Sacral neuromodulation (SNM) has been U.S. Food and Drug Administration (FDA) approved for urgency incontinence since 1997, and over the years the indications have expanded to overactive bladder, urinary retention and fecal incontinence. It was Tanagho and Schmidt who demonstrated that stimulation of the sacral nerves could modulate detrusor and sphincter function.¹ Until recently, InterStim® was the only approved device on the market for sacral neuromodulation. The implantation initially required a large sacral cutdown, incision of the dorsolumbar fascia with identification of the S3 sacral foramen, placement of a quadripolar electrode through the foramen and securing of the lead to the periosteum. Once the lead was placed, it was tunneled to a subcutaneous pocket and connected to an implantable pulse generator (IPG). Due to the complexity of the procedure, most urologists were hesitant to adopt this technology. In 2002 a quadripolar tined lead was FDA approved, resulting in a simple percutaneous procedure to implant the lead, thus expanding the number of implanters and patients being offered this technology.² Unfortunately, other than the introduction of InterStim II in 2006–a smaller implantable pulse generator that lasted half as long (4 to 5 years)–there were no advances in sacral neuromodulation until approval of the Axonics® Rechargeable Sacral Neuromodulation System in September 2019.³

Figure 1. Axonics implantable pulse generator and patient remote. Reproduced with permission from Axonics.

Figure 2. Size comparison of Axonics, InterStim Micro and InterStim II implantable pulse generators. Reproduced with permission from Axonics and Medtronic.

Figure 3. InterStim Smart Programmer. Reproduced with permission from Medtronic.

Figure 4. Recharger disc and belt. Reproduced with permission from Medtronic.

Introduction of the Axonics SNM system addressed several limitations to traditional sacral neuromodulation. The device is full-body, 3 T magnetic resonance imaging (MRI) compatible, rechargeable with a 15-year battery life and significantly smaller (5.5 cc) than the InterStim II IPG (14 cc), and it has a simplified patient remote control that looks like a key fob (fig. 1). The remote can turn on and off the system, adjust stimulation levels and notify the patient when the battery needs to be recharged, all using changes in lights rather than a numeric readout. The system is current-controlled and adjusts automatically to maintain constant current when changes in impedance occur. The clinician programmer wirelessly connects to the implantable pulse generator without the need for a separate transmitter over the IPG. Finally, the implant technique feels very similar to the Medtronic® InterStim system, making the transition simple for the experienced implanter.

The Axonics SNM system was the first new product in this space since 1997, and the technology is very exciting. However, there are a couple of limitations. First, it offers only a rechargeable product, and not all patients are willing or able to charge a device. Thus, a fixed-battery option is needed, which apparently is being developed. Second, although its patient remote is very simple, it only holds a single program, and patients would need to return to the office to have their program changed if efficacy wanes. This may be difficult, especially if patients travel a long distance. Some patients maintain efficacy with sacral neuromodulation and rarely require changes to their stimulation parameters. However, others require frequent program changes to maintain clinical efficacy. Expanding patient-controlled programming options would be a definite benefit.

Not to be outdone, Medtronic introduced its InterStim Micro Rechargeable Sacral Neuromodulation System in August 2020. The InterStim Micro IPG is 80% smaller than the InterStim II (14 cc) and almost 50% smaller than Axonics (2.8 vs 5.5 cc, fig. 2). Like Axonics, it has a current-controlled system, maintaining constant current with changes in impedance. The Micro has a 15-year battery life using the Medtronic Overdrive™ battery technology, which reportedly has zero battery fade, more rapid charging than traditional lithium-ion batteries and no battery damage if fully discharged. The device is full-body, 3 T MRI compatible using the SureScan™ leads, and these MRI-safe leads are now part of the InterStim II fixed battery system, making that MRI compatible. The Interstim Micro connects to Medtronic Smart Programmer, which has the familiarity of a smartphone and gives the patient the ability to try up to 11 distinct programs without an office visit (fig. 3). This can be helpful for patients who have more complex pelvic floor issues such as dual incontinence and pelvic pain, but may be overwhelming for the elderly or those who are less tech savvy. In addition, the clinician uses the patient’s Smart Programmer with a unique clinician app to interrogate and reprogram the device. On the downside, the programmer requires a communicator that is placed over the IPG to connect to the Smart Programmer, unlike Axonics, which is a direct connect.

Both Axonics and InterStim Micro require a disc-like charger that is connected to a belt and placed over the IPG until the system is fully charged (fig. 4). That charger itself needs to be charged in a charging dock that is plugged into the wall. Axonics IPG can be charged once per month. Medtronic recommends weekly charging, for 20 to 30 minutes. Patients must be willing and able to charge for successful therapy. If they are not a good candidate, then a fixed battery system should be used.

Sacral neuromodulation is a restorative therapy that is FDA approved for voiding and bowel dysfunction but can provide secondary gains in other pelvic floor disorders such pelvic pain, irritable bowel and sexual dysfunction.⁴ The new rechargeable SNM systems not only reduce health care costs because of their longevity, but also may be more acceptable to patients due to their small size and MRI compatibility. In the end, hopefully these advances in technology will expand the number of patients treated with neuromodulation who suffer from pelvic floor disorders, and improve their quality of life.