Sacral nerve stimulationn or InterStim® is a form of reversible surgical therapy for urinary frequency, urgency, urge incontinence (also known as OAB or overactivity of the bladder) or idiopathic non-obstructive urinary retention.
This therapy was initially approved by the FDA in 1997 for urge incontinence and soon after was approved for urinary frequency, urinary urgency, and idiopathic non-obstructive urinary retention. Future use of sacral nerve stimulation in the United States will include fecal incontinence and possibly chronic constipation. Sacral nerve stimulation is currently in use in Europe for fecal dysfunction.
Candidates for sacral nerve stimulation include patients that have undergone unsuccessful treatment for overactivity of the bladder or non-obstructive urinary retention with either medications or other therapies such as biofeedback, tibial nerve stimulation, hydrodistention of the bladder, urethral dilation, or bladder instillations. Those patients unable to tolerate medications for these conditions may be candidates for sacral nerve stimulation. Some insurance companies require patients to have at least one year of symptoms prior to approving sacral nerve stimulation.
There are often other conditions or symptoms that coexist in patients that are candidates for sacral nerve stimulation. Many patients with urinary issues also have fecal issues. Fecal incontinence, irritable bowel syndrome, spastic colon, and constipation often occur in tandem with urinary frequency, urgency, urge incontinence and idiopathic non-obstructive urinary retention. Also, many patients with these issues will have been diagnosed with interstitial cystitis, urethral syndrome, fibromyalgia and chronic migraine headaches. Frequently this patient group will experience issues with pelvic pain, prostate pain, anorectal pain, bladder pain, and pain with intercourse (dyspareunia). It is not uncommon to see a history of depression or anxiety in patients with significant voiding dysfunction.
Theories regarding the mechanism of action for sacral nerve stimulation are numerous. Central to the theory is that voiding dysfunction is often a neurologic problem and not a problem with the bladder or urethra. The neurological model for voiding dysfunction helps explain the use of sacral nerve stimulation for fecal dysfunction as the bladder, urethra, rectum and anus are innervated by the same portion of the nervous system (second through the fourth sacral nerves with the third sacral nerve providing a majority of input to the bladder). It is currently theorized that there is either increased amplification of sensory information from the pelvis to the brain or inappropriate utilization of primitive nerve fibers in patients with voiding dysfunction. In either case appropriate neurologic sequencing may be disrupted and voiding dysfunction may occur. This could be likened to a car that is out of timing or a computer that is short-circuiting. Sacral nerve stimulation is thought to impact nerve information transmission from the peripheral nerves to the brain improving neural sequencing. Some research suggests that sacral nerve stimulation may have primary action in the brain.
Sacral nerve stimulation is a two step surgical process. This is mandated by the FDA and all insurance companies. The initial operation is broadly described as the test stimulation. If a patient receives greater than 50% improvement in voiding complaints after test stimulation, the patient is considered to be a candidate for implantation of the InterStim® device.
Test stimulation occurs in two forms. One form of test stimulation is the percutaneous nerve evaluation (PNE). This form of testing is the original type of test stimulation and can be performed in the office, surgery center or hospital. It is favored by many surgeons as it can be performed under local anesthesia in the office. When the PNE is completed, the patient will have usually two small flexible wires (leads) in position adjacent to the right and left third sacral nerve exiting from the skin over the sacrum. One of these wires will be connected to an external pulse generator and programmed to give appropriate stimulation during the trial period. This wire may be changed to the opposite side several days later. The PNE usually only lasts several days as these leads will migrate away from the third sacral nerve with routine movement and sitting. Given this, the patient is asked to refrain from exercise or over activity during the trial period. The patient may go to work during this testing period as long as the position is reasonably sedentary. Frontal showers are possible during this testing period but baths and swimming are not possible. The lead can be removed easily in the office after completion of testing.
Another form of test stimulation is the staged implant. The staged implant is performed in either a surgery center or hospital. It can be performed under local anesthesia, sedation or general anesthesia depending on the surgeon, patient and anesthesiologist. The surgery differs from the PNE as a permanent lead with four electrode sites is implanted through a small incision over the sacrum and connected to an external cable to be attached to the external pulse generator during the testing phase. Placement of the permanent lead facilitates longer testing periods as the lead has significantly diminished chance of migration. As there are four electrode sites, many different combinations of stimulation may be employed to gain maximal benefit during the testing period. Similar limitations for activity hold for patients that undergo a staged implant. However, increased activity is possible after the lead has been in position for one week. This lead will remain in position if the testing phase is successful. If the testing period is unsuccessful, the lead will have to be removed surgically.
There are many opinions regarding which type of testing is most appropriate. The PNE is certainly a minimal procedure for most patients and can be performed in the office. Leads can be removed without difficulty if the patient has no success during the test. However lead migration with the PNE is not a question of if but when the lead will move out of position in relation to the third sacral nerve. Therefore an unsuccessful PNE could either be a technical issue (lead migration) or simply that the patient did not respond to sacral nerve stimulation. The physician often has a good idea of which occurred but on occasion the reason for PNE failure is uncertain. Moreover, the position of the PNE lead in relation to the nerve that provided benefit may not be obtained during the implant of the permanent lead necessitating revision of lead position. The staged implant is favored by some surgeons as migration is essentially a non-issue making technical failure less of a concern. Also, the implanted lead placed in during the staged implant is the lead that will be in position indefinitely in the patient that obtains benefit from therapy. There will be no further positioning required at the time of implantation. The trade-off for the patient is the need for a second operation even if the testing period is a failure. Also, there is an increased cost of test stimulation that is only an issue if there is failure of testing to provide significant enough benefit to move to implantation.
With either form of testing, success rates range from 65-85% for urinary indications. Success is defined as >50% improvement in urinary symptoms during the testing phase. Objective and subjective parameters are evaluated to determine success.
After successful PNE or staged implant the patient undergoes implantation of the permanent InterStim components. If the patient underwent a PNE for testing, the permanent lead and implantable pulse generator (IPG) will be placed during the implantation surgery. The lead is placed through a small incision over the sacrum and the IPG is placed through a two inch incision in the upper aspect of the buttocks on the right. The IPG contains all of the necessary electronic components and battery for InterStim® function. A tunnel under the skin connects the lead to the IPG. Approximately 30 minutes are required for implantation of the lead and IPG. If a staged implant was performed for testing, only IPG implantation is necessary. This requires approximately 15 minutes of operative time. Both of these procedures are performed under either local anesthesia, sedation or general anesthesia. Recuperation after implantation is short with approximately one week of pain at the upper buttocks incision site. Generally, the patients are asked to refrain from heavy physical activity for 2 weeks to allow appropriate healing. After this healing period, the patient may engage in any activity desired.
At the time of surgery and in the post-operative period, the IPG will be programmed to provide maximal benefit. The detailed programming is performed by Medtronic personnel or physician office staff. There may be several programming sessions required to give maximal benefit to the patient. More simplistic programming can be performed by the patient with a hand programmer that is provided for use with the IPG. The patient programmer will allow changes in the intensity of stimulation, turning the IPG off or on, and changing between up to 4 stimulation programs. Patients may choose to change programs or intensity of stimulation at different times of the day or at times when symptoms seem to be most prominent.
There are very few potential issues related to InterStim® therapy. The IPG does have a finite battery life that varies with the type of stimulation required. In general the IPG will last for 3-7 years and require surgical replacement at that time. There can be infection develop at the lead or IPG sites requiring antibiotic therapy or in some cases removal of the components. Occasionally, patients will develop pain a the IPG site requiring medical therapy or removal. If there are mechanical issues with the InterStim® unit, surgery will be required to revise the components if problems are found during diagnostic evaluation of the IPG in the office. Most important, there have been no reported nerve injuries with sacral nerve stimulation. Also it is important to note that by turning the unit off or explanting the unit will bring the patient back to their baseline urinary symptoms.