Percutaneous Nerve Evaluation
Percutaneous nerve evaluation (PNE) is the office-based or outpatient test-stimulation technique used to determine whether a patient should proceed to permanent Sacral Neuromodulation (SNM). A temporary monopolar wire is placed through a foramen needle near the S3 sacral nerve root and connected to an external pulse generator; durable implantation is usually offered only when the trial produces at least 50% improvement in the target diary variable.[1][2][3]
See also: OAB & Urgency Urinary Incontinence, Underactive Bladder, Fecal Incontinence, and Sacral Neuromodulation.
Indications
PNE screens patients being considered for SNM. The 2024 AUA/SUFU OAB guideline permits minimally invasive treatment without mandatory behavioral or drug trials , but US InterStim urinary labeling still requires failed or intolerable conservative treatment.[27][28] The selected device's indication must cover the presenting problem:
| Indication | Role of PNE | Practical endpoint |
|---|---|---|
| Urgency urinary incontinence / OAB | Tests symptom response before SNM implantation, subject to the chosen device's narrower label | At least 50% improvement in the prespecified primary diary symptom[2] |
| Urgency-frequency syndrome | Tests whether S3 stimulation improves storage symptoms | Reduced frequency and larger voided volumes |
| Non-obstructive urinary retention | Tests whether neuromodulation improves emptying after mechanical obstruction is excluded | Fewer catheterizations, lower catheterized volumes, or spontaneous voiding |
| Female functional bladder outlet obstruction | A separate selection pathway: EAU 2026 weakly recommends SNM for selected women after diagnostic evaluation[25] | Spontaneous voiding, fewer catheterizations, or lower residual volume |
| Fecal incontinence | Screens for SNM candidacy; ASCRS emphasizes test response as the best predictor | At least 50% reduction in fecal-incontinence episodes[4] |
The therapeutic trial itself, whether PNE or a staged tined-lead trial, remains the most useful predictor of SNM benefit. Baseline selection still starts with phenotype, exam, diary data, postvoid residual, and urodynamics or anorectal physiology when the diagnosis is complex.[3][5]
Pre-Procedure Setup
| Step | Key point |
|---|---|
| Position | Prone on a radiolucent table; tape the buttocks apart so perineal bellows and toe response can be seen |
| Anesthesia | Local anesthesia with or without light sedation; keeping the patient awake preserves sensory feedback[5][6] |
| Baseline diary | Collect urinary or bowel diary data before the trial so the response denominator is real |
| Imaging | Fluoroscopy can help with AP/lateral localization and difficult landmarks; a representative Axonics PNE manual permits bony landmarks and/or fluoroscopy[7][8][9][22] |
| Equipment | PNE foramen needle, temporary monopolar wire electrode, external stimulator, external pulse generator, sterile dressing |
For an Axonics basic PNE trial using the Model 1901 lead and Model 1601 trial stimulator, inability to operate the Axonics trial system is a listed contraindication.[22] The Model 1601 trial-stimulator manual warns against diathermy, calls for preprocedure coordination when a pacemaker or defibrillator is present, and instructs patients to keep the trial stimulator and lead dressing dry.[26] Infection, bleeding risk, obstruction, and inability to complete a useful diary also need assessment. Check the complete current prescriber booklet and model instructions before testing; the Model 1901 implant manual refers elsewhere for fuller warnings.[22]
No catheter or wound drain is placed for PNE itself; the temporary monopolar wire and its dressing are the only indwelling components, removed at the end of the trial. This is a brief, usually local-anesthesia office procedure without a planned catheter or period of immobility, so apply the general risk framework (age, obesity, prior VTE, malignancy, prolonged operation or immobility) on the VTE Prophylaxis page.
Fluoroscopic Landmarking
When fluoroscopy is used, AP imaging can identify the S3 foramina and lateral imaging can help with depth and trajectory. Axonics Model 1901 PNE instructions also permit bony landmarks and/or fluoroscopy, target the superior-medial foramen and confirm placement by motor and/or sensory responses; anatomy can produce a useful response at another sacral level.[9][10][11][22]
The "bullseye" maneuver, borrowed from endourology access, rotates the C-arm roughly 30 degrees so the foramen opens from an ellipse into a more circular target.[7]
Technique
- Prep and drape with the patient prone, buttocks taped apart, and the feet visible.
- Localize the intended foramen with anatomic landmarks and, when used, AP/lateral fluoroscopy; confirm trajectory and response under the selected system's instructions.
- Infiltrate local anesthetic in the skin and subcutaneous tract while avoiding excessive deep anesthetic around the nerve target.
- Plan the skin entry and direct the foramen needle toward the superior-medial aspect of the target foramen; the cited bullseye technique places entry approximately 2 cm cephalad to a horizontal mark at S3.[7][22] With Axonics Model 1901, limit foramen needle passes and depth; stop at the desired response, usually at about 2.5–4.0 cm with the tip at the anterior surface of the foramen.[22]
- Connect the needle to the test stimulator and test at low amplitude.
- Adjust needle depth and trajectory for a reproducible response at the intended root, usually S3. If the response remains unsuitable, the Axonics Model 1901 instructions permit testing an adjacent foramen or the contralateral side, including an appropriate S4 target.[22]
- Thread the temporary monopolar wire electrode through the needle.
- Stimulate the lead and confirm the desired motor or sensory response before removing its stylet; reposition while adjustment remains possible. Then withdraw the foramen needle and stylet without displacing the lead, and reconfirm stimulation.[22]
- Secure the wire with adhesive strips and an occlusive dressing; avoid tension loops that pull during sitting, bending, or toileting.
- Connect the wire to the external pulse generator. For Axonics Model 1901, check impedances between the lead, ground pad if used, and trial stimulator, and resolve faults before proceeding. Teach the patient how to adjust amplitude, protect the dressing, and complete the diary.[22]
Correct S3 Response
Correct placement is confirmed by a concordant motor and/or sensory response at low stimulation amplitude. Published data on sensory-only responses conflict. In 208 PNE procedures for fecal incontinence, outcomes did not differ between sensory-only and combined sensory-motor responses.[5] In a separate 35-patient urinary series using a staged quadripolar test lead, 95% of those with a successful test had a motor response; a sensory-only response carried a 4.7% chance of a positive test.[12] An awake patient remains valuable when the motor response is subtle or absent.
| Finding | Interpretation |
|---|---|
| Bellows response | Pelvic-floor / anal-sphincter contraction with deepening of the gluteal cleft; it can occur with S3 or S4 stimulation, so pair it with great-toe response and sensory field to distinguish the level[22] |
| Great-toe plantar flexion | Supports S3 placement when paired with bellows |
| Perineal, vaginal, scrotal, or rectal sensation | Appropriate sensory field; may be the deciding response in sensory-only patients |
| Lateral foot rotation or calf response | Suggests S2 stimulation |
| Anal-only response without toe response | Suggests S4 stimulation or too-caudal placement |
Aim for a reproducible appropriate response at low amplitude.[10][11][12][22] In the Axonics Model 1901 PNE manual, <2 mA refers to the initial needle opening threshold, whereas an optimal lead motor response is ≤3 mA.[10][11][12][22]
Trial Programming and Diary
| Parameter | Typical setting | Practical note |
|---|---|---|
| Frequency | 14 Hz | Standard starting frequency for SNM test stimulation |
| Pulse width | 210 microseconds | Usual starting pulse width |
| Amplitude | Just below comfortable sensory threshold | Avoid painful stimulation; patients can adjust within the instructed range |
| Trial duration | Often 3-7 days for a temporary PNE wire | Do not extend a specific temporary-lead system beyond its labeled duration; Axonics Model 1901 is intended for no more than 7 days. Longer staged tined-lead testing is a different pathway[22] |
The patient should complete a voiding or bowel diary throughout the test. Success is usually defined as at least 50% improvement in the primary symptom metric compared with baseline: fewer urgency-incontinence episodes, lower voiding frequency, fewer catheterizations or lower catheterized volumes, or fewer fecal-incontinence episodes.[2][3][13]
In a prospective study of 45 patients tested with an implanted tined lead, 29 met the 50% improvement cutoff; mean time to success among responders was 3.3 days (range 1-9), and the authors concluded that testing beyond 2 weeks is unnecessary at that cutoff.[14] In a supervised 3-week test phase with weekly reprogramming (90 patients), cumulative test success rose over time; 42% of first-week nonresponders became successful candidates after reprogramming.[15] A temporary PNE wire must not be left in place beyond its labeled duration; the Axonics Model 1901 PNE lead is intended to be used for no more than 7 days.[22]
PNE vs Staged Tined-Lead Trial
| Feature | PNE | Staged tined-lead trial |
|---|---|---|
| Setting | Office or outpatient procedure | Procedure room or operating room |
| Lead | Temporary monopolar wire | Permanent quadripolar tined lead |
| Anesthesia | Local anesthesia, with or without light sedation | Local, sedation, or general anesthesia |
| Trial duration | Usually 3–7 days with a temporary wire; observe the selected model limit (Axonics Model 1901: no more than 7 days)[22] | Longer tined-lead testing, 1–3 weeks in the cited staged-implant model; duration depends on system and protocol[17] |
| Strengths | Fast, inexpensive, minimal invasiveness, low infection risk | More stable lead position, longer trial, lower false-negative risk |
| Limitations | Lead migration and short trial can create false negatives | More resource-intensive; one retrospective fecal-incontinence series reported infection in 10.5% of staged-test patients versus 0% of PNE patients[18] |
| If successful | Return for permanent tined lead and generator | Return for pulse-generator implantation only |
PNE is efficient and low-risk, but it can miss patients who later respond to a staged tined-lead trial. In one single-center series, 18 of 41 (44%) patients with a failed PNE proceeded to implantation after a successful subsequent tined-lead screen. Long-term success after permanent implantation appeared more related to a successful test phase than to which screening method was used in that series.[16]
Cost and adverse events differ by strategy. A Medicare-based model compared direct lead-and-generator implantation without a test phase with staged tined-lead implantation. Under assumed equal efficacy and modeled infection rates, direct implantation cost less when success exceeded 65.4% in ambulatory surgery centers or 61.3% in outpatient hospitals.[17] In a retrospective fecal-incontinence series, 37/45 PNE patients (82.2%) and 37/41 staged-test patients (90.2%) progressed to complete implantation after greater than 50% improvement in Wexner score during testing (p = 0.36).[18] Infection was reported in 0% of the PNE group and 10.5% of the staged group.[18]
Predictors of Success
| Predictor | Practical meaning |
|---|---|
| Bellows and toe response | Supports correct S3 targeting. A motor response was associated with successful first-stage testing in urinary series[12][13] |
| Concordant perineal sensation | Was associated with PNE success in one contemporary series[13]; in a fecal-incontinence series, sensory-only responders did as well as those with motor responses[5] |
| Younger age | Associated with higher PNE success in contemporary series[13] |
| Urgency-incontinence or fecal-incontinence phenotype | Was associated with PNE success in one 102-patient cohort; pain and constipation were not comparator indications[13] |
| Absence of neurologic diagnosis | Was associated with persistent optimal response at ≥1 month after implantation in one retrospective cohort[13] |
| Anal sphincter EMG in fecal incontinence | Simple electrophysiologic testing has shown useful positive predictive value in selected FI patients[19] |
In a retrospective 2015–2019 series of 102 PNE patients with urgency-frequency, urgency urinary incontinence, and/or fecal incontinence, 78/102 (76.5%) responded to PNE; among responders, 68/78 (87.2%) maintained an optimal response at ≥1 month after implantation.[13] Older series reported PNE response rates of 40–50% before routine fluoroscopy.[13]
Limitations and Complications
| Issue | Why it matters | Mitigation |
|---|---|---|
| Lead migration | Temporary wire is not anchored and can create a false-negative trial | Secure dressing, minimize tension, restrict vigorous bending / twisting, consider staged trial after negative PNE when suspicion remains high |
| False-negative result | PNE is less sensitive than staged tined-lead testing[16] | — |
| Infection | Low with standard PNE but higher with prolonged externalized testing; a 6-week tined-lead test for constipation had a 22% infection rate | Keep standard trials short; avoid extended externalized testing unless the benefit is clear[18][20] |
| Placebo / open-label response | Especially relevant for fluctuating syndromes and constipation | Anchor interpretation to diaries and clinically meaningful endpoints |
| Skin irritation / dressing failure | Can interrupt trial or dislodge the wire | Patient teaching and early troubleshooting |
| Retroperitoneal hemorrhage | A case report described presacral hematoma after PNE in an anticoagulated patient[29] | Assess anticoagulant use and investigate new abdominal or genital ecchymoses[29] |
| MRI during the trial | Medtronic's US InterStim MRI guidelines state that MRI has not been evaluated with test-phase (therapy evaluation) components, that the external neurostimulator is MR Unsafe and that MRI should not be prescribed while evaluation components are in place. Axonics labels its external trial system, including the external pulse generator and percutaneous leads, MR Unsafe | Schedule elective MRI before the trial or after the evaluation components are removed; if MRI is required, remove all evaluation components first[23][24] |
In a review of spinal cord, sacral and peripheral nerve stimulation, hardware-related complications (lead migration, failure, fracture) were more common than biological complications such as infection and pain over the implant.[21]
Practical Pearls
- Keep the patient awake enough to report perineal, vaginal, scrotal, or rectal sensation. A fecal-incontinence series found similar outcomes with sensory-only responses, but a urinary series found sensory-only responses rarely led to a successful test.
- Use fluoroscopy deliberately.[11] In a nine-patient physiologic study, cranial-medial needle positions produced stronger pelvic-floor EMG responses at low current.[11]
- Treat PNE as a screening test with false negatives. If the phenotype is strong and the PNE trial fails because of migration, poor stimulation, or a very short trial window, consider a staged tined-lead trial before abandoning SNM.
- Avoid prolonged externalized trials unless there is a specific reason; infection risk rises as trial duration stretches.
Key Takeaways
- PNE is the office-based trial technique for SNM, not a separate definitive treatment.
- Correct S3 localization uses landmarks or fluoroscopy together with physiology: bellows, great-toe plantar flexion, and concordant perineal sensation.
- A positive trial is usually at least 50% improvement in the target diary variable.[2]
- PNE is lower burden than staged tined-lead testing but has more false negatives because the temporary wire can migrate.
Videos
References
1. Herbison GP, Arnold EP. "Sacral Neuromodulation With Implanted Devices for Urinary Storage and Voiding Dysfunction in Adults." Cochrane Database Syst Rev. 2009;(2):CD004202. doi:10.1002/14651858.CD004202.pub2
2. Van Kerrebroeck PE, van Voskuilen AC, Heesakkers JP, et al. "Results of Sacral Neuromodulation Therapy for Urinary Voiding Dysfunction: Outcomes of a Prospective, Worldwide Clinical Study." J Urol. 2007;178(5):2029-2034. doi:10.1016/j.juro.2007.07.032
3. Noblett KL, Buono K. "Sacral Nerve Stimulation as a Therapy for Patients With Refractory Voiding and Bowel Dysfunction." Obstet Gynecol. 2018;132(6):1337-1345. doi:10.1097/AOG.0000000000002968
4. Bordeianou LG, Thorsen AJ, Keller DS, et al. "The American Society of Colon and Rectal Surgeons Clinical Practice Guidelines for the Management of Fecal Incontinence." Dis Colon Rectum. 2023;66(5):647-661. doi:10.1097/DCR.0000000000002776
5. Govaert B, Melenhorst J, van Gemert WG, Baeten CG. "Can Sensory and/or Motor Reactions During Percutaneous Nerve Evaluation Predict Outcome of Sacral Nerve Modulation?" Dis Colon Rectum. 2009;52(8):1423-1426. doi:10.1007/DCR.0b013e3181a91241
6. Butrick CW. "Patient Selection for Sacral Nerve Stimulation." Int Urogynecol J. 2010;21 Suppl 2:S447-S451. doi:10.1007/s00192-010-1274-2
7. Liem SS, Demus T, Jivanji D, Palmerola R. "Bullseye Technique to Optimize S3 Foramen Access: Applying a Trusted Endourology Technique to Pelvic Medicine." Urology. 2023;171:252-254. doi:10.1016/j.urology.2022.10.011
8. Luchristt D, Amundsen CL. "Strategies for Difficult Fluoroscopic Landmarking During Sacral Neuromodulation Lead Placement." Urology. 2023;174:218-220. doi:10.1016/j.urology.2022.12.029
9. Hendrickson WK, Amundsen CL. "Sacral Neuromodulation: Sacral Anatomy and Optimal Lead Placement." Int Urogynecol J. 2021;32(9):2545-2547. doi:10.1007/s00192-020-04615-8
10. Dodge NA, Linder BJ. "Techniques for Optimizing Lead Placement During Sacral Neuromodulation." Int Urogynecol J. 2020;31(5):1049-1051. doi:10.1007/s00192-019-04208-0
11. Vaganée D, Voorham J, Voorham-van der Zalm P, De Wachter S. "Needle Placement and Position of Electrical Stimulation Inside Sacral Foramen Determines Pelvic Floor Electromyographic Response-Implications for Sacral Neuromodulation." Neuromodulation. 2019;22(6):709-715. doi:10.1111/ner.12953
12. Cohen BL, Tunuguntla HS, Gousse A. "Predictors of Success for First Stage Neuromodulation: Motor Versus Sensory Response." J Urol. 2006;175(6):2178-2181. doi:10.1016/S0022-5347(06)00315-6
13. Kocher NJ, Derisavifard S, Rueb J, Goldman HB. "Predictive Factors of PNE Success in a Contemporary Series: A Single Institution Experience." Neurourol Urodyn. 2021;40(1):376-383. doi:10.1002/nau.24571
14. Jairam R, Drossaerts J, Marcelissen T, van Koeveringe G, van Kerrebroeck P. "Onset of Action of Sacral Neuromodulation in Lower Urinary Tract Dysfunction-What Is the Optimal Duration of Test Stimulation?" J Urol. 2018;199(6):1584-1590. doi:10.1016/j.juro.2017.12.053
15. Tilborghs S, Van de Borne S, Vaganée D, De Win G, De Wachter S. "A Supervised 3 Weeks Test Phase in Sacral Neuromodulation With a 1-Year Followup." J Urol. 2021;205(1):206-212. doi:10.1097/JU.0000000000001317
16. Marcelissen T, Leong R, Serroyen J, van Kerrebroeck P, de Wachter S. "Is the Screening Method of Sacral Neuromodulation a Prognostic Factor for Long-Term Success?" J Urol. 2011;185(2):583-587. doi:10.1016/j.juro.2010.09.103
17. Sun AJ, Harris CR, Comiter CV, Elliott CS. "To Stage or Not to Stage?-a Cost Minimization Analysis of Sacral Neuromodulation Placement Strategies." Neurourol Urodyn. 2019;38(6):1783-1791. doi:10.1002/nau.24075
18. Rice TC, Quezada Y, Rafferty JF, Paquette IM. "Percutaneous Nerve Evaluation Versus Staged Sacral Nerve Stimulation for Fecal Incontinence." Dis Colon Rectum. 2016;59(10):962-967. doi:10.1097/DCR.0000000000000668
19. Altomare DF, Rinaldi M, Petrolino M, et al. "Reliability of Electrophysiologic Anal Tests in Predicting the Outcome of Sacral Nerve Modulation for Fecal Incontinence." Dis Colon Rectum. 2004;47(6):853-857. doi:10.1007/s10350-004-0524-0
20. Yiannakou Y, Etherson K, Close H, et al. "A Randomized Double-Blinded Sham-Controlled Cross-Over Trial of Tined-Lead Sacral Nerve Stimulation Testing for Chronic Constipation." Eur J Gastroenterol Hepatol. 2019;31(6):653-660. doi:10.1097/MEG.0000000000001379
21. Eldabe S, Buchser E, Duarte RV. "Complications of Spinal Cord Stimulation and Peripheral Nerve Stimulation Techniques: A Review of the Literature." Pain Med. 2016;17(2):325-336. doi:10.1093/pm/pnv025
22. Axonics. Sacral Neuromodulation System: PNE Lead Implant Manual, Model 1901/Kit 1701, 110-0131-001 Rev D (2020), pp. 4-10 (model-specific; complete current prescriber/MRI manuals still govern). Manufacturer manual.
23. Medtronic. MRI Guidelines for InterStim Systems (neurostimulator Models 97810, 97800, 3058 and 3023), US instructions for use. M980291A040 Rev A, 2025-04-15. Warnings: MRI during therapy evaluation. Current MRI manual library.
24. Axonics. MRI Patient Guidelines for Patients With the Axonics SNM System, US instructions for use. 110-0093-001 Rev R (05/19/2021). Section 3.2, MR Unsafe devices.
25. European Association of Urology. EAU Guidelines on Non-neurogenic Female LUTS. EAU Guidelines Office; 2026, section 4.5.4.b.3.c. Guideline.
26. Axonics. Sacral Neuromodulation System: Trial Stimulator Manual, Model 1601, 110-0049 Rev R (2023), pp. 4-8. Manufacturer manual.
27. Cameron AP, Chung DE, Dielubanza EJ, et al. "The AUA/SUFU guideline on the diagnosis and treatment of idiopathic overactive bladder." Neurourol Urodyn. 2024;43(8):1742-1752. doi:10.1002/nau.25532
28. Medtronic. InterStim Basic Evaluation System: Indications, Safety, and Warnings. US healthcare-professional labeling. Manufacturer labeling.
29. Aldossary NA, Hassouna M. "Retroperitoneal hematoma post percutaneous sacral nerve evaluation: A case report." Urol Case Rep. 2022;43:102068. doi:10.1016/j.eucr.2022.102068