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]
This page is the PNE technique companion to the broader SNM article. It should be read as a screening / trial procedure, not as a definitive therapy separate from SNM.
See also: OAB & Urgency Urinary Incontinence, Underactive Bladder, Fecal Incontinence, and Sacral Neuromodulation.
Indications
PNE screens patients being considered for SNM in the same indication set as permanent sacral neuromodulation:
| Indication | Role of PNE | Practical endpoint |
|---|---|---|
| Refractory urgency urinary incontinence / OAB | Confirms symptom response before SNM implantation | At least 50% reduction in urgency-incontinence episodes, urgency, or frequency |
| 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 obstruction is excluded | Fewer catheterizations, lower catheterized volumes, or spontaneous voiding |
| 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 | Use AP and lateral fluoroscopy; difficult landmarks can be managed with oblique adjustments and sacral-anatomy checks[7][8][9] |
| Equipment | PNE foramen needle, temporary monopolar wire electrode, external stimulator, external pulse generator, sterile dressing |
Contraindications are the same practical barriers that make SNM unsafe or uninterpretable: untreated infection, uncorrected coagulopathy, inability to manage the external generator or diary, uncontrolled obstruction, or an indication where symptom fluctuation makes a short open-label test uninterpretable.
Fluoroscopic Landmarking
Use AP fluoroscopy to identify the medial borders of the S3 foramina and mark the S3 level. Lateral fluoroscopy confirms depth and trajectory. The needle path should aim for the cephalad-medial S3 foramen, because stimulation at the cranial and medial aspect of the foramen produces stronger pelvic-floor recruitment at clinically useful thresholds.[9][10][11]
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. It is useful when sacral slope, obesity, bowel gas, or degenerative anatomy makes AP foramen access uncertain.[7]
Technique
- Prep and drape with the patient prone, buttocks taped apart, and the feet visible.
- Mark the S3 foramina on AP fluoroscopy, then confirm sacral depth and trajectory on lateral view.
- Infiltrate local anesthetic in the skin and subcutaneous tract while avoiding excessive deep anesthetic around the nerve target.
- Insert the PNE foramen needle about 2 cm cephalad to the S3 skin mark and direct it toward the cephalad-medial foramen.
- Connect the needle to the test stimulator and test at low amplitude.
- Adjust needle depth and trajectory until the response pattern is consistent with S3 rather than S2 or S4.
- Thread the temporary monopolar wire electrode through the needle.
- Withdraw the needle while leaving the wire adjacent to the S3 root.
- 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 and teach the patient how to adjust amplitude, protect the dressing, and complete the diary.
Correct S3 Response
Correct placement is confirmed by a concordant motor and/or sensory response at low stimulation amplitude. Sensory-only responses can still predict success, so an awake patient is valuable when motor response is subtle or absent.[5][12]
| Finding | Interpretation |
|---|---|
| Bellows response | Pelvic-floor / anal-sphincter contraction with deepening of the gluteal cleft; strongest visual clue for S3 |
| 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 the lowest reproducible threshold that produces a clinically appropriate response. Contemporary lead-placement literature generally treats low-amplitude responses, often at or below about 2 mA, as a marker of closer sacral-root proximity and better technical placement.[10][11][12]
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 | Usually 3-7 days; sometimes 7-14 days | Longer trials can capture delayed responders but increase dressing and infection burden |
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]
Onset is often early but not instantaneous. In one onset-of-action study, the mean time to response was about 3.3 days, with a range of 1-9 days, supporting a test window long enough to avoid premature false-negative interpretation.[14] A supervised 3-week test phase with weekly reprogramming increased cumulative success because some first-week nonresponders converted after reprogramming.[15]
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 days to 1-2 weeks | Usually 2-4 weeks |
| 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; infection risk is higher because hardware is implanted during the test |
| If successful | Return for permanent tined lead and generator | Return for pulse-generator implantation only |
PNE is efficient and low-risk, but it is less sensitive than a staged tined-lead trial. Patients who fail PNE may still respond to staged testing; in one long-term comparison, 44% of PNE failures responded to subsequent tined-lead testing.[16] Long-term success after permanent implantation appears more related to a successful test phase than to which screening method was used.[16]
Cost and adverse-event tradeoffs matter. A cost-minimization analysis found that starting with PNE can reduce total cost in appropriately selected patients, while fecal-incontinence data showed lower infection with PNE than staged testing but higher risk of false-negative screening.[17][18]
Predictors of Success
| Predictor | Practical meaning |
|---|---|
| Bellows and toe response | Supports correct S3 targeting and is associated with successful first-stage neuromodulation[12] |
| Concordant perineal sensation | Predictive even when motor response is limited; do not dismiss sensory-only success[5] |
| Younger age | Associated with higher PNE success in contemporary series[13] |
| Urgency-incontinence or fecal-incontinence phenotype | Often more predictable than pain-predominant or constipation phenotypes |
| Absence of neurologic diagnosis | Predicts more durable success after implantation in some series[13] |
| Anal sphincter EMG in fecal incontinence | Simple electrophysiologic testing has shown useful positive predictive value in selected FI patients[19] |
In a contemporary fluoroscopy-era PNE series, about 76.5% of patients responded to PNE and most responders maintained response after permanent implantation.[13] Historical PNE success was lower, partly because earlier series used less consistent fluoroscopic targeting and less refined lead-placement technique.
Limitations and Complications
| Issue | Why it matters | Mitigation |
|---|---|---|
| Lead migration | Most important PNE limitation; 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 | Do not equate failed PNE with absolute SNM failure in a good candidate[16] |
| Infection | Low with standard PNE but rises with prolonged externalized testing | Keep standard trials short; avoid extended PNE 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 |
Overall neuromodulation complications are usually hardware-related rather than biologic: migration, fracture, loss of efficacy, pain at the implant or wire site, and infection dominate the complication profile.[21]
Practical Pearls
- Keep the patient awake enough to report perineal, vaginal, scrotal, or rectal sensation; sensory-only responders can still benefit from SNM.
- Use fluoroscopy deliberately. A cranial-medial S3 target and low-amplitude response are more important than simply entering "a foramen."
- 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.
- Interpret response by the symptom that brought the patient to SNM, not by a generic global impression.
- 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 both fluoroscopy and physiology: bellows, great-toe plantar flexion, and concordant perineal sensation.
- A positive trial is usually at least 50% improvement in the target diary variable.
- PNE is lower burden than staged tined-lead testing but has more false negatives because the temporary wire can migrate.
- Failed PNE should not automatically exclude SNM when the phenotype and technical circumstances still support candidacy.
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