In the SNS studies reviewed, 55–100 % percent of patients had a successful peripheral nerve evaluation (PNE) test as defined by >50 % improvement in FI severity during the testing phase. A successful PNE test is highly predictive of a successful permanent implant. Preoperative anal physiology testing and ultrasonography do not appear to be predictive of SNS success for the management of fecal incontinence [19]. Factors associated with failure of PNE testing phase include increased age [20, 21], defects in the external anal sphincter [20, 22], and repeated PNE attempts [20, 22]. However, if a PNE test was successful, the aforementioned factors were not associated with reduced success of a permanent implant [22].

Of the SNS studies reviewed, the majority were prospective case studies with only two randomized trials [23, 24]. Most studies were of moderate to low quality evidence by the GRADE approach [18] and were limited by the lack of a direct comparator. In a randomized double blind crossover trial, there was a significant improvement in frequency of episodes, symptom severity, and quality of life during the device ON versus device OFF phase, indicating that improvement was due to the device and not due to placebo [23]. When SNS was compared to optimal medical management in a randomized controlled trial, those treated with SNS had a statistically significant improvement in weekly fecal incontinence episodes (from 9.5 to 3.1) and an improvement in quality of life [24]. Further, 47.2 % of patients achieved perfect continence with SNS. In contrast, the optimal medical management group had no improvement in fecal incontinence, nor quality of life scores. Meurette et al. compared SNS to artificial bowel sphincter (ABS) and noted that the SNS had higher postoperative CCIS scores (9.4 vs. 4.7), but less constipation and a similar improvement in quality of life [25]. Additionally, there was no significant morbidity in the SNS group while 53 % of patients in the ABS group required further surgical revision due to mechanical failure or ulceration/erosion of the anal canal. Aside from these studies, there are no other direct comparative studies of SNS versus alternative therapies.

The success rates for SNS based upon an improvement of at least 50 % in FI severity following permanent implantation are shown in Table 38.2, in a per protocol analysis (success of patients who received a full-system implantation). Overall, 54–100 % of patients undergoing permanent implantation experienced a statistically significant greater than 50 % improvement of FI in all follow up stages. Perfect continence was achieved in 4–73 % of patients. Table 38.2 demonstrates an improvement in CCIS score across all follow up lengths. SNS therapy for FI was shown to be effective in studies with follow-up as long as 9 years [51, 78], though patients need ongoing follow up; many patients will need a battery change or lead revision over time [82].