11 Sacral Neuromodulation
Sacral neuromodulation (SNM) received approval from the U.S. Food and Drug Administration in 1997 (InterStim; Medtronic, Inc, Minneapolis, MN). SNM is indicated for the treatment of refractory urge urinary incontinence, frequency-urgency syndrome, and idiopathic urinary retention and chronic fecal incontinence. Although the exact mechanism of action has not been fully determined, SNM appears to modulate bladder behavior through electrical stimulation of somatic afferent axons in the spinal roots, which modulate voiding and continence reflex pathways in the central nervous system, likely by inhibiting interneuronal transmission in the bladder reflex pathway (Amend et al., 2011a). Neuromodulation also may have a direct effect on the pelvic floor.
The InterStim device has three components: a battery-powered neurostimulator (also known as the implantable pulse generator [IPG]), an extension cable, and a tined electrical lead (Figure 11-1). The tined lead is a semipermanent, insulated electrical stimulation lead with four contact points near the tip (quadripolar) and four plastic collapsible projections (tines), which help to anchor the lead to the surrounding tissue. The IPG is a remotely programmable, battery-operated device that generates an electrical stimulus transferred to the lead contact points. With SNM, the electrical lead is implanted in close proximity to the third sacral nerve root (S3) at the level of the S3 spinal foramen (Figure 11-2). Stimulation of the S3 nerve root has demonstrated the best efficacy for SNM because S3 provides innervation to the bladder itself. Appropriate positioning of the electrical lead is verified by motor or sensory responses to electrical stimulation at the time of implantation (Figure 11-3). The S3 nerve root and neighboring S2 and S4 roots exhibit characteristic responses to electrical stimulation (Table 11-1). Ipsilateral great toe plantar flexion and pelvic floor “bellows” response are the predictable motor responses seen with S3 stimulation.
(Courtesy Medtronic, Inc, Minneapolis, MN.)
Figure 11-2 Final position of the four electrical contact points of the stimulation lead in close proximity to the third sacral nerve root (S3) and the four plastic projections or tines embedded in and securing the lead to the tissue overlying the sacral foramen. A, Lateral view. B, Posterior view.
Most commonly, the device is implanted after a test phase (i.e., nerve evaluation) initially confirms therapeutic response. The test phase may be accomplished using a temporary, disposable lead implanted in the office as part of a basic evaluation (i.e., percutaneous or peripheral nerve evaluation [PNE]) or with a tined lead as part of a planned two-phase procedure. For PNE, a temporary, unipolar electrical lead is percutaneously implanted under local analgesia, typically in the office setting, and a short-term trial period (7 days) ensues to confirm response. With positive results, the temporary lead is easily removed, and the patient can progress directly to single-stage InterStim implantation (i.e., quadripolar lead and IPG) in the operating room. If results are inconclusive, an advanced evaluation approach may be tried using implantation of a tined, quadripolar lead (discussed later).
Alternatively, the InterStim system may be implanted in first-time patients with a staged, two-step process involving an initial percutaneous placement of the tined lead with an external impulse generator for a short trial period followed by permanent implantation of the IPG device in patients who respond to the trial period. The tined electrical lead placement is typically done with fluoroscopic guidance but may also be done without, using palpable bony landmarks. A temporary, external electrical stimulator is attached, and a clinical trial period of 1 to 4 weeks ensues, during which the patient evaluates his or her response to the therapy. If appropriate benefit occurs (defined as a >50% improvement in symptoms as measured by a voiding diary), the IPG is connected to the previously placed lead and surgically implanted in the upper buttocks during a second surgical stage procedure. If there is not a significant response, the implanted lead is removed without implanting an IPG. Adjustments to the impulse generator settings can be made with a remote programming device.
A 55-year-old woman who has had urge incontinence for more than 3 years, without known neurologic disease or other significant morbidities, with a normal physical examination, and with negative urinalysis and cytology requests therapy for her symptoms. She has received two different antimuscarinic agents, each for 1 month, with dose escalation being used and with no significant reduction in symptoms despite medication combination with optimized behavioral therapy. Urodynamics reveals a bladder capacity of 175 mL, with detrusor overactivity occurring at 100 mL with incontinence and no evidence of obstructed voiding and minimal post-void residual. Diagnostic cystoscopy is unremarkable. The patient is very bothered by her symptoms and desires an attempt at more definitive therapy if possible. After a detailed discussion of options, including botulinum toxin (Botox) injection and SNM, she decides to proceed with a PNE test stimulation.
1. Patient positioning, setup, and analgesia. Temporary lead placement during PNE may be performed in the outpatient clinic under local analgesia. The patient is positioned prone on the procedure table with slight flexion at the hips. The patient’s shoes and socks are also removed to allow observation of the feet. The nonsterile ground pad is affixed to the skin away from the site of lead placement (sterile field). The skin overlying the sacrum, buttocks, and perineum is sterilely cleansed, and surgical drapes are placed to allow visualization and inspection of the buttocks and gluteal crease. Analgesia is achieved through infiltration of the skin and subcutaneous tissues down to the bone in the vicinity of the desired sacral foramina with local anesthetic (e.g., lidocaine or bupivacaine).
2. S3 localization and lead placement. To locate the S3 foramen, anatomic landmarks or fluoroscopy may be used (Figure 11-4). When the approximate location of the S3 foramen is identified, a foramen needle is inserted percutaneously into the foramen at a 60-degree angle relative to the skin. The test stimulator is connected to the foramen needle via the test stimulation cable, and stimulation is applied. With electrical stimulation applied, the clinician observes for responses to stimulation (see Table 11-1 and Figure 11-3). If the desired responses are not seen, the needle may be moved up and down to change the depth, or a foramen needle may be placed on the contralateral side. If responses are consistent with different sacral nerves (i.e., S2 or S4), the needle may be reinserted into the appropriate foramen and retested. If the desired responses are seen, the test stimulation lead is inserted into the foramen needle until the lead electrode exits the needle tip (determined by markings on the lead). The lead is retested for appropriate responses, and if confirmed, the foramen needle is removed over the lead, leaving the lead in place in the S3 foramen.