Suboptimal outcome can be divided into two main categories:

Lack or loss of efficacy is one of the most common problems after implantation, and its incidence is estimated to be as high as 12 % when assessing available data concerning the use of SNS for fecal incontinence [4]. One of the potential reasons for early failure is suboptimal location of the permanent device (quadripolar foramen electrode, usually a tined lead). This is often prematurely interpreted as a problem inherent in non-staged techniques during PNE. In a ­non-staged procedure, a thin, monopolar electrode is used during PNE and a permanent electrode and stimulator are subsequently implanted as a separate procedure. This may result in the permanent electrode not being placed in a ­position identical to that which resulted in an initially good response during PNE. This problem may be avoided with a staged process in which a permanent electrode is used for PNE; if this is clinically beneficial, a second stage procedure will merely involve connecting the lead that is already in place for conversion to an implant, which would then be attached to an added stimulator. This latter technique is more costly and usually is limited to the placement of one electrode during PNE. However, no study has yet specifically addressed comparative clinical outcomes between these two approaches.

Another possible cause for lack of efficacy is instability of the permanent electrode in the postoperative period and stabilization by fibrosis. As a consequence, it can be difficult to find an optimal stimulation setting without uncomfortable side effects.

Eventual loss of efficacy is less well understood and likely due to multiple causes often acting together. Mechanical factors relating to the device, such as electrode migration, a broken fiber within the electrode, a loose connection, or surrounding fibrosis with a loss of conduction, are speculated to play a larger role in lack of effectiveness with time. Nerve injury or damage during prolonged stimulation may contribute to alterations in longer-term efficacy, but our current knowledge of variations in neurophysiological and neurochemical mechanisms involved in SNS is ­limited. Furthermore, any progressive neurological disease may account for an initially positive therapeutic effect and potential worsening of a preexisting condition over time which may be beyond the therapeutic benefit of SNS implantation.

Pain can be caused by the mechanical presence of the device or may be an adverse effect of stimulation. The estimated incidence is 13.0 % [4, 5]. Pain is most commonly reported at the site of the implanted stimulator [3]. The presence of the impulse generator itself can result in skin erosion, hematoma, cellulitis, local allergic reaction, seroma, or wound dehiscence. If the patient loses a significant amount of weight and a substantial amount of subcutaneous tissue, the device can protrude. Pain in other sites, including the perineum, leg, or foot, also has been reported and in most cases seems to be an adverse effect of stimulation.

The reported incidence of infection is 3.9 % [6, 7]; however, a large multicenter study of SNS in fecal incontinence reported a rate of 10.8 %, with most infections occurring within 3 weeks of device implantation [8]. Staphylococcus aureus is the most frequent causative bacterium, which may be detected on tined leads [9].

During SNS in fecal incontinence, adverse functional effects have been reported, including the need for deactivation during defecation and urination, respectively [10, 11]. Sleep disturbance may necessitate switching off stimulation at or during periods of increased sexual drive [12]. In a single report, one patient complained of a sensation of a minor electric shock when passing through an ambient electric or magnetic field (such as a store’s antitheft security system). The effect of SNS on pregnancy also is poorly understood or not well documented, for example, in one report a patient who had stimulation until 9 weeks of gestation prematurely delivered an infant with Down’s syndrome [13].