Sacral nerve stimulation (SNS) is a widespread option in the treatment of defecation disorders (DDs) such as fecal incontinence (FI) and chronic constipation (CC). SNS consists in a peripheral nerve stimulation using a tined lead quadripolar electrode positioned in a caudo-lateral direction along the targeted sacral nerves, mainly the third sacral nerve. Routinely, the electrode position is checked by intraoperative antero-posterior and latero-lateral X-ray projections and specific responses to the electric stimuli given through the electrode. Generally, a trial-and-error approach is used to assess the clinical response to the therapy. However, the success rate may decrease over time and further reprogramming of the SNS settings are needed. It has been hypothesized that a reduction in SNS efficacy could be related to an incorrect position or displacement of the electrode. Recently, a new magnetic resonance imaging (MRI)-compatible SNS system was approved by FDA. The aim of this study was to assess the utility of postoperative MRI evaluation in patients submitted to SNS for DDs.

This is a prospective single-center observational study on consecutive patients undergoing definitive implant of MRI-compatible SNS system (Interstim II, Medtronic, USA) for DDs at a tertiary academic center from December 2022 to February 2024. Exclusion criteria were malignancies under treatment, acute anorectal sepsis, anorectal strictures, chronic diarrhea unresponsive to medical treatment, progressive neurological disease, pregnancy, inadequate response along the SNS stimulation test period and inability to use mobile devices. SNS implant was performed adopting a standardized technique, under local anesthesia and the guidance of intraoperative X-ray imaging and sensory/motor responses to the electrostimulation. After the definitive SNS implantation, patients underwent a routine MRI assessment of the electrode position using a specific time-controlled safety protocol with T2-weighted sequences. Patients’ baseline characteristics (age, sex, DDs etiology, any vaginal deliveries, obstetric anal sphincter injuries, comorbidities, previous anal or abdominal surgeries), SNS settings, intra- and postoperative complications were collected. FI severity was assessed collecting the number of FI episodes per week and the Cleveland Clinic Fecal Incontinence (CCFI) score, while symptoms of CC were evaluated using the Cleveland Clinic Constipation Scoring System (CCSS).

In total, 8 patients [6 females, 75%, median age 52.5 (IQR: 45.0-58.0) years] who underwent MRI-compatible SNS implant for DDs (6 for FI and 2 for CC) were included in the study. Among the FI group, 3 patients suffered from FI secondary to obstetric anal sphincter injuries (less than 30 degrees), other 2 patients reported persistent FI after lumbar herniated disc and rectal prolapse surgery, and 1 patient had idiopathic FI. One patient was affected by a neurological disorder after mumps also causing CC, while another patient reported persistent CC even after surgery for rectal prolapse. The standardized protocol of intraoperative stimulation and radiological projections documented an adequate electrode placement in all patients. No electrode damage or response reduction was reported after MRI assessment. After performing MRI, the explant of an SNS system due to loco-regional infection was necessary in one patient. Median follow up was 6.0 (IQR: 2.5-10.0) months. At last follow up, a significant reduction in mean FI episodes [5.0 (IQR: 4.0-10.7) vs. 1.0 (IQR: 0-3.5), p=0.046] and CCFI score [12.0 (IQR: 9.2-14.2) vs. 8.0 (IQR: 4.7-10.5), p=0.027] was reported in patients suffering from FI. However, in these patients, when evaluated with postoperative MRI, only 2 electrodes were detected in the correct position. Moreover, one electrode was placed too medial to the sacral nerve root, into the mesorectal fat (Figure 1), while an electrode displacement was observed in two patients (Figure 2, although the electrode was in the proximity of the target sacral nerve, patients reported a reduction of efficacy over time). Therefore, based on the MRI findings, in these patients, a reprogramming of the SNS settings was needed involving the most distal electrode poles in the electrical stimulating field. Conversely, in the CC patients, MRI assessment showed an optimal tined lead position along the third sacral nerve. However, at follow up, no significant differences were reported between pre- and postoperative CCSS scores [11.5 (IQR: 4.5-12.7) vs. 13.0 (IQR: 9.7-13.0), p=0.655].

MRI showed to be a safe and useful diagnostic tool in the follow up of patients undergoing SNS implant for DDs. In this study, surprisingly, even if a standardized placement technique was adopted and provided an adequate intraoperative stimulation, only 5 of the 8 implanted electrodes were detected in the correct position at MRI assessment. Although SNS demonstrated to be effective in FI treatment regardless optimal electrode position, suboptimal position required reprogramming the SNS settings. Therefore, MRI evaluation showed to be helpful in choosing the correct electric field of stimulation. Unfortunately, although a correct position of the tined lead was documented with MRI, no significant clinical improvement was reported in the CC patients. 
Preliminary data from this study underline the utility of a systematic MRI evaluation of patients submitted to the new MRI-compatible SNS system in order to check the electrode position and identify any variations over time. Moreover, any foci of infection could be accurately detected. Therefore, either a tailored programming of the SNS settings based on a change in electrode position or the removal of the SNS system could be decided more precisely. However, before considering the inclusion of the MRI in any guidelines on SNS, the timing of MRI postoperative evaluation should be assessed. On the other side, as showed in our experience, the electrode placement could be furtherly improved, probably by adopting technical and surgical adjustments that would be able to ensure the best targeted stimulation of the sacral nerve with less energy consumption of the stimulator device.

A systematic postoperative MRI assessment could be able to explain suboptimal or lack of SNS efficacy, and guide clinicians for reprogramming the SNS parameters.

Continence 12S (2024) 101672
DOI: 10.1016/j.cont.2024.101672