose the period of stimulation they had preferred. Outcomes were reported separately for 19 participants who preferred the 'on' and five who preferred the 'off' period. For the group of 19, the median (range) episodes of faecal incontinence per week fell from 1.7 (0 to 9) during the 'off' period to 0.7 (0 to 5) during the 'on' period; for the group of five, however, the median (range) rose from 1.7 (0 to 11) during the 'off' period compared with 3.7 (0 to 11) during the 'on' period. Four of 27 participants experienced an adverse event resulting in removal of the stimulator. In the crossover trial by Sørensen and colleagues, participants did not experience any FI episodes in either the one-week ‘on’ or ‘off’ periods. In the crossover trial by Vaizey, participants reported an average of six, and one, episodes of faecal incontinence per week during the 'off' and 'on' periods respectively in two participants with FI. Neither study reported adverse events. In the crossover trial by Kahlke, 14 participants with FI experienced significantly lower episodes of FI per week during the stimulator 'on' (1 (SD, 1.7)) compared with the 'off' period (8.4 (SD, 8.7)). Adverse events reported include: haematoma formation (n = 3); misplacement of tined lead (1); and pain at stimulator site (n = 1). Two trials assessed SNS for constipation. In the Kenefick trial, the two participants experienced an average of two bowel movements per week during the 'off' crossover period, compared with five during the 'on' period. Abdominal pain and bloating occurred 79% of the time during the 'off' period compared with 33% during the 'on' period. No adverse events occurred. In contrast, in the trial by Dinning with 59 participants, SNS did not improve frequency of bowel movements and 73 adverse events were reported, which included pain at site of the implanted pulse generator (32), wound infection (12), and urological (17) events. The limited evidence from the included trials suggests that SNS can improve continence in a proportion of patients with faecal incontinence. However, SNS did not improve symptoms in patients with constipation. In addition, adverse events occurred in some patients where these were reported. Rigorous high quality randomised trials are needed to allow the effects of SNS for these conditions to be assessed with more certainty.

Plain-language summary: This review evaluated the published evidence for the use of SNS for patients with faecal incontinence or constipation from six trials of SNS for faecal incontinence (219 participants) and two trials of SNS for constipation (61 participants). Two of the faecal incontinence trials had a 'parallel group design', which means that one group of participants received SNS and the other control group did not receive SNS throughout the trial. The remaining six trials had a 'crossover design', in which the participants experienced equal periods with stimulation 'off' then 'on', or vice versa. The level of stimulation was such that participants could not tell whether the system was 'on' or 'off'. In the two 'parallel group' trials, 53 and 15 participants with faecal incontinence who were in the SNS group experienced fewer episodes of faecal incontinence compared to the control group at 3 and 12 months. In the first crossover trial, 24 participants who completed the trial chose the period of stimulation they had preferred while still unaware whether this was 'on' or 'off'. Nineteen participants who preferred the 'on' period experienced 59% fewer episodes of FI per week during the 'on' period, and 5 participants who preferred the 'off' period experienced 118% more episodes of FI per week. In the second crossover trial, the participants did not experience episodes of FI during either the 'on' or the 'off' periods. In the third trial, participants experienced 83% fewer episodes of faecal incontinence during the 'on' compared with the 'off' period. In the fourth crossover trial participants experienced 88% fewer episodes of faecal incontinence during the 'on' period compared with the 'off' period. —adverse effects:Not all trials reported adverse effects after SNS. The two 'parallel group' trials reported only minor complications, in 10% of SNS participants in the first study, and in 3 participants in the second study. In the first crossover study 7 out of 34 participants were excluded from crossover due mainly to complications. Four out of 27 participants with an implanted system in this study experienced a problem that led to the device being removed. The participants in the fourth crossover trial experienced some complications with the SNS implanted electrode such as pain (one person), misplacement of the tined lead (one person) and haematoma (swelling containing blood) (three people). In one trial assessing SNS for constipation, two participants reported an increase of 150% in the frequency of passing stools per week, and time with abdominal pain and swelling went down from 79% during the 'off' period to 33% during the 'on' period. However, in the much larger second trial assessing SNS for constipation, in 59 participants SNS did not improve frequency of bowel movements. The limited evidence suggests that SNS can improve continence in some people with faecal incontinence. SNS did not improve symptoms in patients with constipation. Larger, good-quality trials are needed to provide more reliable evidence on the effectiveness of SNS for these two conditions.