Background Ilioinguinal (IL) and iliohypogastric (IH) nerve blocks are used in patients with chronic postherniorrhaphy pain. The present study tested the hypothesis that our method, previously developed in cadavers, blocks the nerves separately and selectively in human volunteers. Methods We blocked the IL and the IH nerves in 16 volunteers in a single-blinded randomized cross-over setting under direct ultrasound visualization, by injecting two times the ED95 volume of 1% mepivacaine needed to block a peripheral nerve. The anaesthetized skin areas were tested by pinprick and marked on the skin. A digital photo was taken. For further analysis, the parameterized picture data were transformed into a standardized and unified coordinate system to compare and calculate the overlap of the anaesthetized skin areas of the two nerves on each side. An overlap <25% was defined as selective block. Results Fifty nerve blocks could be analysed. The mean volume injected to block a single nerve was 0.9 ml. Using ultrasound, we observed spread from one nerve to the other in 12% of cases. The overlap of the anaesthetized skin areas of the nerves was 60.3% and did not differ after exclusion of the cases with visible spread of local anaesthetic from one nerve to the other. Conclusions The IL and IH nerves cannot be selectively blocked even if volumes below 1 ml are used. The most likely explanation is the spread of local anaesthetic from one nerve to the other, although this could not be directly observed in most case
