528 CHAPTER 13 Neuropathic bladder
Neuromodulation in lower urinary tract dysfunction
Neuromodulation is the electrical activation of afferent nerve fibers to modulate their function. To treat urinary tract conditions, sacral and nonsacral neuromodulation can be employed.
Electrical stimulation applied anywhere in the body preferentially depolarizes nerves (higher current amplitudes are required to directly depolarize muscle). In patients with LUT dysfunction, the relevant spinal segments are the sacral nerves S2–4.
Neuromodulation is a second-line treatment for refractory lower urinary tract dysfunction, such as nonobstructive chronic urinary retention, urgency–frequency syndrome (overactive bladder), and urgency incontinence, when behavioral and drug therapy has failed.
Several sites of stimulation are available, with the electrical stimulus being applied directly to nerves, or as close as possible:
•Sacral nerve stimulation (SNS) is most widely studied.
•Pudendal nerve—direct pelvic floor electrical stimulation (of bladder, vagina, anus, pelvic floor muscles) or via stimulation of dorsal penile or clitoral nerve (DPN, DCN)
•Posterior tibial nerve stimulation (PTNS)
SNS
Sacral nerve stimulation, also known as sacral neuromodulation (SNM), uses continuous or cycling mode of electrical pulses to activate or inhibit neural reflexes associated with lower urinary tract function via stimulation of the sacral nerves.
The mechanism of action is not conclusive. One theory is that indirect stimulation of the pudendal nerve and direct inhibition of the preganglionic neurons suppress detrusor overactivity and therefore improve symptoms.
An alternate theory is that stimulation may inhibit involuntary reflex voiding by altering the transmission of sensory input from the bladder to the pontine micturition center, inhibiting ascending afferent pathways but not the descending pathways. In the patient with nonobstructive urinary retention, SNM most likely causes an inhibition of the guarding reflex, with a reduction in sphincteric overactivity that may reduce bladder outlet and urethral resistance.
Patients should have failed conservative management with medications and or behavioral therapies and should undergo extensive evaluation including urodynamics. The procedure involves implanting a programmable device that delivers a pulse low-amplitude stimulation to the sacral nerves (InterStim, Medtronic).
The procedure is typically performed in two stages. Percutaneous placement of the electrode is performed under fluoroscopic guidance near the S3 foramina. Position is tested by either verbal response from the patient or motor responses (tightening of the levators, bellows response in the anal area, plantar flexion of the great toe).
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