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Indications & Patient Selection
Pudendal neuralgia, Nantes criteria, block vs. ablation
Indications
- Pudendal neuralgia — chronic perineal pain (perineum, labia/scrotum, penis, anorectal region)
- Pain worse with sitting, better standing or lying
- Burning, electric shock quality, or hypersensitivity in pudendal distribution
- Entrapment at Alcock’s canal (by sacrospinous ligament, between sacrotuberous and sacrospinous ligaments)
- Post-surgical pudendal neuropathy (hernia repair, pelvic surgery)
- Post-radiation or childbirth injury to pudendal nerve
- Oncologic pain in pudendal distribution (vulvar, penile, rectal malignancy)
- Diagnostic block (Nantes criterion 5) before proceeding to ablation
- Cryoablation for refractory cases after positive diagnostic block
Contraindications
- Uncorrectable coagulopathy
- Active pelvic infection / gluteal abscess
- Pregnancy
- Negative diagnostic block (cryoablation unlikely to help)
- Structural cause not excluded (endometriosis, pelvic tumor, Tarlov cyst) — treat underlying cause first
Nantes Criteria for Pudendal Nerve Entrapment (All 5 Required)
| # | Criterion | Notes |
|---|---|---|
| 1 | Pain in the territory of the pudendal nerve | Perineum, anorectal, genitalia |
| 2 | Pain worsened by sitting | Key feature; relief standing or lying |
| 3 | Patient not woken by pain at night | Distinguishes from central sensitization |
| 4 | No objective sensory loss on examination | Objective loss suggests structural nerve damage |
| 5 | Positive diagnostic pudendal nerve block | ≥50% pain reduction lasting ≥24 h = positive |
Block vs. Cryoablation Decision
- Diagnostic/therapeutic block: first-line; local anesthetic ± steroid; positive block = ≥50% pain reduction for ≥24 h
- Repeat therapeutic block: up to 3–4 per year; avoid repeat same nerve injection within 4–6 weeks
- Cryoablation: indicated after definite positive block(s) in refractory disease; superior to chemical neurolysis for benign pain (reversible, lower dysesthesia risk)
- Pulsed RFA: alternative for ablation of this mixed motor/sensory nerve (42°C × 120 s) — preferred over continuous RFA to limit motor fiber injury
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Pre-Procedure Planning
Imaging, labs, diagnostic block first, consent
Imaging
- MRI pelvis (rule out structural cause: endometriosis, pelvic tumor, Tarlov cyst, pelvic floor dysfunction)
- MR neurography if available — increased T2 signal in pudendal nerve = inflammation/entrapment
- CT for procedural planning — visualize ischial spine, sacrospinous ligament, and pudendal canal geometry
- Review prior pelvic surgery operative notes for relevant anatomy distortion
Labs & Assessment
- INR, platelet count (SIR Cat 1 — thresholds: INR ≤2.5, plt ≥25K)
- Pudendal nerve terminal motor latency (PNTML) testing if available — abnormal (>2.2 ms) supports entrapment
- Baseline VAS pain score — document standing vs. sitting
- Prior pain management history: PT, pelvic floor therapy, prior blocks, medications
- Anesthesia consult if MAC planned (cryoablation cases)
MRI pelvis reviewed. Structural cause excluded or documented.
Diagnostic block performed first (for cryoablation planning). Document pain response and duration.
Consent obtained. Discuss temporary perianal/scrotal anesthesia, urinary retention, rectal injury, hematoma, AV fistula (rare), and infection.
Bilateral vs. unilateral block planned. Bilateral if bilateral symptoms; unilateral reduces side-effect risk if lateralized pain.
Driver arranged post-procedure. Motor block may prevent driving for up to 48 h.
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Anatomy
Pudendal nerve course, Alcock's canal, CT landmarks
Nerve Course
- Origin: ventral rami of S2–S4 nerve roots (mixed motor and sensory)
- Exits pelvis through greater sciatic foramen, below piriformis muscle
- Crosses ischial spine; enters perineum via lesser sciatic foramen
- Travels through Alcock’s canal (pudendal canal): between medial surface of obturator internus (lateral) and ischiorectal fat (medial), under the obturator fascia
- Entrapment most common between sacrospinous and sacrotuberous ligaments at ischial spine
Terminal Branches
- Inferior rectal nerve — cutaneous innervation of external anal sphincter and perianal skin
- Perineal nerve — superficial branch: posterior scrotal/labial skin; deep branch (muscular): bulbospongiosus, ischiocavernosus, external urethral sphincter
- Dorsal nerve of penis / clitoris — sensory innervation
CT Landmarks for Targeting
- Primary target (Alcock’s canal approach): medial border of obturator internus muscle in the pudendal canal — nerve runs as posterior-most structure of the neurovascular bundle; intermediate density compared to adjacent vessels
- Alternative target (ischial spine approach): nerve located between sacrospinous and sacrotuberous ligaments where they converge at ischial spine; injectate distributes anteromedially into Alcock’s canal
- Dual-site targeting: both ischial spine AND Alcock’s canal in same session increases block effectiveness
- Pudendal artery and vein run adjacent — aspirate before injection; always confirm no vascular opacification with dilute contrast
- Rectum lies medial — CT confirms needle tip position prior to injection
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Technique
CT-guided block and cryoablation — step by step
Supplies
22G Chiba needle
Dilute contrast (10%)
0.5% bupivacaine 5–10 mL
Triamcinolone 40 mg (therapeutic block)
Extension tubing
Standard sterile tray + lidocaine 1%
17G IceSense3 / IcePearl cryoprobe (ablation only)
Cryoablation console + argon supply (ablation only)
CT fluoroscopy suite
CT-Guided Block at Ischial Spine / Alcock’s Canal
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Positioning & CT Planning
Prone positioning. CT planning scan through pelvis — identify ischial spine and obturator internus bilaterally. Mark skin entry point for transgluteal approach (via gluteal fat, medial and posterior to ischial tuberosity).
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Skin Prep, Local Anesthesia
Standard sterile prep over gluteal region. 1% lidocaine skin wheal and subcutaneous track down to gluteal fascia. Note: periosteum of ischial spine is sensitive — adequate local anesthesia improves patient tolerance.
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Needle Advancement
Advance 22G Chiba needle via transgluteal approach to pudendal canal, targeting the medial border of the obturator internus. For ischial spine target: advance to midpoint between ischial tuberosity and ischial spine, just deep to the sacrotuberous ligament. Intermittent CT confirms trajectory.
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CT Confirmation of Needle Position
Confirm needle tip in perineural fat adjacent to pudendal nerve. Needle should NOT be within the nerve fascicles (patient reports sharp shock-like pain on approach if intraneural — withdraw 2–3 mm).
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Aspiration + Contrast Injection
Aspirate — must be negative for blood. Inject 0.5 mL dilute contrast (10%). CT confirms perineural spread along pudendal canal and absence of vascular opacification. If arterial blush or rapid washout seen, reposition needle before proceeding.
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Medication Injection (Block)
Inject 3–5 mL 0.5% bupivacaine for diagnostic block. For therapeutic block: add triamcinolone 40 mg. Total injectate 5–10 mL per side. Observe for injectate spread anteromedially along canal on post-injection CT.
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Bilateral Injection (if Indicated)
Repeat contralateral side using identical technique. Bilateral injections for bilateral or midline symptoms. Unilateral preferred if pain is clearly lateralized — reduces risk of bilateral urinary retention or perineal numbness.
Cryoablation Variant (After Positive Diagnostic Block)
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Probe Placement
17G cryoprobe (IceSense3 or equivalent) advanced via transgluteal approach parallel to pudendal canal, with tip positioned adjacent to nerve at Alcock’s canal entry. CT confirms position. One probe per side.
▶ CT-guided pudendal nerve cryoablation probe
Pudendal nerve cryoablation: probe at Alcock's canal — target the nerve between the obturator internus fascia and sacrospinous ligament; ice ball must encompass the nerve without involving adjacent neurovascular structures.
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Freeze-Thaw Cycles
Two freeze cycles: 8 minutes freeze → 4 minutes passive thaw → 8 minutes freeze (8/4/8 min protocol). Alternatively: 10/5/10 protocol for larger target volume. Intermittent CT scans during freeze confirm ice ball development encompassing nerve.
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Ice Ball Monitoring
Ice ball appears as low-density zone on CT. Confirm ice ball encompasses nerve and Alcock’s canal. Monitor proximity to rectum (medial) — ice ball must NOT contact rectal wall. If ice ball tracking toward rectum: pause freeze, consider probe repositioning.
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Active Thaw & Probe Removal
After final freeze: active thaw (helium gas) 3–5 minutes until probe freely mobile. Do NOT remove probe while frozen — tissue avulsion risk. Final CT confirms no hematoma or rectal injury.
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Troubleshooting
Intraoperative issues and solutions
Needle Position
Intravascular Injection
Aspiration of blood OR rapid contrast washout on CT injection test → do not inject medication. Withdraw needle 2–3 mm, reposition medially, repeat aspiration and contrast test before injecting.
Needle Position
Rectal Wall Proximity
CT confirms needle tip or cryoprobe approaching rectal wall (<5 mm clearance). Redirect needle laterally. For cryoablation: if ice ball tracking toward rectum during freeze, active thaw and reposition probe before resuming. Air injection into perirectal fat (pneumodissection) can create protective buffer.
Spread Pattern
Inadequate Perineural Spread on Contrast
Contrast pools locally without tracking along canal — suggest fibrous entrapment or incorrect plane. Add hyaluronidase 100 units to injectate to dissolve fibrosis if suspected post-surgical scar. Alternatively, target both ischial spine AND Alcock’s canal to improve coverage.
Cryoablation
Ice Ball Not Encompassing Nerve
CT demonstrates ice ball offset from target nerve. Active thaw, reposition probe 2–3 mm closer to nerve fascicles, resume freeze. Two probes placed in parallel 1 cm apart can be used for larger ablation zone if single probe insufficient.
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Complications
Expected effects, minor, and major complications
Expected / Acceptable Effects
- Temporary perianal and perineal numbness — expected with any block; resolves 4–12 h after local anesthetic; may persist weeks after cryoablation
- Temporary urinary retention — advise patient to void before procedure; monitor ≥1 h post-block; Foley if unable to void
- Vaginal/scrotal numbness — expected and desired effect for pain control
- Mild gluteal soreness at probe insertion site (cryoablation)
Complications
- Rectal injury — rare; full-thickness injury requires surgical consultation; most rectal proximity issues resolve without treatment
- Gluteal hematoma — most self-limited; serial CT if expanding
- Pudendal arteriovenous fistula — rare post-cryoablation complication (reported); presents as pulsatile hematoma; CT angiography; endovascular treatment
- Infection / gluteal abscess — rare; antibiotics; CT-guided drainage if organized
- Sciatic nerve inadvertent block — if needle too lateral; transient foot weakness; resolves with local anesthetic metabolism
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Post-Procedure Care
Recovery, sensory exam, pain diary, follow-up
Immediate Recovery (Block)
- Observe 1–2 h post-procedure; confirm ability to ambulate safely
- Perianal sensory exam (light touch) to confirm expected sensory change in pudendal distribution
- Sitting test: assess pain level with sitting post-block — immediate improvement supports pudendal origin
- Voiding trial before discharge; Foley if urinary retention (>600 mL on bladder scan)
- No driving for 24–48 h (motor block risk); arrange driver
Recovery (Cryoablation)
- Observe 2–4 h; vital signs monitoring
- Expect perineal numbness for days to weeks — counsel patient this is expected
- Nerve regeneration begins 1–2 mm/day; full motor/sensory recovery 6–8 weeks (epineurium preserved)
- NSAIDs (ibuprofen 600 mg TID × 5 days) to manage post-cryoablation inflammation
- CBC not routinely required unless hematoma concern
Follow-up Schedule
| Timepoint | Assessment | Action |
|---|---|---|
| 24–48 h | Phone check (block cases) | Document pain score; confirm no urinary retention |
| 2 weeks | Clinic visit — pain reassessment, sensory exam | VAS score vs. baseline; document sitting vs. standing; repeat block if partial response |
| 6 weeks | Cryoablation follow-up | Assess motor/sensory recovery; pain diary review |
| 3 months | Outcome assessment | If pain returns: repeat cryoablation safe (epineurium intact = nerve can regenerate and be retreated) |
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Pearls & Pitfalls
Technique refinements and critical errors to avoid
Technique Pearls
✦
Sitting test post-block: have the patient sit in a hard chair immediately after the block. Significant immediate pain improvement strongly supports pudendal nerve as the pain generator and validates the diagnosis before proceeding to ablation.
✦
Dual-site injection (ischial spine + Alcock’s canal in same session) increases effectiveness vs. single-site injection alone — address both entrapment zones simultaneously.
✦
Cryoablation is superior to chemical neurolysis (alcohol, phenol) for benign pudendal neuralgia: reversible, repeatable, lower dysesthesia risk, and no risk of neuritis from chemical injury.
✦
The pudendal nerve is intermediate in density relative to the adjacent pudendal artery and vein on CT — fascicles may be directly visible. The nerve typically sits as the posterior-most structure in the neurovascular bundle within Alcock’s canal.
✦
Hyaluronidase 100–150 units added to the injectate in cases with suspected post-surgical fibrosis — fibrinolytic action improves spread through scarred perineural tissues.
✦
Use pulsed RFA (42°C × 120 s) rather than continuous RFA if RF modality is chosen for this mixed motor-sensory nerve — continuous high-temperature RFA risks permanent motor fiber injury to pelvic floor and sphincter muscles.
Critical Pitfalls
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Never skip diagnostic block before cryoablation. Negative block = cryoablation unlikely to provide benefit. Diagnostic confirmation is Nantes criterion #5 and essential for patient selection.
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Always confirm no vascular opacification with dilute contrast before injecting medication. Pudendal artery runs immediately adjacent. Intravascular local anesthetic injection can cause systemic toxicity.
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Never remove cryoprobe while frozen. Probe-tissue adhesion during active freezing causes tissue avulsion and hemorrhage on removal. Always complete active thaw (helium) until probe is freely mobile before withdrawal.
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Do not treat bilateral pudendal neuralgia with bilateral cryoablation in one session without discussion. Bilateral complete motor block risks bilateral urinary/fecal incontinence during recovery period. Consider staged approach or patient counseling.
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References & Resources
Source material and related procedures
Primary References
- Prologo JD, Ray CE Jr., eds. Advanced Pain Management in Interventional Radiology: A Case-Based Approach. Thieme; 2024. DOI: 10.1055/b000000387
- Tran DQ, et al. Cryoneurolysis: a scoping review of the evidence. Reg Anesth Pain Med. 2021;46(3):255–263.
- Labat JJ, et al. Diagnostic criteria for pudendal neuralgia by pudendal nerve entrapment (Nantes criteria). Neurourol Urodyn. 2008;27(4):306–310.
- Prologo JD, et al. Percutaneous CT-guided cryoablation for the treatment of refractory pudendal neuralgia. Skeletal Radiol. 2015;44(5):709–714.
- Kastler A, et al. Dual site pudendal nerve infiltration: more than just a diagnostic test? Pain Physician. 2018;21(1):83–90.
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References & Resources
Primary sources · Key data · Related procedures
Key Guidelines
- ASRA Practice Guidelines for Regional Anesthesia
- International Pudendal Nerve Entrapment Society Guidelines
Primary References
- Prologo JD, Ray CE Jr., eds. Advanced Pain Management in Interventional Radiology. Thieme; 2024.
- Benson JT, Griffis K. Pudendal neuralgia, a severe pain syndrome. Am J Obstet Gynecol. 2005;192(5):1663-1668.
- Labat JJ et al. Diagnostic criteria for pudendal neuralgia by pudendal nerve entrapment (Nantes criteria). Neurourol Urodyn. 2008;27(4):306-310.