Percutaneous Nerve Evaluation

Sacral nerve stimulation is indicated for patients with medication and/or biofeedback refractory urinary frequency, urinary urgency, urge incontinence, and idiopathic non-obstructive urinary retention.

Past the indications for use of neuromodulation, use of a percutaneous nerve evaluation (PNE) versus a staged implant approach to the testing phase has to be considered for all patients. In my practice, I reserve the PNE for those patients whom are thought to have limited chance of success with sacral nerve stimulation or in patients that are thought to be poor surgical candidates. Most important is that the patient makes the decision about which form of testing they desire. A very large percentage of our patients go to staged implant over the PNE, primarily based on my biases but also as many of our patients do not want to undergo any discomfort during the procedure.

Generally patients can present to the office with little preparation. Patients are not required to fast prior to the PNE. However, it is best to have the patients collect information for 1-2 days in a voiding diary prior to the day of lead placement. Those patients that are on some form of anticoagulation (aspirin, NSAIDs, etc) should stop these at least one week prior to the procedure. Hibiclens scrub is not required but certainly can help decrease infection risk. We generally schedule the PNE on days when I am operating and fit the PNE in between cases as I am waiting for the operating room to turn-over. An exam room table is more than adequate for the procedure. Essentially all of the equipment and materials needed for the PNE are supplied in the Medtronic kit or by the Medtronic representative. Only Lidocaine, extra sponges and Tegaderm dressings are needed for the procedure beyond what is supplied in the kit.

Procedure

The patient is placed on the examination table in the prone position with pillows under the face, pelvis and shins. The sacral area, buttocks, and feet will need to be exposed for the procedure. The supplied grounding pad is placed on the heel of one of the exposed feet. Testing cables provided in the Medtronic kit are connected to the grounding pad and later to the hook electrode. The red tipped cable is connected to the grounding pad and the black tipped electrode into the hook electrode. Iodine skin preparation is performed over the sacrm, lower back, and area of the gluteal crease. Drapes in the Medtronic PNE kit are used mark the field of the procedure with the bottom drape across the lower to mid point of the gluteal crease, lateral drapes just past the edge of the sacrum, and the upper drape at the lower lumbar area. Approximately 9 cm from the coccyx is marked with the supplied surgical pen as the probable level of the S3 foramen. Given the angulation of the needle into the foramen the anticipated skin entry site is at least one centimeter cephalad to this point and is marked at the midline of the sacrum. Marks are also make approximately one centimeter lateral to this midline mark as the beginning sites for skin entry. These sites and possible entry sites caudad and cephalad are infiltrated with Lidocaine. The subcutaneous tissues are infiltrated as well.  Care must be taken in thin patients to be sure the infiltration is not entering the foramen.  Advancing the needle so as to tap the sacrum with the needle used to infiltrate the skin will confirm that the foramen is not in the field of infiltration. Usually 5-7 cc of Lidocaine is used per side.

A foramen needle is then placed into one of the marked sites at a 60 degree angle in relation to the skin with a medial tilt to the hub of the needle that places the hub approximately over the midline. The needle is advanced until the sacrum is encountered or the foramen is entered. If the sacrum is impacted by the needle, another 2 cc of Lidocaine is used to anesthetize the sacral table. Obviously, if the foramen is entered no further Lidocaine will be needed at that site. It is unusual to immediately enter the foramen and further exploration of the sacrum with the needle is needed to access the foramen. Passing the tip of the needle in both a caudal and cephalad fashion along the same vertical plane is a technique first used to find the foramen. If the foramen is not encountered, changing the position of the needle, moving either more medial or lateral, with exploration of the sacrum in both the caudad and cephalad direction in the new vertical plane, may eventually assist in finding the foramen.

After locating the foramen, the needle is passed just past the opening of the foramen into position adjacent to the nerve. Patients often will have a minor reaction to passing through the foraminal membrane. Do not infiltrate with Lidocaine while inside the foramen. The depth of the needle at this point varies with the body habitus of the patient.  In the average patient, the 5 cm mark with be at the skin level when entering the foramen. The supplied hook electrode is attached to the non-insulated portion of the needle and connected via the supplied cable to the external pulse generator. With slow increases in stimulation increments, the nerve is stimulated and attention is given to the great toe on the side of stimulation and to the area of the gluteal crease. Flexion of the great toe and a pulling in of the levator muscles at the upper area of the gluteal crease suggest position of the needle near S3. Also, sensation of stimulation in the perineum, rectum, vagina, or scrotum suggest appropriate position of the needle. If given the choice of muscular responses versus sensory responses only, I will generally choose the position that gives the best muscular responses. If responses are poor or non-existent, the position of the needle will need to be changed either in the same foramen (moving deeper or more superficial) or to another foramen. If excessive toe response, foot response or foot rotation is noted the needle will need to be positioned in a more caudad position. Leaving the previously placed needle in position as a guide for placement of the other foramen needle either further cephalad or caudad on the same side will lessen the time needed to enter the next foramen.

As appropriate stimulation parameters are obtained, the test lead is advanced into position through the foramen needle. Two areas on the lead have slightly less tight weave to signifiy the 3 and 5 inch points on the lead. The lead should be advanced to the appropriate mark based on use of either the 3 or 5 inch needle. The lead is tested for position by connecting the hook electrode to the non-insulated portion of the lead at the tip. With confirmation of position, the needle is carefully removed with care being taken not to change the position of the lead. Testing is performed once again to confirm position. A small Tegaderm dressing in the kit is used to fix the lead to the skin. The opposite side then has a lead placed in similar fashion.

Post-Procedure

After placement of the leads, a lead will need to be connected to the supplied short cable and to the remaining grounding pad. The excess cable of the remaining lead will be taped to the skin with a gauze dressing. Programming of the lead is performed with the external pulse generator after connected the cable. Patients are given instructions on use of the external pulse generator and some simple troubleshooting instructions. Also, the patients are encouraged to keep a voiding diary during the time of testing. If possible, a lateral sacral film and/or pelvic X-ray is performed to show position. Patients are seen in the office 3-4 days later and changed to the other lead for further testing if desired or needed. We do not prescribe pain medication or antibiotics following the PNE.

Notes and Surgical Pearls

Test stimulation with the PNE can be a difficult procedure, especially for the surgeon with little experience in sacral nerve stimulation. Patients are very much aware of a surgeons lack of confidence and become anxious in turn. Those surgeons who are early in their experience curve should consider PNE in a sedated setting in a surgery center or hospital operating room.

Patients who have a significant pain component in addition to the voiding problems may be difficult patients for a PNE in the office under local anesthesia.  Consider this patient group for sedation as well.

My experience with PNE placement is that I am usually too far caudad with my first needle placement. Consider this if there is minimal to no response with the first needle placement.

Obviously, most surgeons do not have fluoroscopy in the office. Placement of foramen needles and leads is greatly facilitated with fluoroscopic imaging. Obtaining a KUB and/or lateral sacral film will give future guidance for lead placement if success is obtained with the PNE.