Neuromodulation

Neuromodulation uses electrical stimuli to alter neurotransmission in refractory storage and emptying disorders. The mainstay is sacral neuromodulation (SNM) — the implanted InterStim system, placed in two stages or trialled by a percutaneous nerve evaluation (PNE) — with percutaneous tibial nerve stimulation (PTNS) as a less invasive alternative. The underlying physiology, drug therapy, and OAB algorithm are covered in the Functional Urology topic; this page is the operative reference.

Indications and Patient Selection

SNM is a third-line option for overactive bladder (after conservative therapy and medications fail) and is tried before invasive surgery such as augmentation cystoplasty or diversion — no urodynamic finding predicts response.

• FDA-approved indications — urge urinary incontinence (1997), non-obstructive urinary retention and urinary frequency (1999), overactive bladder (2002), and fecal incontinence (2011). It is not approved for neurogenic bladder or symptoms from known neuropathy (MS, Parkinson, spinal-cord injury).
• Mechanism — activation of somatic sacral afferent inflow at the sacral root modulates the storage/emptying reflexes: in OAB it modulates sensory outflow to the pontine micturition centre to prevent involuntary contractions, and in retention it inhibits the guarding reflex.
• Non-obstructive retention — a staged trial is preferred over PNE because these patients need a longer testing period. Fowler syndrome (young women with retention, impaired sphincter relaxation) responds to neuromodulation.
• Interstitial cystitis / bladder pain syndrome — not an approved indication, but SNM is a fourth-line option (AUA); a staged implant (with sedation) and an ipsilateral-to-pain lead are preferred.
• Fecal incontinence — assess with gastroenterology first and offer SNM after conservative measures fail.

Contraindications and Precautions

• Absolute — significant sacral/spine anatomic abnormalities, cognitive dysfunction, physical limitations preventing pelvic-organ function, and non-compliance.
• MRI is contraindicated except for head MRI (magnetic fields heat the lead), and diathermy (shortwave, microwave, therapeutic ultrasound) is contraindicated (device interaction can cause permanent nerve/tissue injury).
• Pregnancy — deactivate the device once pregnancy is confirmed (counsel women of reproductive age); continued therapy may be reasonable in retention to avoid catheter-associated UTI.
• Cardiac pacemakers — SNM appears safe with a pacemaker that lacks cardioversion/defibrillation (cross-talk risk otherwise).

Nerve-Root Responses

Lead placement targets the S3 nerve root, confirmed by motor (and, in an awake patient, sensory) responses to stimulation — aim for an opening threshold under 2 volts, eliciting a bellows response first, then the great-toe response.

Nerve root	Motor response	Sensory response
S2	Plantarflexion of the entire foot with lateral rotation; anal clamp	Leg and thigh
S3	Plantarflexion of the great toe + bellows reflex (anal wink/levator contraction)	Pulling in the rectum, scrotum, or vagina
S4	Bellows reflex only	Pulling in the rectum only

Note: Hinman Ch104 describes the S3 great-toe response as dorsiflexion rather than plantarflexion — a frequently muddled point in the literature; this table follows the study-note/standard teaching. The ideal sensory response is genital and perineal.

Sacral Neuromodulation Technique

The system is placed either as a two-stage implant (Stage 1 tined-lead trial, then Stage 2 generator) under monitored anaesthesia/local, or trialled by an office PNE under local anaesthesia.

Stage 1 — Tined-Lead Placement

1. Preparation — baseline voiding diaries; stop blood thinners ≥ 7 days before; chlorhexidine skin cleansing; IV cefazolin or vancomycin before incision. Position prone on a fluoro-friendly table with the feet uncovered to watch motor responses; prefer MAC/local (if general anaesthesia is needed, use only a short-acting paralytic).
2. Access the S3 foramen — on AP fluoroscopy mark the medial edge of the foramina; on lateral fluoroscopy the S2 level is the sacroiliac-joint fusion shadow, and the first "hillock" below it is the S3 disc space. Enter ~1 cm above this with the foramen needle (3.5-inch / 9 cm standard; 5-inch / 12.5 cm for larger habitus), "walking" it into the foramen so the tip just enters the anterior bony plate; stimulate to confirm the S3 response at < 2 volts.
3. Deploy the tined lead — make a 1-cm incision (deep enough to avoid superficial, fracture-prone placement); pass the introducer sheath over the directional guide (radiopaque marker halfway through the bone table); advance the quadripolar tined lead (electrodes 0–3, model 3889) with the curved stylet aimed "down and out," straddling the anterior bone table, and test all four electrodes (bellows then great-toe at 1–2 volts); deploy the tines with electrode 3 just distal to the anterior sacrum.
4. Connect and tunnel — site the future generator pocket on the buttock below the iliac crest and lateral to the sacrum; tunnel the extension cable subcutaneously, connect it to the lead (setscrew tightened to an audible click, boot secured with a polypropylene tie), and bury the connection hub deep and medial under the future generator site.
5. Close and trial — two-layer closure; run a ~3-week trial with voiding diaries; > 50% symptom improvement qualifies for Stage 2.

Stage 2 — Generator Implant

Extend the incision medially, externalise and disconnect the lead from the extension cable, and create a subcutaneous pocket ≥ 1 inch deep, just large enough to seat the generator flat (an oversized pocket lets it flip and risks lead migration). Connect the lead to the generator, coil excess lead behind it, and close in layers to obliterate dead space.

Percutaneous Nerve Evaluation (PNE)

An office trial of temporary leads under local anaesthesia — preferred for most patients as it avoids anaesthesia and an operating room. The PNE lead is non-tined (migration-prone) and unipolar (not steerable), so it is placed under fluoroscopic guidance, usually as bilateral test leads. Sensory responses (vaginal/perineal in women, scrotal/perineal in men) are relied on more than motor. Each lead is tested ~3 days; after a 7-day trial, > 50% improvement qualifies for a full implant (or proceed to Stage 1 for longer testing).

Explantation

For < 50% improvement, remove the tined lead and extension under local or in theatre — because the tines sit outside the foramen, the lead pulls out with constant pressure and no risk of nerve injury.

Percutaneous Tibial Nerve Stimulation (PTNS)

The tibial nerve (L4–S3) is stimulated to modulate the pelvic-floor/bladder/sphincter nerves. PTNS improves OAB comparably to antimuscarinics with a better adverse-effect profile and is a third-line OAB therapy (2019 AUA); its main limitation is the need for frequent sessions. Adverse effects are minor (stimulation discomfort, insertion-site bleeding).

Complications

• Infection — about 3%; treatment requires removal of the lead and generator, with at least 6 weeks before reattempting. Cefazolin is standard prophylaxis; reserve vancomycin for penicillin allergy or prior MRSA.
• Revision / removal — generator or lead revision in 7–32% and device removal in 8.5–18%. Consider generator revision for site pain, a flipping device, or too-superficial/belt-line placement; lead revision for loss of efficacy (after trauma or weight change), uncomfortable stimulation/lower-limb radiation, or a full implant less effective than the PNE. Evaluate with reprogramming, impedance checks (high impedance suggests lead fracture), and sacral radiographs.