Evaluation of Postblock Hypersensitivity Using Quantitative Sensory Testing before, during, and after Axillary Brachial Plexus Block Resolution in Healthy Volunteers

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BACKGROUND: The problem of a sharp increase in pain after resolution of a nerve block, known as rebound pain, has been increasingly recognized. It is currently unknown whether rebound pain simply reflects the unmasking of untreated, residual surgical pain, or if it represents a de novo hyperalgesic state driven by a mechanistic resetting of pain thresholds in response to regional anesthesia. In this study, the authors assessed nociceptive processing after a single-injection axillary brachial plexus block in healthy volunteers to detect whether a previously blocked arm displayed objective hypersensitivity in the absence of surgical pain. METHODS: Healthy volunteers received a single-injection axillary brachial plexus block using 15 cc mepivacaine 1.5%, with their contralateral unblocked arm serving as a control. Pain sensitivity between arms was compared using a range of quantitative sensory testing, comparing punctate, pressure, and heat pain using paired t tests or Wilcoxon signed-rank tests. The primary outcome was heat pain threshold to 1 of 10 pain, compared between previously blocked and control arms at 60 min after motor resolution (MR), with secondary outcomes being other modalities compared at this timepoint. Additionally, the authors explored differences in maximally sensitized values during the entire postblock period. RESULTS: All 40 participants (age 19 to 64 yr, 60% female) completed the study. At 60 min after MR, heat pain threshold to 1 of 10 pain was not significantly different between arms (block, 40.8° ± 2.2°C; control, 41.2° ± 2.6°C; P = 0.160). Testing of several other sensory modalities revealed only a slightly lower temperature to reach 5 of 10 heat pain in the block versus control arm. When examining the maximally sensitized values including 3 h after MR, the control arm showed greater sensitivity in some tested modalities (punctate pain threshold and painful after sensations) but not others (heat and pressure pain thresholds and tolerances, temporal summation of pain). CONCLUSIONS: Multimodal pain testing revealed no evidence of general postblock hypersensitivity during resolution of a single-injection axillary brachial plexus block.

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