Comparison of Motor and Sensory Response With Interstim Stimulation

Overview

Sacral nerve stimulation (SNS) implantation is a minimally invasive procedure which has current FDA approval for urinary urge incontinence, urgency-frequency syndrome and non-obstructive urinary retention, and has been available in the United States since 1997. The SNS delivers non-painful, mild electrical pulses to the sacral nerves to modulate the reflexes that influence the bladder, sphincter, and pelvic floor to improve or restore normal voiding function. While SNS has been shown to have efficacy for the aforementioned conditions, the exact mechanism of action is unknown, but it is believed to work primarily through the somatic afferent system in promoting inhibitory reflex pathways to facilitate urine storage. The degree of stimulation is thought to be at a level that only evokes a sensory and not a motor response; however this has not been tested.

SNS involves a two-stage procedure. The initial phase is considered the test stimulation period where the patient is allowed to evaluate whether or not the therapy is effective in controlling her symptoms. There are two techniques that exist in performing the test stimulation. The first is an office-based procedure termed the percutaneous nerve evaluation (PNE). This involves placing a temporary electrode wire through the S3 sacral foramen under local anesthesia. The wire is secured with tape and connected to an external generator the patient wears for a trial period of 3-7 days. If patients have at least 50% improvement in their symptoms during the test phase, they are candidates for chronic implant of the lead and implantable pulse generator (IPG). The advantage of the PNE is that it is an incision free procedure performed in the office utilizing local anesthesia, and does not require hospitalization. The disadvantage comes from the fact that the wire is not securely anchored in place, and has the propensity to migrate away from the nerve with physical activity. The second alternative is known as a staged implant. This is typically performed as an outpatient procedure using local anesthesia, intravenous sedation, and intra-operative fluoroscopy. This procedure involves placement of the chronic quadripolar lead wire adjacent to a sacral nerve root (typically S3). The lead is self-anchoring and therefore reduces the potential for migration. The patient goes through a test phase that can last from 7-21 days. The advantage of this technique is that it allows for a longer trial period with minimal risk of lead migration. The chronic wire also has 4 electrodes that can each be trialed as the active electrode to achieve optimal improvement in patients' symptoms. In addition, during the 2nd stage, or final implant the previously placed tined-lead remains in place and is simply connected to the IPG. This eliminates the chance of variable lead placement from the test and implantation phases. The disadvantage of the staged implant is that it requires two visits to the operating room and may be more costly to the health care system. However, in a prospective study comparing the PNE to the staged implant, there was a significantly higher rate of conversion to implant with the staged procedure vs. the PNE (88% vs.46%). In addition, infection rates are not higher with the staged implant when compared to the PNE.