The impact of conditioned pain modulation on the development of secondary hyperalgesia

Background and aims: Conditioned Pain modulation (CPM) is the proposed human equivalent of diffuse noxious inhibitory controls, a spinal event that underlies the ‘pain inhibits pain’ phenomenon. Secondary hyperalgesia, mechanistically underpinned by central sensitisation, is also reported to be a spinal event. Thus, our aim is to investigate the impact of CPM on the development of secondary hyperalgesia as it would provide mechanistic credence to their spinal origin.

Methods: In healthy human volunteers, to elicit secondary hyperalgesia we applied noxious high frequency electro-cutaneous stimulation (HFS) to the volar forearm. A modified version of the DFNS Mechanical Pain Sensitivity test was used to assess sensory changes, while a 10g von Frey filament was used to assess spread. Following, and in a separate session, to elicit CPM, we applied (concurrent to the HFS) conditioning pressure at 70% the individual’s pain tolerance using a cuff algometer.

Results: Initial pilot data (N = 5) revealed no significant difference in either hyperalgesia intensity or area of secondary hyperalgesia, suggesting that CPM does not act to inhibit the spinal events that underlie its development/progression. Following further protocol refinements, formal testing is now underway and recruitment is ongoing.

Conclusions: Pain perception, unique and non-linear, is influenced by multiple physiological and psychological factors. Understanding the mechanisms that underlie pain modulatory processes is vital if novel pharmacotherapeutic strategies are to be identified. Spinal events take centre stage in the transition from acute to chronic pain and understanding how spinal manifestations of the pain state interact is crucial.