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Indications & Patient Selection
Indications
- Peripheral neuropathy — nerve entrapment syndromes causing focal pain or numbness
- Oncologic nerve pain — tumor invasion or compression of peripheral nerves
- Post-surgical neuropathy — scar entrapment following hernia repair, hip or groin surgery
- Diagnostic block — lidocaine 1–2 mL confirms nerve as pain source (≥50% relief) before ablation or definitive treatment
- Therapeutic block — bupivacaine + corticosteroid for medium-term relief
- Neurolysis — absolute ethanol (2–5 mL) for oncologic or refractory neuropathic pain after positive diagnostic block
Workup & Selection
- CT or MRI to identify nerve anatomy, mass lesions, and prior surgical anatomy before intervention
- EMG / NCS for entrapment confirmation — helps localize the affected segment
- MR neurography (3T) for complex or post-surgical cases with distorted anatomy — order before CT-guided intervention
- Failed conservative management (≥6 weeks) typically required before interventional approach
- Contraindications: Active infection at needle path · Uncorrectable coagulopathy · Allergy to injectate components · Absent diagnostic block confirmation before neurolysis
Common Nerve Targets
| Nerve | Syndrome | Symptoms |
|---|---|---|
| Lateral Femoral Cutaneous (LFCN) | Meralgia paresthetica | Lateral thigh numbness / burning |
| Genitofemoral (GFN) | Inguinal / scrotal pain | Groin / inner thigh pain post-hernia repair |
| Iliohypogastric / Ilioinguinal | Post-hernia pain | Lower abdominal / inguinal pain |
| Femoral | Femoral neuropathy | Anterior thigh pain / weakness |
| Obturator | Hip / medial thigh pain | Groin pain |
| Posterior Femoral Cutaneous (PFCN) | Sitting / perineal pain | Posterior thigh / perineum |
| Superior Gluteal | Piriformis syndrome | Buttock pain radiating down leg |
| Cluneal Nerves | Posterior iliac crest pain | Low back / buttock pain |
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Pre-Procedure Checklist
Choose guidance modality. CT guidance required for deep nerves (genitofemoral, obturator, PFCN, cluneal). Ultrasound is adequate for superficial targets (LFCN, ilioinguinal/iliohypogastric). Default to CT if anatomy uncertain or prior surgery has distorted tissue planes.
Review cross-sectional imaging. Identify nerve anatomy on pre-procedure CT or MRI. Confirm absence of mass lesion along needle path. For post-surgical cases, order 3T MR neurography to map distorted anatomy before the procedure.
Determine block intent. Diagnostic (short-acting lidocaine 1% only, 1–2 mL) vs. therapeutic (bupivacaine 0.25–0.5% + triamcinolone 40 mg, 1 mL each) vs. neurolysis (absolute ethanol 98%, 2–5 mL — only after positive diagnostic block). Document intent clearly before starting.
Needle selection. 22G, 2.5–3.5" spinal needle for most targets. 22G Chiba for deeper retroperitoneal targets (GFN). Avoid larger-gauge needles — 22G minimizes hematoma risk and intravascular injury.
Injectate preparation. Diagnostic: 1–2 mL lidocaine 1%. Therapeutic: 1 mL bupivacaine 0.25–0.5% + 1 mL triamcinolone 40 mg. Neurolysis: 2–5 mL absolute ethanol (98%) — draw up in labeled syringe, confirm before injecting. Maximum bupivacaine dose: 2 mg/kg.
Patient positioning. Supine or prone depending on target nerve. Confirm positioning plan on pre-procedure CT scout. LFCN: supine or prone. GFN: supine. PFCN / cluneal: prone. Have positioning aids (bolsters, foam) available.
Sedation. No IV sedation required for most peripheral nerve blocks. Mild anxiolysis (oral anxiolytic or 1–2 mg midazolam IV) for highly anxious patients. Ensure patient can report paresthesias during needle placement — do not over-sedate.
Consent. Discuss: temporary motor block (genitofemoral/obturator — thigh weakness, no driving), intravascular injection risk, hematoma, infection, non-target spread, and limited duration of relief. For neurolysis: discuss irreversible nerve injury risk.
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Anatomy by Nerve
Lateral Femoral Cutaneous Nerve (LFCN)
▶ CT — lateral femoral cutaneous nerve block
LFCN block: needle at the fascial plane medial to ASIS beneath the inguinal ligament — small volume (3–5 mL) spreads along the nerve; confirm position with contrast before injection.
- Arises from L2–L3 nerve roots; purely sensory nerve
- Exits pelvis under inguinal ligament medial to ASIS; runs superficial to tensor fascia lata into the thigh
- Target: 1 cm medial and 1 cm inferior to ASIS — where nerve exits into thigh between fascia layers
- Approach: CT or US; supine or prone; 22G needle
- Syndrome: meralgia paresthetica — lateral thigh burning/numbness
Genitofemoral Nerve (GFN)
▶ CT — genitofemoral / iliohypogastric nerve block
Genitofemoral/iliohypogastric block: needle at psoas fascia for GFN and iliohypogastric nerve — CT-guided approach prevents femoral nerve spread with targeted small-volume injection.
- Arises from L1–L2; runs on anterior surface of psoas muscle
- Divides into femoral branch (anterior thigh) and genital branch (scrotum/labia)
- Target: CT-guided; anterior to psoas at L3–L4 level (medial to iliopsoas, lateral to ureter)
- Approach: CT supine or lateral decubitus; 22G Chiba needle; lateral retroperitoneal approach
- Syndrome: groin/scrotal pain post-hernia repair
Iliohypogastric / Ilioinguinal Nerve
- Both arise from L1; run between transversus abdominis and internal oblique muscles
- Exit inguinal canal; innervate lower abdominal wall and inguinal region
- Target: between muscle layers at ASIS level; US or CT guidance
- Approach: in-plane US or CT axial at ASIS; transverse plane injection into fascial plane
- Syndrome: post-hernia repair lower abdominal/inguinal pain
Obturator Nerve
▶ CT — femoral and obturator nerve block
Femoral and obturator nerve block: femoral nerve lateral to artery at inguinal ligament; obturator nerve at obturator canal — combined block for hip/knee pain.
- Arises from L2–L4; exits pelvis through obturator foramen
- Divides into anterior branch (hip adductors, medial thigh sensation) and posterior branch
- Target: CT-guided; obturator foramen or medial to femoral vessels
- Approach: CT supine; coronal plane most useful; 22G needle
- Motor risk: adductor weakness — counsel about driving
Posterior Femoral Cutaneous Nerve (PFCN)
- Arises from S1–S3; exits greater sciatic foramen inferior to piriformis
- Runs deep to gluteus maximus; innervates posterior thigh and perineum
- Target: CT subgluteal approach; prone positioning
- Syndrome: sitting-related perineal/posterior thigh pain
Cluneal Nerves (Superior)
▶ CT — bilateral cluneal nerve block
Bilateral superior cluneal nerve block: needles at posterior iliac crest where nerves cross — CT confirms position at iliac crest margin; inject 2–3 mL per side.
- Cross posterior iliac crest approximately 7 cm lateral to midline
- Innervate low back, buttock, and posterior iliac crest region
- Target: CT posterior approach at posterior iliac crest; prone positioning
- Syndrome: posterior iliac crest / low back pain, sometimes confused with SI joint pain
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Technique
Default RadCall approach · share your own below
Supplies
22G 2.5–3.5" spinal needle
22G Chiba needle (GFN/deep targets)
CT guidance
Dilute contrast (Omnipaque 300, 1:10)
1 mL + 3 mL + 5 mL syringes
Lidocaine 1% (diagnostic)
Bupivacaine 0.5% (therapeutic)
Triamcinolone 40 mg/mL
Absolute ethanol 98% (neurolysis only)
ChloraPrep
Sterile drape + gloves
Sterile dressing
Approach A — LFCN Block (Prototype)
Steps
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CT planning
Identify ASIS on CT scout. Locate LFCN on reformatted images — often visible as small nerve 1–2 cm medial to ASIS between fascia iliaca and inguinal ligament. Plan needle path to fascial plane between tensor fascia lata and iliopsoas.
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Position + skin prep
Supine or prone positioning per pre-procedure plan. CT scout to confirm landmarks. Sterile prep and drape. Local anesthetic at skin entry site.
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Advance 22G needle to target
Advance needle to ASIS region, targeting 1 cm medial and 1 cm inferior to ASIS. Goal: needle tip in fascial plane between iliac fascia and inguinal ligament. Confirm depth and position with CT image.
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Contrast injection
Inject 1 mL dilute contrast under CT fluoroscopy or intermittent CT. Confirm fascial plane spread — contrast should track along the plane (not vascular, not intraperitoneal). Vascular uptake: withdraw 2 mm and recheck before injecting medication.
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Inject medication
Diagnostic: 1–2 mL lidocaine 1%. Therapeutic: 1 mL bupivacaine 0.5% + 1 mL triamcinolone 40 mg. Small volumes are key — 1–2 mL is sufficient; larger volumes increase non-target spread.
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CT confirm and withdraw
Obtain final CT image confirming needle position and injectate distribution. Withdraw needle. Apply sterile dressing. Observe patient 30 minutes for pain response and motor changes.
Approach B — Genitofemoral Nerve Block
Steps
1
CT planning — identify psoas margin
Supine positioning. Review pre-procedure CT to identify psoas margin at L3–L4 level. Locate the anterior psoas surface. Identify ureter (medial) to avoid. Plan lateral retroperitoneal approach.
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Lateral retroperitoneal approach
22G Chiba needle from flank through retroperitoneal fat to anterior psoas surface. Advance incrementally with CT confirmation. Target: anterior surface of psoas at L3–L4 level, lateral to ureter.
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Contrast to confirm psoas fascia
Inject 1 mL dilute contrast. Confirm spread along psoas fascia plane — perineural position. No vascular uptake. No spread into peritoneum. Adjust if spread is diffuse (volume too large).
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Inject medication
2 mL bupivacaine 0.5% + 1 mL triamcinolone 40 mg. For neurolysis: 2–5 mL absolute ethanol after confirmed positive diagnostic block. Total volume should not exceed 3–5 mL for most targets.
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CT Landmarks
Nerve Target Reference
| Nerve | CT Target | Best CT Plane | Key Anatomy |
|---|---|---|---|
| LFCN | Medial-inferior to ASIS; deep to inguinal ligament | Axial | Between fascia iliaca and tensor fascia lata |
| GFN | Anterior psoas surface at L3–L4 level | Axial | Medial to iliopsoas; lateral to ureter |
| Iliohypogastric / Ilioinguinal | Between transversus abdominis and internal oblique at ASIS | Axial at ASIS level | Fascial plane between muscle layers |
| Obturator | Obturator foramen; medial to femoral vessels | Coronal (most useful) | Anterior / posterior division at foramen |
| PFCN | Inferior to piriformis at greater sciatic foramen | Axial prone | Deep to gluteus maximus |
| Cluneal (superior) | Posterior iliac crest ~7 cm lateral to midline | Axial / coronal prone | CT posterior approach; subcutaneous at crest |
Fascial Plane Confirmation
- Contrast should track along the fascial plane containing the nerve — linear spread confirms perineural delivery
- Diffuse spread → too much volume injected; increases non-target effects
- Vascular filling (branching vessels opacify rapidly) → withdraw 2 mm, recheck before injecting
- Intraperitoneal spread (GFN) → withdraw; needle too deep; confirm retroperitoneal position before reinjecting
Positioning Tips
- LFCN / GFN / Obturator / Iliohypogastric: Supine — standard CT table positioning
- PFCN / Cluneal / Superior gluteal: Prone — place foam bolster under pelvis to flatten gluteal contour and open posterior approach
- Confirm target slice on CT scout before prepping and draping
- Mark skin entry point with CT-compatible marker before sterile prep
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Troubleshooting
Problem
Intravascular contrast uptake on test injection
Likely cause: Needle tip in periforaminal vessel, retroperitoneal vessel, or fascial plane vasculature. Uncommon with 22G needle but always check before injecting medication.
Next step: Withdraw needle 2 mm, aspirate, reposition. Repeat contrast injection before any medication. Arterial or venous injury is extremely rare with 22G; manual pressure for 5 minutes if concern. Never inject medication — especially ethanol — with active vascular uptake.
Problem
Cannot identify small nerve on CT
Likely cause: Small nerve caliber (LFCN, ilioinguinal are 1–3 mm); post-surgical distortion; insufficient soft tissue contrast.
Next step: Use nerve anatomy landmarks (fascial planes) rather than attempting direct nerve visualization — target the fascial compartment containing the nerve. Consider ordering 3T MR neurography before procedure for precise anatomic mapping in post-surgical or complex cases.
Problem
Inadequate pain relief after block
Likely cause: Incomplete perineural spread (small volume, fascial plane missed); contributing nerve not targeted; incorrect nerve identified as pain source.
Next step: Consider slightly higher volume to ensure perineural spread (max 2–3 mL). Reassess whether an alternative contributing nerve is present (e.g., obturator and LFCN both contributing to thigh pain). Revisit EMG/NCS data and imaging.
Problem
Pain returns quickly after initial relief
Likely cause: Short duration of local anesthetic with early washout; underlying ongoing nerve irritation; structural cause not addressed.
Next step: Consider longer-acting agent (bupivacaine vs. lidocaine). If relief was ≥50% even briefly — positive diagnostic result; proceed to neurolysis (absolute ethanol), cryoablation, or radiofrequency ablation for durable relief.
Problem
Intraperitoneal spread on GFN block
Likely cause: Needle advanced too far anteromedially beyond retroperitoneal fat; peritoneum entered.
Next step: Withdraw needle; re-confirm on CT that tip is in retroperitoneal fat anterior to psoas before reinjecting. Intraperitoneal injection of a small amount of local anesthetic is not dangerous, but the block will be ineffective. Monitor for peritoneal signs briefly before discharging.
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Complications
Immediate / Periprocedural
- Intravascular injection: Rare with 22G needle. Always aspirate before injecting; use contrast test dose under CT fluoroscopy. No epinephrine in block mixture. Absolute ethanol intravascular injection is catastrophic — confirm negative aspiration and fascial spread before neurolysis.
- Local anesthetic systemic toxicity (LAST): Use minimal effective dose; maximum bupivacaine 2 mg/kg. Symptoms: tinnitus, metallic taste, perioral numbness, seizure. Have resuscitation available. Lipid emulsion rescue for severe LAST.
- Temporary motor block: Genitofemoral / obturator blocks → adductor or thigh weakness. Counsel patient before procedure — no driving until resolved (typically 4–6 hours).
- Non-target spread: Large-volume LFCN or GFN block → femoral nerve involvement → anterior thigh weakness. Use minimal volumes (1–2 mL) to prevent this.
Delayed
- Infection: Rare with sterile technique (<0.1%). Fever and progressive pain post-procedure → CT with contrast; broad-spectrum antibiotics if soft tissue infection identified.
- Hematoma: Apply compression after procedure; monitor for expanding hematoma especially with anticoagulated patients. Usually self-limited.
- Neurolysis complications (ethanol): Permanent motor deficit, dysesthesia, deafferentation pain. Perform diagnostic block first — always. Hydrodissect nerve from adjacent structures (5–10 mL saline) before ethanol injection to protect skin, vessels, and bowel.
- Corticosteroid effects: Transient glucose elevation in diabetics; limit steroid injections per region per year. Not a routine concern for peripheral nerve blocks given small volumes used.
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Critical Pearls
✶
CT for deep nerves, US for superficial — use the right tool. CT guidance is required for obturator, genitofemoral, PFCN, and cluneal nerves. Ultrasound is sufficient for LFCN and ilioinguinal. Using ultrasound for deep retroperitoneal nerves risks inadequate visualization and non-target injection; defaulting to CT for uncertain anatomy is always safer.
✶
Small volumes are key — less is more. 1–2 mL is sufficient for most peripheral nerve blocks. Larger volumes increase spread to non-target nerves (e.g., femoral nerve from LFCN block causing anterior thigh weakness) and reduce diagnostic specificity. If relief is absent despite correct positioning, increase volume modestly — do not default to large-volume injections.
✶
Fascial plane spread confirms perineural delivery. On CT, contrast should track linearly along the fascial plane containing the nerve. Diffuse globular spread means too much volume or incorrect plane. Vascular filling pattern (rapid branching opacification) means needle tip is intravascular — withdraw before injecting any medication.
✶
Always perform diagnostic block before ablation. A positive diagnostic block (≥50% pain relief) validates the nerve as the target. Proceeding to cryoneurolysis, RFA, or ethanol neurolysis without diagnostic confirmation wastes a permanent treatment and can cause permanent injury to the wrong nerve. Never skip this step for ablative procedures.
✶
MR neurography maps anatomy before intervention in complex cases. Especially for post-surgical cases (hernia repair, hip arthroplasty) with distorted tissue anatomy. Order 3T MR neurography protocol before CT-guided intervention when anatomy is uncertain — it identifies nerve course, entrapment site, and any perineural pathology that changes the target.
✶
Hydrodissection isolates nerve before ablation. Inject 5–10 mL normal saline around the target nerve immediately before cryoneurolysis or ethanol to create a fluid buffer separating the nerve from adjacent skin, vessels, and bowel. This is mandatory for safe neurolysis and reduces the risk of collateral tissue injury.
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References
Citations
- Prologo JD, Ray CE Jr., eds. Advanced Pain Management in Interventional Radiology: A Case-Based Approach. Thieme; 2024. Ch. 31 (Pezeshk P, Wadhwa V, Chhabra A).
- Chhabra A, Andreisek G, Soldatos T, et al. MR neurography: past, present, and future. AJR Am J Roentgenol. 2011;197(3):583–591.
- Chhabra A, Soldatos T, Subhawong TK, et al. The application of three-dimensional diffusion-weighted PSIF technique in peripheral nerve imaging of the distal extremities. J Magn Reson Imaging. 2011;34(4):962–967.
- Chhabra A, Faridian-Aragh N, Chalian M, et al. High-resolution 3T MR neurography of the lumbosacral plexus. Radiographics. 2012;32(3):967–983.
- SIR Standards of Practice Committee. Consensus Guidelines for Periprocedural Management of Coagulation Status. J Vasc Interv Radiol. 2012;23(6):727–736.