Clinical Review

Interstim: An implantable device for implacable urinary symptoms

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Urge incontinence, urgency–frequency, and retention may yield to electrical stimulation


Sacral neuromodulation stimulation (SNS) offers a less invasive alternative treatment for a difficult challenge: how to improve quality of life for patients with refractory lower urinary/pelvic floor disorders. In the past, the options were limited to radical surgical procedures such as urinary diversion, augmentation cystoplasty, or cystectomy.

The neuromodulation technique has been used for treatment of other disorders such as deep brain stimulation for Parkinson’s disease and vagal nerve stimulation for epilepsy. SNS stimulates the sacral nerves to modulate the neural reflexes that influence the bladder sphincter and pelvic floor.

The implantable InterStim device is FDA approved for the treatment of both storage and release disorders:

This article discusses several clinical trials that investigated its effectiveness, indications, and adverse events.

InterStim: The backstory

What is InterStim?

InterStim is an implantable system comprising a lead with 4 electrodes, an extension cable, and a programmable impulse generator. Usually, the lead is implanted into the S3 sacral nerve root, and the impulse generator is placed in the upper buttock region.

FDA approval

SNS for lower urinary tract disorders has been under investigation in clinical trials since 1981 and in multicenter trials conducted during the late 1980s. In 1997, the sacral neuromodulation system, InterStim (Medtronic Corp, Minneapolis, Minn), was FDA approved for treatment of urge incontinence, and, in 1999, for treatment of urinary frequency and urinary retention.

For tough cases

This method of therapy has proven to be an alternative treatment for a difficult patient group—those in whom pharmacological and behavioral management have failed. Therefore, considerable interest continues in the development of this method of therapy.

The delicate balance of stimulation and inhibition

A review of the neuroanatomy helps to explain the possible underlying mechanism of the workings of SNS. In the human, stimulation of the parasympathetic system through the pelvic nerves (S2–S4) leads to bladder emptying, and stimulation of the sympathetic system through the hypogastric nerves (T10–L2) leads to inhibition of bladder emptying.

The pudendal nerve receives information from Onuf’s nucleus in the S2–S4 level and innervates the skeletal muscle of the pelvic floor/external urethral sphincter.

The contraction and relaxation of these muscles are integral to micturition and bladder storage. A physiologic coordination exists between the somatic and the autonomic nervous system; it is when this delicate balance of stimulation and inhibition is changed by disease states that problems of urinary urgency, urge incontinence, and urinary retention result. Two guarding reflexes also work to prevent incontinence:

Bladder afferent pathways are mediated by unmyelinated C fibers (which detect noxious stimuli and trigger voiding) and small myelinated A fibers (which detect bladder muscle tension or fullness). Pudendal afferent input can turn on voiding reflexes by suppressing the guarding reflex pathways, and pudendal afferent input can also turn off overactive voiding by blocking ascending sensory pathways.

Thus, stimulation of the sacral nerves through pudendal afferent signaling influences bladder storage and emptying. Sacral neuromodulation affects these mechanisms by stimulating the somatic afferent inhibition of sensory processing in the spinal cord.

Does SNS successfully treat unrelenting urgency–frequency?


Hassouna M, Siegel S, Nyeholt A, et al. Sacral neuromodulation in the treatment of urgency–frequency symptoms: a multicenter study on efficacy and safety. J Urol. 2000;163:1849–1854.

All patients in this small, prospective, multicenter randomized trial had refractory urinary urgency–frequency. They completed extensive urologic evaluation consisting of voiding diaries, urodynamic testing, physical examination, and symptom questionnaires. Of the 51 patients, 90% were female and 10% were male. All patients underwent an office-based percutaneous stimulation trial of 3 to 7 days, to determine response to sacral nerve modulation. As practice guidelines dictate, a successful trial was determined if 50% or more of any urinary symptoms were relieved.

The patients were randomly assigned to 2 groups: the control group (n=26) that received no SNS and the study group (n=25), which underwent InterStim implantation. At 6 months, urinary symptoms in all patients were reassessed by urodynamic testing, SF-36 health survey, and voiding diaries.

In the control group, patients’ voiding diaries, symptoms, and urodynamic testing results remained unchanged. In the treatment group, 56% of patients had a greater than 50% reduction in the number of voids (<8) per day, compared with the control group. The treatment group had a significant increase in average voided volume from 118 to 226 cc, and a reduced degree of urgency. The treatment was associated with improved quality of life measured using the SF-36 health survey.

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