The role of peripheral afferents in persistent inguinal postherniorrhaphy pain: a randomized, double-blind, placebo-controlled, crossover trial of ultrasound-guided tender point blockade

Background: Severe, persistent inguinal postherniorrhaphy pain (PIPP) is a debilitating condition that develops in 2-5% of patients. PIPP may be neuropathic in nature, yet the lesion in the peripheral nervous system has not been located. Most PIPP-patients demonstrate a tender point (TP) in the medial aspect of the inguinal region that triggers pain upon minimal pressure. As TPs may play a role in the pathophysiology of PIPP, the aim of this trial was to investigate the analgesic effects of local anaesthetic TP-blockade. Methods: A randomized, double-blind, placebo-controlled, crossover trial was performed in 14 PIPP-patients and six healthy volunteers. All participated in two sessions, seven days apart, receiving 10 ml of 0.25% bupivacaine or normal saline via an ultrasound-guided fascial plane block at the TP. The TP-area was used for pain assessments (at rest, on movement, with 100 kPa pressure-algometry) and quantitative sensory testing (pressure pain thresholds, thermal detection/pain thresholds, supra-threshold heat perception), before and after the TP-blockade. Results: The median (95% CI) reduction in pain was 63% (44.1 to 73.6%) after bupivacaine compared with 36% (11.6 to 49.7%; P=0.003) after placebo. Significant increases in cool detection (P=0.01) and pressure pain thresholds (P=0.009) with decreases in supra-threshold heat pain perception (P=0.003) were seen after bupivacaine only. In four out of six volunteers, increased thermal and evoked-pain thresholds after bupivacaine compared with placebo, was demonstrated. Conclusions: This trial demonstrates that peripheral afferent input from the TP-area is important for maintenance of spontaneous and evoked pain in PIPP.