Mechanism of femoral nerve palsy complicating percutaneous ilioinguinal field block.

Femoral nerve palsy has been reported after percutaneous ilioinguinal field infiltration with general anaesthesia for inguinal herniorrhaphy. The mechanism whereby this could occur was studied in cadaver dissections. It was found that the plane between the transversus abdominis muscle and the transversalis fascia was continuous laterally with the tissue plane deep to the iliacus fascia, which is the plane containing the femoral nerve. Injection of methylene blue 1 ml into this plane resulted in pooling of dye around the femoral nerve. Femoral nerve palsy may result from infiltration of a sufficient volume of local anaesthetic into the plane between the transversus abdominis muscle and the transversalis fascia with tracking of the injectate deep to the iliacus fascia to affect the femoral nerve. This finding has important implications for the performance of a percutaneous ilioinguinal field block particularly in day surgery provision.