Pudendal nerve block (technique)

Pudendal nerve blocks are performed on those with suspected or proven pudendal neuralgia. Using CT will ensure accurate needle placement, which aims for a perineural pudendal nerve injection in the pudendal canal, also known as Alcock's canal. 

Indications

• pudendal neuralgia

• diagnostic

Contraindications

Absolute

• anaphylaxis to contrast/injectates

• active local/systemic infection 

Relative

• bleeding diathesis

• recent injection with steroid in same/other body parts

• unable to remain still for the procedure

• young age

Procedure

The general principle of pudendal nerve blocks is to:

• identify the pudendal canal and pudendal neurovascular bundle 

• use image guidance to access and confirm needle position with iodinated contrast

• give injectate, often steroid containing

Pre-procedural evaluation

Relevant imaging should be reviewed, and the details of the patient confirmed.  The patient should have an opportunity to discuss the risks and benefits and consent obtained.

Risks include 

• infection

• bleeding

• allergy 

• focal fat necrosis / skin discolouration at the injection site (less likely as a deep injection)

• failure of the procedure to relieve pain

Equipment

• CT biopsy grid and a skin marker

• skin cleaning product

• sterile drape

• sterile field and tray for sharps

• syringe selection i.e. 10 mL, 5 mL and 3 mL

• larger bore drawing up needle

• needle to administer local anesthetic i.e. 25-gauge needle

• needle to cannulate pudendal canal i.e. 22-gauge 90 mm Quincke needle

• sterile gauze

• adhesive dressing

Syringe selection

A suggested syringe and injectate selection for CT-guided pudendal nerve block -

• 10 mL syringe:10 mL of local anesthetic i.e. 1% lidocaine

• 5 mL syringe: iodinated contrast

• 3 mL syringe: 40mg triamcinolone acetonide (40 mg/1 mL) and 1 mL 0.5% ropivacaine

Needle selection

Pre-procedure planning should calculate the distance required to reach the pudendal canal, as larger patients will require longer needles. 

• pudendal nerve block:  22-gauge 90mm or 150mm Quincke needle

Technique

CT

• check for allergies and if on blood thinners

• consent

• optimize patient positioning by lying prone and place CT biopsy grid 

• perform planning CT, from the superior aspect the of hip joints, and inferiorly to include all of the inferior pubic rami

• identify the pudendal canal and pudendal neurovascular bundle; the canal is medial to the lesser sciatica foramen at the point the obturator internus passes from the pelvis ¹

• clean skin and draw up appropriate medications

• give local anesthesia along the proposed needle path

• under CT guidance, pass the needle into the pudendal canal, avoiding the neurovascular bundle

• inject a small amount of iodinated contrast to confirm needle tip position which should extend down the pudendal canal

• administer injectate, usually steroid containing 

• removed the needle and apply dressing/band-aid as required

• pain diary to be given

Complications

Nerve injury has been described, with smaller gauge needles preferred when possible ². The most serious complication is infection. Steroid containing injections should be postponed if there are signs and/or symptoms of local and/ or systemic infection. Possible fat necrosis causing skin dimpling and skin discolouration can occur due to steroid leaking into the surrounding soft tissues ³.