Introduction
Neuromodulation in the form of sacral nerve modulation (SNM) for the treatment of refractory voiding dysfunction was first described by Tanagho and Schmidt in 1998. A commercially available device, the InterStim System (Medtronic, Minneapolis, MN) was approved by the Food and Drug Administration for the treatment of urge urinary incontinence in 1997, for the treatment of nonobstructive urinary retention and urinary frequency in 1999, for the treatment of overactive bladder in 2002, and for the treatment of fecal incontinence in 2011. The procedure involves a trial phase and a subsequent implantation, either as a two-staged procedure under monitored anesthesia or using temporary leads in an office setting under local anesthesia, referred to as a percutaneous nerve evaluation (PNE).
Patient Selection
Urge Urinary Incontinence or Overactive Bladder
Patients presenting with overactive bladder should have a thorough history and physical examination. In treating overactive bladder, InterStim should be considered as a third-line option after conservative therapies and medications have failed.
Nonobstructive Urinary Retention
Sacral nerve modulation therapy is a safe and durable treatment option for nonobstructive urinary retention. One can consider a staged approach to testing as opposed to a PNE because these patients typically require a longer duration of testing to determine success.
Fecal Incontinence
Patients presenting with fecal incontinence should be appropriately assessed by a gastroenterologist and offered InterStim if all other conservative measures have failed.
Contraindications
Sacral nerve modulation is not approved in the treatment of neurogenic bladder and should be avoided in patients with significant abnormalities of the sacrum. Additionally, patients with significant mental or physical disabilities are not appropriate candidates for SNM.
Magnetic resonance imaging (MRI) studies, except for MRI examinations of the head, are contraindicated in InterStim patients because magnetic fields can produce electrode currents and subsequent heating of the lead. Additionally, shortwave diathermy, microwave diathermy, and therapeutic ultrasound diathermy are contraindicated in InterStim patients because device interaction can result in permanent nerve or tissue injury.
Special Considerations
Pelvic Pain
InterStim therapy is not approved for the treatment of chronic pelvic pain. However, SNM is listed as a fourth-line treatment option for interstitial cystitis or painful bladder syndrome per American Urological Association (AUA) guidelines. We prefer a staged implant to allow for procedural sedation as central sensitization with pain hypersensitivity is common in this population. We also see an improvement in lateralizing pain if the InterStim lead is also placed on the ipsilateral side.
Pregnancy
The safety and efficacy of SNM has not been established for pregnancy, an unborn fetus, or delivery. All women of reproductive age must be counseled about contraception and the need for device deactivation as soon as pregnancy is confirmed per manufacture recommendations. Continued InterStim therapy during pregnancy may be beneficial in certain patients with urinary retention because it minimizes the risk of urinary tract infections from intermittent catheterization.
Cardiac Pacemakers
There is a risk of possible cross-talk between the sacral neurostimulator and cardiac devices, although InterStim placement appears safe in the setting of pacemakers without cardioversion or defibrillation technology.
Stage 1: Operative Technique
Preoperative Preparation
Preoperatively, the patient should have completed baseline voiding diaries documenting symptoms and should ideally stop blood thinners at least 7 days before the procedure. We have a preoperative cleansing regimen in place for patients undergoing any part of the staged approach or full implantation. This involves Hibiclens (4% chlorhexidine gluconate) skin cleansing the night before and Sage cloths (2% chlorhexidine gluconate) cleansing in the preoperative area right before surgery. A prophylactic dose of intravenous cephazolin or vancomycin is administered before incision.
Positioning
The patient is placed prone on a “fluoro-friendly” table to allow for imaging of the sacrum ( Fig. 104.1 ). Bilateral chest rolls are placed to assist with respiration during anesthesia. Pillows are placed under the hips, knees, and shins to pad pressure points. The feet are left uncovered for monitoring responses. The lower back and buttocks are prepped widely with an alcohol-based antiseptic solution. Surgical drape with Ioband should be wide enough to allow full access to the lower back and visualization of the entire buttocks.
We prefer heavy monitored anesthesia care (MAC)/local anesthesia for our staged InterStim procedures and therefore rely solely on motor responses to determine intraoperative lead placement. If general anesthesia with placement of an endotracheal tube is required for patient safety, it is imperative that only a short-acting paralytic is used during induction.
Accessing S3 Spinal Foramen
Ideal lead placement is located medial and superior within the S3 foramen along the course of the S3 nerve root. An anteroposterior (AP) fluoroscopic view is obtained to determine and mark the medial edge of the sacral foramina bilaterally. An imaginary line parallel to the spine at the level of the either mark is where the foramina align ( Fig. 104.2 ). A lateral fluoroscopic view is next obtained to locate the target foramen and appropriate entry point of the needle. The S2 level ( Fig. 104.3, B ) is readily identified as where the sacroiliac joint fuses, forming a characteristic shadow ( Fig. 104.3, A ). The first anterior protrusion or “hillock” from the surface of the sacrum below this shadow is typically S3 intervertebral disc space ( Fig. 104.3, C ). The foramen needle should enter approximately 1 cm above and parallel to this line.
After the starting point has been estimated, the skin is anesthetized to create a dime-sized wheel. A specialized foramen needle is then inserted and “walked” up or down until it drops in to the S3 foramen. Fluoroscopy is kept in the lateral view to aid in finding the optimal entrance into the S3 foramen. The standard needle size is 3.5 inch (9 cm); however, for patients with a larger body habitus, a 5-inch (12.5-cm) needle may be necessary. When the needle enters the foramen at the desired location, fluoroscopy is used to gauge the depth. The tip of foramen needle should just barely be inside the anterior bony plate at the location of the S3 nerve root ( Fig. 104.4 ). The foramen needle is stimulated with the screening device to check motor responses ( Table 104.1 ).
| Level | Motor | Sensory |
|---|---|---|
| S2 | Bellows (inward going of intergluteal fold), clamp (AP pinching of perineum or coccyx), dorsiflexion of foot, heel rotation, and calf cramping | Genital |
| S3 | Bellows, dorsiflexion of great toe, bottom of foot | Genital, perineal, anal |
| S4 | Bellows | Anal |
Proper placement should elicit a bellows response first followed by great toe dorsiflexion at low-voltage thresholds. We aim to achieve an “opening threshold” or motor response at less than 2 volts. If responses are not appropriate at this level (i.e., consistent with S2 or S4), the needle direction or the point of introduction at the skin level is readjusted to aim for the correct S3 foramen. If the responses are less than ideal within the S3 foramen, then the needle needs to be adjusted medially or laterally within the foramen to improve contact with the nerve. If the patient is awake enough to report sensation, the ideal location of stimulation is genital and perineal.
Deployment of Tined Lead
After the appropriate response to the S3 nerve with the foramen needle has been attained, a 1-cm skin incision through the subcutaneous fat is made at the entry point of the foramen needle. If this incision depth is inadequate, the lead will be positioned too superficially, resulting in patient discomfort and an increased risk of lead fracture. The inner foramen needle stylet is removed and replaced with the directional guide. There are two depth markers on the directional guide. The specific depth mark will depend on the length of the foramen needle used ( Fig. 104.5 ). The foramen needle is removed over the directional guide, taking care to keep the guide in place. The dilator and introducer sheath is next placed over the directional guide and passed into the foramen under continuous fluoroscopy. The radiopaque marker located on the sheath should be placed halfway through the bone table for optimal lead deployment ( Fig. 104.6 ).
The tined lead is quadripolar with four electrode sites labeled 0 to 3. There are four tines just proximal to the electrode sites; radiopaque markers straddle these tines and help to determine lead depth and deployment fluoroscopically. The distal end of the lead has four contact points that correspond to the four electrode sites ( Fig. 104.7 ). These contact points are used to test the lead placement before deployment. We only use lead model 3889 with its equal and equidistantly spaced leads because its ability to closely follow the curved path of the S3 nerve root. We prefer to use the curved lead stylet because it is softer, less likely to force the electrode out of the proper plane, and easier to glide upon the path of the nerve.
