Assessing the influence of non-ischaemic A-fiber conduction blockade on offset analgesia: an experimental study

Offset analgesia (OA) is believed to reflect the efficiency of the endogenous pain modulatory system. However, the underlying mechanisms are still being debated. Previous research suggested both, central and peripheral mechanisms, with the latter involving the influence of specific A-delta-fibers. Therefore, this study aimed to investigate the influence of a non-ischaemic A-fiber conduction blockade on the OA response in healthy participants. A total of 52 participants were recruited for an A-fiber conduction blockade via compression of the superficial radial nerve. To monitor fiber-specific peripheral nerve conduction capacity, quantitative sensory testing was performed continuously. Before, during and after the A-fiber block, an individualized OA-paradigm was applied to the dorsum of both hands (blocked and control side were randomized). Pain intensity of each heat stimulus was evaluated by an electronic visual analogue scale. A successful A-fiber conduction blockade was achieved in thirty participants. Offset analgesia has been verified within time (before, during, after blockade), and condition (blocked and control side) (p < 0.01, d > 0.5). Repeated measurements ANOVA showed no significant interaction effects between OA within condition and time (p = 0.24, η² = 0.05). Hence, no significant effect of A-fiber blockade was detected on OA during noxious heat stimulation. The results suggest that peripheral A-fiber afferents may play a minor role in OA compared to alternative central mechanisms or other fibers. However, further studies are needed to substantiate a central rather than peripheral influence on OA. PERSPECTIVE: This article presents the observation of offset analgesia before, during and after a successful A-fiber conduction blockade in healthy volunteers. A better understanding of the mechanisms of offset analgesia and endogenous pain modulation in general may help to explain the underlying aspects of pain disorders.