Introduction

In 1982, the first sacral nerve neuromodulation (SNM) was performed by Tanagho and Schmidt at the University of California in San Francisco ( ). SNM, delivered by the InterStim system and developed by Medtronic (Minneapolis, MN, USA), was first introduced in Europe in 1994. In 2002, Medtronic released a new implantable tined lead (model #3889), making the procedure less invasive. In 2006, Medtronic released a smaller neuromodulator called InterStimII. The US Food and Drug Administration (FDA) approved SNM for three chronic, voiding dysfunction conditions: intractable urge incontinence in 1997, urgency/frequency syndrome, and nonobstructive urinary retention in 1999 for patients who had failed to respond or could not tolerate conservative treatments ( ). In 2011, the FDA approved SNM for chronic, fecal incontinence in patients who had failed or could not tolerate conservative treatment. The indications for SNM have expanded from intractable urge incontinence and urgency/frequency syndrome ( ; ) and idiopathic chronic urinary retention ( ; ), to include reestablishment of pelvic floor awareness, resolution of pelvic muscle tension and pain ( ), interstitial cystitis ( ), and neurogenic detrusor over activity ( ).

Mechanism of Action (MOA)

The mechanism of action (MOA) by which SNM works to alter bladder functions are not yet clearly understood; however, there are two proposed hypotheses:

• 1.
  

  SNM may restore balance between excitatory and inhibitory impulses to and from the pelvic organ at sacral and suprasacral levels.

  
  
• 2.
  

  Afferent and efferent pathways are modulated to provide a balance that results in improved bladder function ( ).

As reported by , the MOA of SNM is most likely multifactorial and impacts the neuroaxis at several different levels. One hypothesis for the MOA of SNM is that indirect stimulation of the pudendal nerve and direct inhibition of the preganglionic neurons suppresses detrusor over activity. Another hypothesis is that stimulation alters the transmission of sensory input from the bladder to the pontine micturition center by inhibiting involuntary, reflex voiding. Sacral neuromodulation most likely inhibits the guarding reflex, leading to a reduction in sphincter over activity and therefore a reduction in outlet resistance in cases of nonobstructive urinary retention. In fecal incontinence, Vitton et al. described evidence of a somatosympathetic reflex pathway, which might explain the reason why fecal incontinence treatment using SNM, leading to reduced colonic activity and increased tone of the anal sphincter complex, may be successful ( ). See the chapter by DeGroat in this book for a more detailed discussion of mechanisms of SNM.

Sacral Neuromodulation Procedure

Before permanent implantation for SNM, a screening test, the percutaneous nerve evaluation (PNE) is performed to assess the clinical efficacy or lack thereof. SNM implantation can be performed as a one-stage procedure or a two-stage procedure (implantation).

One-Stage Implantation

The PNE test is performed in an outpatient setting under local anesthesia with the patient in the prone position and involves a nonanchored test lead placed into the S3 foramen and connected to an external stimulator, in a monopolar fashion ( ). The test period is usually between 5 and 7 days, according to implanter preference. The patient is asked to keep a voiding diary, after which the test lead is removed. The procedure is performed bilaterally by cannulating the S3 foramen and stimulating the S3 sacral nerves on each side to elicit the desirable responses of tingling, numbness, or vibration-like sensations of the pelvic floor (rectum/vagina/scrotum/perineum) and big toe dorsiflexion. The side eliciting the desirable response and comfort for the patient is selected for the nonanchored, percutaneous, lead insertion.

The site of S3 foramen can be localized directly by fluoroscopy or by palpating the upper borders of the sciatic notches bilaterally and then drawing a line connecting them. This line intersects the midline of the sacrum. One fingerbreadth, on either side of the midline of the sacrum, at this intersection, is the location of the S3 foramen. Alternatively, the foramen may be localized by measuring 9 cm cephalad to the drop-off of the sacrum. The S3 foramina are about 1–2 cms lateral to the midline of the sacrum, on either side. The electrode is then introduced through the foramen via a needle and cannula.

Once through the foramen, the electrode is connected to an external pulse generator and electrically stimulated in a monopolar fashion. The external pulse generator gives the patient the ability to control stimulation intensity. After an adequate test period for efficacy or lack, thereof (efficacy defined as a ≥50% subjective and/or objective response), the electrode is removed. The objective response is evaluated and the responses are kept in a voiding diary, completed by the patient, before insertion, and compared to the diary after insertion. A response of ≤50% voiding after PNE indicates failure of the trial. If there is ≥50% response, the trial is successful. The overall response rate for PNE is about 55%. Lead migration is considered the main factor that leads to false-negative results. If the patient has a positive trial, he or she will undergo permanent implantation of the tined lead and internal pulse generator ( ). On the other hand, patients who do not respond to PNE may undergo a two-stage, implantation procedure.

Two-Stage Implantation

If the patient does not respond to PNE testing, is not a candidate for office-based test stimulation or the indication of the treatment is fecal incontinence, stimulation may be performed in the operating room, under local or general anesthesia, using a permanent tined lead with aid of fluoroscopy to decreases the incidence of technical-related test failures. Immediate implantation of the permanent, tined, four contact lead aims to avoid lead migration and allows prolonged patient testing/screening. The permanent lead has self-anchoring tines that reduces the risk of migration. The lead is placed into the S3 foramen under local or general anesthesia; the exact position into the foramen is guided by fluoroscopy. Appropriate placement into the correct foramina is tested for appropriate motor responses, which are dorsiflexion of the great toe and bellows contraction of the perineal area, representing contraction of the levator muscles (bellows reflex). If the patient is awake and the procedure is performed under local anesthesia, the patient will feel tingling, numbness, or vibration like sensations of the pelvic floor (rectum/vagina/scrotum/perineum). An S3 response should be noted, using at least one electrode combination (one anode, one cathode). After eliciting the appropriate responses, the lead is tunneled to the opposite side from its foramen, entry site to cross the midline, is connected subcutaneously to a temporary extension lead that exits the skin at the future site of the implantable pulse generator (IPG) and then connected to an external pulse generator. This procedure enables test periods of up to 2–4 weeks. If the patient has a good response during this period, the implanted lead is connected to an implanted IPG after removal of the externalized lead extension. This procedure is usually performed under local anesthesia injected subcutaneously in the buttocks area (A 3–4 cm incision in the upper gluteal crease below beltline is made for a deep subcutaneous pocket to allow implantation of the IPG). In the United States, the two-stage implant is widely used as a standard procedure for implanting InterStim ( ).

Due to the lower risk of migration and longer test evaluation period, this test has a higher response rate. Prolonged screening with the tined lead has a response rate of 67%, when compared with the 43% response rate, using PNE testing ( ). The costs for the test protocol with the tined leads are much higher when compared to PNE testing. Currently, the use of either of the two screening options is arbitrary and left up to the desires of the implanting physician. Postoperatively, the IPG is turned on and the SNM is programmed, using different parameters of electrodes used, pulse width, frequency, and amplitude until the desired responses are elicited. Programming of the device is performed by the physician or the physician’s agents.