Sacral Neuromodulation

CHRISTOPHER F. TENGGARDJAJA

SANDIP VASAVADA

Overactive bladder (OAB) symptoms are projected to affect 34 million people in the United States, with a projected national cost of $76 billion in 2015 (1,2). OAB is defined as urinary urgency with or without incontinence commonly associated with frequency and nocturia (3). Patients treated for OAB symptoms undergo behavioral modification in addition to pharmacotherapy. By the time these patients reach the urologist, they usually have refractory symptoms. Sacral neuromodulation (SNM) has proven to be a durable and successful treatment for refractory OAB patients.

Our knowledge of electrical stimulation for neuromodulation is dependent on prior neurophysiologic studies. In the 1800s, work in canine models localized preservation of sensation versus motor function to respective intact dorsal or ventral spinal nerve roots. Selective transection of pelvic and hypogastric nerves helped to further determine their effects on bladder function and micturition. These physiologic observations laid the foundation for our present day techniques. By 1960, several studies began exploring the feasibility of selective nerve root activation with electrical activity. Schmidt et al. (4) demonstrated the ability to induce voiding in canine models with stimulation of sacral nerve roots. Brindley et al. (5) demonstrated improved voiding in spinal cord injury patients with implantation of sacral anterior root stimulators. The placement of neurostimulators has evolved from morbid open cut-down procedures involving laminectomies to minimally invasive percutaneous image-guided techniques. First approved in Europe in 1994, SNM was approved by the U.S. Food and Drug Administration (FDA) for urgency urinary incontinence (UUI) in 1997 followed by urgency-frequency syndrome and nonobstructive urinary retention (UR) in 1999. Most recently, SNM was approved for the treatment of fecal incontinence (FI) in 2011 (InterStim, Medtronic, Inc). This chapter focuses on the indications, techniques, and outcomes for implantation.

MECHANISMS OF ACTION OF SACRAL NEUROMODULATION

Urinary storage and emptying are facilitated by complex interactions between the central and peripheral nervous systems. Urinary storage is facilitated by lumbar sympathetics and pudendal efferent activation. Micturition is activated by sacral parasympathetics and pudendal relaxation of the external urinary sphincter. SNM is thought to affect both afferent and efferent activity to allow for detrusor relaxation and also cause selective inhibition at the spinal and supraspinal levels (6). Given that SNM is effective in both OAB and nonobstructive UR, it is thought that different pathways are modulated in each patient group with a return to “normal neurophysiology.” For patients with OAB who possibly have an unmasking of a primitive reflex or abnormal sensory afferent discharge, SNM may act on pudendal nerve afferents regulating abnormal activation and inhibiting interneuronal transmission leading to a modulation of sensory outflow to the pontine micturition center. This subsequently prevents involuntary detrusor contractions by modulating the abnormal activation of the micturition reflex. Patients with nonobstructive UR are hypothesized to have an erroneous disinhibition of the spinal guarding and bladder afferent loop reflexes. SNM restores a normal voiding pattern by inhibiting the constant activation of these guarding reflexes (7).

DIAGNOSIS

An initial visit includes a comprehensive history and physical examination. Important aspects of the history include onset and duration of symptoms, exacerbating factors, prior failed therapies, and an assessment of quality of life. Questionnaires such as Urogenital Distress Inventory Short Form (UDI-6) and Incontinence Impact Questionnaire Short Form (IIQ-7) can aid in establishing baseline symptom scores of severity and bother. Age of the patient and onset of symptoms play an important role in the initial assessment. A young patient with sudden onset of symptoms accompanied by other neurologic complaints may warrant further workup for underlying neurologic disease such as multiple sclerosis. Past medical and surgical history are noted because these may be contributing factors to the patient’s problems. OAB symptoms can often be traced to other comorbidities such as poorly controlled diabetes or history of a stroke. UR can be preceded by radical pelvic surgery or a long-standing history of voiding dysfunction.

Physical examination focuses on evaluation of concurrent anatomic abnormalities such as demonstrable pelvic organ prolapse and/or stress urinary incontinence (SUI) during a supine stress test in female patients. These issues can complicate successful treatment of urgency by SNM, which is rendered moot by profound SUI. Saddle sensation should be evaluated and anal sphincter tone assessed. Testing of the bulbocavernosus reflex evaluates the S2-S4 pelvic nerve root. Absence suggests an underlying neurogenic process that may need further evaluation (8). In male patients, a digital rectal exam can suggest a prostatic etiology for outlet obstruction. Initial evaluation tests include a urinalysis to screen for infection and a postvoid residual in patients who complain of incomplete emptying. A 3-day voiding diary is very useful in identifying patients who are candidates for SNM as well as educating patients on their fluid intake and habits. This also serves as an objective measure of urgency and incontinence. If the patient performs
clean intermittent catheterization, a voiding/catheterization diary gives a baseline of the patient’s symptoms as well as the patient’s storage capacity. Urodynamics is reserved for cases where mixed urinary incontinence and abnormal compliance in a neurogenic bladder confound the diagnosis. Fluorourodynamics evaluates whether the patient has obstructive voiding amenable to surgical correction versus non-neurogenic voiding dysfunction, which may be ameliorated by pelvic floor training or SNM. Cystoscopy is selectively performed if there is a concern for bladder outlet pathology.

INDICATIONS FOR SURGERY

After a thorough workup, patients diagnosed with either refractory UUI, urgency-frequency or nonobstructive UR can be considered candidates for SNM. Contraindications exist for SNM. Although SNM can be done in short outpatient staged procedures or with an in-office peripheral nerve evaluation followed by one-stage implantation, all patients should be cleared for surgery. Candidates for therapy must be able to lie in the prone position throughout the duration of the procedure. SNM is not currently recommended in patients who need to undergo spinal magnetic resonance imaging (MRI) due to the theoretical risk of lead heating/dislocation and/or implantable pulse generator (IPG) torquing/malfunction/displacement. Nevertheless, a retrospective review of nine patients who underwent 15 MRIs including that of the lumbar spine showed no adverse effects to MRI interpretation or device functioning (9). Although this review had similar results to other studies, SNM placement in patients with known neurologic disease necessitating frequent spinal MRIs cannot be advocated at this time. Magnetic resonance (MR) of the head at 1.5 Tesla is conditionally approved in patients with InterStim. MR studies should not be performed in patients who are in the testing phase (10). Given the unknown risk of teratogenicity, SNM is relatively contraindicated during pregnancy. Patients with a functioning SNM should have their IPG turned off for the duration of their pregnancy. All patients should have cognition which allows them to properly operate the iCon controller. Performing a mini-mental status exam or clock test to evaluate baseline cognition can be helpful in evaluation for candidacy. Lastly, complex anatomy or prior history of surgery or trauma in the area of the sacral foramina may preclude a patient from being a candidate for lead placement.

ALTERNATIVE THERAPY

All patients are counseled on therapy options based on their initial complaint. For those with UUI or urgency-frequency, first-line therapy consists of behavioral modification. Secondline therapy for OAB includes pharmacotherapy. Alternative third-line therapy options such as percutaneous tibial nerve stimulation (PTNS) and onabotulinumtoxinA injection into the detrusor should be discussed. PTNS is a minimally invasive stimulation of the posterior tibial nerve with weekly 30-minute sessions for an initial period of 12 weeks. OnabotulinumtoxinA has been approved for use in both idiopathic and neurogenic detrusor overactivity patients. For the patient who has failed all therapy options, augmentation cystoplasty exists as a fourth-line treatment. Treatment options for patients with nonobstructive UR include intermittent catheterization or surgical management with urinary diversion when appropriate (11). Conservative treatment with fecal incontinence includes physical therapy with biofeedback. For the refractory patient, surgical management including sphincteroplasty, artificial sphincter, and SNM can be considered as other options.

SURGICAL TECHNIQUE

The implantation of SNM has evolved with time. In 2002, the tined lead was introduced, which obviated the need for larger incisions to anchor the lead to the thoracolumbar fascia. (Fig. 47.1) The IPG was originally placed in the lower anterior abdominal wall and now is placed in the gluteal region. The size of the InterStim II 3058 IPG is also smaller with the tradeoff of decreased battery longevity (Fig. 47.2).

SNM involves test stimulation and then implantation. This is one of the benefits of SNM as a therapeutic option. The patient tests the therapy prior to proceeding to implantation. The test stimulation can be done with either peripheral nerve evaluation (PNE) or implantation of a tined lead. The PNE involves placement of a nonpermanent lead followed by a single operation for permanent lead and IPG implantation. The staged operation consists of test stimulation with a tined lead placed in the operating room and then subsequent IPG implantation if successful.

Peripheral Nerve Evaluation Test Stimulation

PNE is an office-based procedure that can be used to test patients prior to implantation of the tined lead and IPG during a single surgery. PNE offers the evaluation of efficacy without the risks and inconvenience of two separate operating room anesthetic procedures. In addition, the standard placement of a PNE lead in the right and left foramen during the trial allows the surgeon to test both sides. Efficacy needs to be seen on only one side to lead to single-stage implantation. Historically, initial permanent lead placement required a morbid cutdown procedure to the thoracolumbar fascia or sacrum for securing the lead. Patient selection is important prior to justifying PNE versus a staged implantation due to the increased risk of lead migration with a non-tined lead and shorter trial period. The PNE lead is placed under local anesthetic. Fluoroscopy may or may not be used. Sensory and motor responses are evaluated. Given the non-tined lead, the trial is set for 6 to 7 days versus the longer trial period in a staged evaluation. Although up to 50% of patients will have a successful PNE trial, numerous patients fail either due to lead migration, accidental lead discontinuation, inappropriate length of trial, or inability to perform complex programming (12). Thus, patients who fail PNE or have equivocal results may still undergo a test stimulation with a tined lead.

We prefer to perform PNE in our office setting with local anesthetic. The patient is asked to shower with a chlorhexidine scrub the night before. In the office, the patient changes into a gown and is placed in the prone position with flexion at the hips and knees. A towel roll or support can be used to aid with flexion. The lower back, presacral area, and buttock are prepped with a skin prep solution. The patient is then draped with the anus exposed to evaluate for bellows. If the patient’s body habitus makes bellows difficult to observe, taping of the
buttocks prior to prepping can aid with visualization. It is often useful to have an assistant in the room to help the patient and the surgeon. A grounding pad for the lead is placed on the calf or heel of the patient. The red cord from the screener cable is connected to the grounding pad. The white connector to the screener cable is connected to the screener box. The black plug of the screener cable is then connected to the patient cable.

FIGURE 47.1 A and B: 3889 InterStim tined lead. Note the four electrodes that are conventionally labeled 0 to 3 from the furthest lead to the closest lead as appearing in this figure. In the artist’s rendition, the electrodes are labeled 0 to 3 and correspond to the areas on the distal portion of the lead that are tested with a J-hook. Also note the two white lines, which are used as guides when the sheath is pulled back for testing of the electrodes. (Reprinted with the permission of Medtronic, Inc. Copyright © 2015.)

FIGURE 47.2 InterStim I IPG (bottom) versus the current Inter-Stim II IPG (top). Although smaller in size, the InterStim II IPG has a shorter battery life. (Reprinted with the permission of Medtronic, Inc. Copyright © 2015.)

Localization of the S3 Foramen

The procedure can be performed with or without fluoroscopic localization. If no fluoroscopy is used, palpable bony landmarks include the tip of the coccyx, the sciatic notches, and the curve of the sacrum. The approximate location of the S3 foramen is found by measuring 9 cm cephalad from the tip of coccyx in the midline and 2 cm lateral from the midline to identify the foramen on either side. An additional mark is made at a level 11 cm from the tip of the coccyx because this will allow for a perpendicular entry of the finder needle to the sacrum and optimal positioning of the finder needle through the S3 foramen (Fig. 47.3). Another method of localizing the S3 foramen is based on the sciatic notch. The S3 foramen is typically at the level of the sciatic notch and a line can be drawn from the left and right notches if they can be palpated. The sacrum defines midline and the foramina should be 2 cm lateral on either side of the midline. A less reliable landmark that can be used for localization is the curve of the sacrum. Theoretically, the sacral crest is S4. Thus, 2 cm cephalad of S4 would be the plane for the S3 foramen.

If fluoroscopy is used during the procedure, a “cross-hair” technique has been described by Chai and Mamo (13). In an anteroposterior (AP) view, the inferior border of the sacroiliac joints is identified. A foramen needle is placed over this area. The spinous process is then marked midline with a directional guide (Fig. 47.4). A mark is made, and the foramen should be 2 cm lateral to the mark on either side. The finder needle should be inserted into the dermis above this marking to achieve an angle of entry perpendicular to the bony plate. This equates to a 60-degree angle placement through the skin. In contrast, if the finder needle were placed directly at the mark on fluoroscopy, a 90-degree angle through the
skin would correlate to a more difficult entry into the foramen given the curve of the sacrum (Fig. 47.5).

FIGURE 47.3 Using landmarks such as the anal verge to estimate the location of the S3 foramen. This ruler demonstrates the 9 and 11 cm marks measured out midline from the anal verge.

Placement of Peripheral Nerve Evaluation Lead

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