The most common nerve targeted for stimulation is the S3 nerve root. Benson (Benson JT. Human sacral neuroanatomy: electrophysiologic determination. Presented as a poster at the 21st annual AUGS meeting, October 6–10, 2000, Vancouver, British Columbia, Canada) demonstrated that the third sacral nerve root provided the majority of innervation to the pelvic organs of interest. However, the sacral anatomy is extremely variable and thus occasionally the S4 or S2 nerve roots are stimulated. To determine optimal lead placement, both motor and sensory responses are desired. The typical sensory response is a tapping or vibratory sensation in the vagina, rectum, or perineum. The motor response includes a pulling in of the pelvic floor muscles known as a bellows response or flexion of the great toe. The ideal goal with placement is to obtain motor responses at all four electrodes at thresholds under 2–3 volts. There has been no definitive study to determine whether sensory or motor responses are more predictive of success. Recent investigation comparing patients evaluated with both sensory and motor response with those evaluated with motor response alone demonstrated similar rates of successful trial stimulation and device explant between groups with 4-year follow-up.26 Pizarro-Berdichevsky27 demonstrated a reduced risk of lead wire revision in patients with higher total motor response scores and great toe scores, but not bellows scores, in a recent retrospective review. The authors suggest that a great toe response reflects stimulation of a more therapeutically relevant target nerve, whereas a pelvic bellows response reflects stimulation of any portion of a broader target, which could include multiple nerves.
Since its introduction, sacral nerve stimulation has undergone significant improvements in design and application. The initial implantation technique required general anesthesia, a larger incision over the sacrum (often 5–7 cm), and dissection to the periosteum. With the introduction of the tined lead wire in 2003, currently the procedure requires only a 2-mm incision and can be performed under local anesthesia.24 In 2006, a smaller, second-generation implantable pulse generator was introduced. Additionally, it was postulated that a curved stylet would mimic the natural path of the sacral nerve root (Fig. 3) and was added to the tined lead kit after a randomized crossover trial demonstrated superiority of the curved over the straight stylet in achieving motor response at lower amplitudes.28
Another important component of sacral nerve stimulation device implantation is infection prevention. The American Urological Association recommends perioperative antibiotic prophylaxis with a first-generation cephalosporin before placement of the lead wire to cover for skin flora; however, data supporting this recommendation are lacking.29 A large retrospective cohort analysis of sacral nerve stimulation procedures noted a significant reduction in surgical site infections after the implementation of a chlorhexidine scrub the night before and the morning of the procedure. The infection rate decreased from 7.4% to 1.7% after institution of the chlorhexidine scrub.30 Additionally, 82% of devices that were explanted as a result of an infection were noted to be colonized with methicillin-resistant Staphylococcus aureus, which may influence the selection of prophylactic postoperative antibiotics.
Before gaining approval from the FDA, Medtronic conducted a prospective, multicenter, randomized study evaluating the efficacy and safety of sacral nerve stimulation between 1993 and 1997 (MDT-103). This study included 23 centers worldwide and recruited 76 patients with urge incontinent, 50 with urgency–frequency, and 51 with urinary retention. All patients were required to have had a successful percutaneous nerve evaluation trial, defined as 50% or greater improvement from their baseline symptoms. They were randomized to implantation either immediately or 6 months after the percutaneous nerve evaluation trial.22
Regardless of the indication for sacral nerve stimulation, findings in the implant groups were encouraging. For the 38 participants with urge incontinence randomized to immediate implantation, the mean number of leakage episodes decreased from 10.8 to 2.9 per day. Overall, 45% of the patients remained completely dry and 34% achieved a 50% or greater reduction in leaking episodes. When the stimulation was turned off at 6 months, the mean number of baseline incontinent episodes went back up to 10.2. For those with urinary urgency–frequency, the mean number of daily voids decreased from 16.9 to 8.8 and 33% experienced a 50% or greater reduction in voids. At 6 months with the stimulation turned off, the number of voids increased to 13.5 voids per day. In patients with urinary retention, 61% were able to eliminate catheter use.
In a systematic literature review of sacral nerve stimulation therapy for the treatment of UUI, Brazelli33 found that, in the four controlled trials in the literature, there was an 80% success rate, whereas, in the 37 case series, there was an overall success of 67%. Amundsen34 conducted a prospective study to determine whether there were factors predictive of complete cure and found that age older than 55 years and the presence of more than three chronic conditions were independently associated with lower cure rates for patients with intractable UUI.
The InSite Trial was a randomized multicenter study evaluating the success rate of sacral nerve stimulation compared with standard medical therapy in patients with OAB. Standard medical therapy was defined as antimuscarinic therapy, and inclusion criteria included failure of at least one medication without having tried every available antimuscarinic medication. The InSite Trial, which was implemented in two phases, reflects current standard practices including minimally invasive technique, routine fluoroscopy use, and exclusive use of a staged implantation of the tined lead.
Phase 1 was a prospective, multicenter, randomized trial comparing sacral nerve stimulation with standard medical therapy and the primary objective was to compare the therapeutic success rate at 6 months, which was defined as 50% or greater improvement from baseline symptoms.35 In contrast to the MDT-103 trial in which all available therapies had failed and patients had more severe baseline symptoms, the patient population in the InSite trial had milder symptoms. Overall, 147 patients were randomized. For patients who were incontinent at baseline, 71% of those treated with sacral nerve stimulation responded compared with 47% of patients treated with standard medical therapy. Rates of complete continence were higher in those treated with sacral nerve stimulation (39% vs 21%) and this group showed statistically significant improvement over the standard medical therapy group in all measures of quality of life. The patient population was a less refractory group than previously studied, suggesting that sacral nerve stimulation is a successful option for patients throughout the OAB spectrum. The implication of the study is that after unsuccessful treatment with anticholinergic medication, patients with OAB may be more likely to benefit from sacral nerve stimulation than an additional anticholinergic medication.
Phase 2 of the InSite trial was a prospective evaluation of safety and efficacy of sacral neuromodulation over 5 years.36 During the study period, 340 patients received test stimulation and of those, 272 (80%) had the sacral nerve stimulation system implanted. The overall therapeutic success rate at 5 years was 82%. Given that the therapeutic success rate was 85% at 1 year,37 this demonstrates the long-term durability of sacral nerve stimulation for patients with OAB. Complete continence at 5 years was achieved in 45% of patients. Sustained improvements in quality-of-life measurements were reported from baseline to 5 years in all domains. The 5-year cumulative rate of adverse events that required surgical intervention after the full sacral nerve stimulation system was implanted was 22% with the most common adverse event being an undesirable change in stimulation. This study, which is one of the first to evaluate outcome data and continuation rates of sacral nerve stimulation beyond 36 months, demonstrates that this therapy provides a durable treatment effect 60 months after implantation of the system.
The Relax OAB trial is the first study describing outcomes of a rechargeable sacral nerve stimulation device. This was a prospective, single-stage implant of the lead wire and implantable pulse generator in patients with refractory OAB.38 The results reported at 3 months are comparable with results in the literature and an infection rate of less than 2% (Fig. 4). Ninety-eight percent of patients were able to recharge the device within the first month.
In 2013, the FDA approved intradetrusor onabotulinum toxin A (Botox) injections for the treatment of UUI. The Rosetta Trial, a randomized multicenter trial comparing intradetrusor Botox injections with sacral nerve stimulation in patients with refractory UUI, demonstrated that intradetrusor Botox injections resulted in a greater daily reduction of urgency incontinence episodes (−3.9 vs −3.3)39 (Fig. 5). Although this finding was noted to be statistically significant, the authors of the trial comment that this finding is of uncertain clinical importance. Additionally, urinary tract infections and the need for self-catheterization postoperatively were higher among the patients who underwent intradetrusor Botox injections. A secondary analysis of the Rosetta Trial revealed that older women with multiple comorbidities and decreased functional and health-related quality of life had decreased treatment response and satisfaction with intradetrusor Botox injections compared with sacral nerve stimulation.40 Recently the American Urological Association updated its guidelines in 2015 to include intradetrusor Botox injections as a standard third-line therapy for patients with refractory OAB.41
In 2011, the FDA approved sacral nerve stimulation for the indication of fecal incontinence. In the multicenter, nonrandomized trial precipitating its approval, 133 participants underwent a 2-week trial of sacral nerve stimulation with 120 (90%) progressing to full device implantation.42 That group experienced a drop in weekly symptoms from 9.4 at baseline to 1.9 at 1 year. Forty-one percent were completely continent and 83% were at least 50% improved, which was sustained over 5 years after implantation.43 Surgical device revision, replacement, or explant, however, occurred in 35.5% of patients. Given the high recurrence and morbidity of other surgical options for the treatment of fecal incontinence, sacral nerve stimulation may be an appealing alternative for patients.44
Another study supporting the use of sacral nerve stimulation for fecal incontinence is a randomized prospective trial of this therapy compared with optimal medical management (dietary therapy and pelvic floor strengthening).45 Ninety percent of those randomized to sacral nerve stimulation had a successful trial and went on to full implantation. Patients were monitored for 12 months; the sacral nerve stimulation group demonstrated a reduction in weekly fecal incontinence episodes from 9.5 to 3.1, whereas the medical therapy group demonstrated no change from baseline, having more than nine incontinent episodes per week at the end of the trial. Interestingly, patients with sphincter defects up to 120° were included, and two thirds of those with sphincter defects demonstrated at least a 75% response rate. This supports a central mechanism rather than direct stimulation of the anal sphincter.
A recent systematic review of sacral nerve stimulation during pregnancy evaluated eight studies of 22 patients who underwent one or more pregnancies after implantation.46 Sacral nerve stimulation was continued during eight of the pregnancies and in the remaining 18 pregnancies in which the device was deactivated, seven patients experienced recurrent urinary tract infections and two patients requested reactivation during the second trimester. Sixteen patients were delivered by cesarean and nine were delivered vaginally. Sacral nerve stimulation functioned appropriately after 15 of the 25 pregnancies. Four patients required reprogramming, three required replacement, and three decided to remove the device after delivery as a result of lack of symptoms after deactivation. Based on this limited evidence, the use of sacral nerve stimulation during pregnancy appears to be safe and did not result in significant maternal or fetal morbidity.
A review article from 2011 demonstrated a 40–72% reduction in pelvic pain after implantation of a sacral nerve stimulation device.47 Martellucci48 published a cohort study of 16 patients with chronic pelvic pain who underwent sacral nerve stimulation implantation. The mean preoperative visual analog scale was 8.1, which decreased to 2.1 after 6 months. This improvement persisted for up to 24 months for 13 patients and up to 60 months for 3 patients. A recent case report of patients with a history of surgically treated deep infiltrating endometriosis who underwent staged sacral nerve stimulation testing49 demonstrated that three of the four patients went on to permanent implantation and all three patients continued to report improved quality of life 30 months after the implantation.
Although not currently approved for patients with interstitial cystitis, sacral nerve stimulation may provide an alternative therapy. In a prospective study of 37 patients with interstitial cystitis, 70% had a successful trial and went on to have the permanent lead wire implanted. Overall 24-hour voids decreased from 24.7 to 12.2 and 96% of the participants said they would undergo the procedure again.50 In a subsequent study of patients with interstitial cystitis, Peters51 demonstrated a 36% reduction in narcotic use with 22% stopping altogether. Zabihi52 prospectively evaluated 30 patients with interstitial cystitis and chronic pelvic pain and found a 41% improvement in symptoms 6 months after sacral nerve stimulation implantation.
The original trials with sacral nerve stimulation excluded patients with underlying neurologic disease. However, patients with neurologic conditions included in Bosch’s study had equivalent success during the trial as neurologically intact patients.32 A report of 33 patients with various neurologic conditions (primarily multiple sclerosis) demonstrated a similar implant rate as neurologically intact patients.53 More recently, a study of 62 patients with neurogenic bladder dysfunction demonstrated that two thirds had a positive response to the trial.54 Among the 37 participants implanted for a mean of 4.3 years, investigators found the effect of sacral nerve stimulation to be sustained in 75.7%, partially altered in 8.1%, and lost in 16.2%.
More peripheral sites of stimulation are currently being evaluated. In a randomized crossover trial of sacral compared with pudendal stimulation, the pudendal implant was found to be superior to a sacral implant in 79% of the cohort.55 All patients had successful placement of the pudendal lead through a posterior approach and there was no difference in time to implantation when compared with the sacral lead.
Some members of the health care community have expressed concern regarding the cost of sacral nerve stimulation. In a retrospective review of patients receiving sacral nerve stimulation, health care utilization was determined for the year before and after implantation.56 The investigators found that sacral nerve stimulation was associated with a 92% reduction in outpatient physician visits, diagnostic and procedure costs along with a 30% reduction in drug expenditures. Dutch investigators modeled the cost of adding sacral nerve stimulation to the treatment algorithm for fecal incontinence and found that adding this therapy decreased cost and improved treatment success, resulting in higher quality-adjusted life-years.57 A recently published cost–consequence model predicted that, over a 15-year period, the per-patient cost of treatment with a rechargeable, as compared with a nonrechargeable, sacral nerve stimulation device would result in an estimated payer cost savings of $27,121.58 Adoption of a rechargeable sacral nerve stimulation device was projected to result in health care expenditure savings up to $12 billion.
The advent of sacral nerve stimulation for the treatment of refractory voiding and bowel dysfunction has provided an effective and safe alternative therapy for patients for whom more conservative treatments have failed. Emerging research indicates that sacral nerve stimulation may be a future therapeutic option for patients with chronic pelvic pain and interstitial cystitis. As sacral nerve stimulation therapy continues to advance with the possible expansion to more direct sites of stimulation and widening therapeutic indications, more women with pelvic floor disorders may benefit from this effective, minimally invasive treatment option.
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