Ultrasound guided pudendal nerve block: A cadaveric study

Though needles were accurately placed during ultrasound-guided pudendal nerve block (PNB) in a cadaver study, results show only 20% of the injectate was found in the interligamentous space, which was the target site. This type of finding has gone largely unexplored in the previous literature, the researchers said.

Lead study author Yuexiang Wang, MD, of the Mayo Clinic's Department of Anesthesiology in Rochester, Minn., said 4-5 mL of injectate is the most commonly used volume in PNB, either guided by fluoroscopy, computed tomography (CT) or ultrasound.

"Although favorable clinical results have been reported by using this amount of injectate, unintended sciatic nerve block is not a rare complication," Dr. Wang said. "Thus, it is unclear as to whether 4 mL, while being effective in blocking the nerve, may be too much volume for the space into which it is being injected."

The investigators searched previous studies and found none addressing the spread pattern of 4 mL injectate under ultrasound guidance, although the spread pattern of a larger amount of injectate (10 mL) was reported (Prat-Pradal et al, Surg Radiol Anat 2009; 31(4):289-93). They decided to study the spread of 4 mL injectate at the level of the ischial spine to better understand what the appropriate volume might be.

For the study, investigators performed PNB bilaterally in 2 cadavers. Guided by ultrasound via a 5 MHz curvilinear transducer, they injected 4 mL of iodine-based contrast into the space between the sacrotuberous ligament and sacrospinous ligament, just medial to the ischial spine.

Using cone-beam CT to assess the spread, investigators observed contrast had spread to the following areas:

• The perirectal fat in 3 injections

• Alcock's canal in 2 injections

• The gluteus maximus and piriformis muscles in 1 injection

No spread to surrounding blood vessels was observed.

Dr. Wang said the effects of the spread outside the interligamentous plane are unpredictable and could be beneficial or detrimental for clinical patients.

"On one hand, the spread of the contrast into the Alcock's canal and the perirectal fat space may cause additional blockage of the inferior rectal nerve, which arises from the pudendal nerve at the beginning of the pudendal canal, then enter the proximal part of the ischiorectal fossa in most cases," Dr. Wang said. "On the other hand, the spread into the gluteus maximus and piriformis muscle may cause an unintended sciatic nerve block which is considered a potential risk for PNB."

Ultrasound PNB has been shown to be effective in treating pudendal neuralgia, which is a common cause of chronic pelvic pain (Rofaeel et al, Reg Anesth Pain Med 2008;33(2):139-45). Using ultrasound to guide the procedure is touted by supporters as advantageous for clearly visualizing each anatomical landmark during treatment, thus increasing accuracy and avoiding the risk of unintended intravascular injection and sciatic nerve block.