Pain inhibition is unrelated with autonomic responses to pain in patients with acute and chronic whiplash associated disorders

Patients with acute whiplash associated disorders (WAD) demonstrate an inefficient endogenous pain inhibition and possibly a dysfunction in autonomic nervous system. The goal of this study was to compare the autonomic response to painful stimuli between patients with acute WAD to chronic WAD and healthy controls. And to explore the role of the autonomic parameters in the inefficient endogenous pain inhibition in acute WAD together with the possible role of post-traumatic stress response. Seventeen patients with acute WAD, thirty patients with chronic WAD and thirty-one healthy controls were subjected to an experiment evaluating the autonomic nervous system at rest and during experimental painful stimuli. Skin conductance, heart rate and heart rate variability (HRV) parameters were monitored continuously during the evaluation of conditioned pain modulation. The paradigm of heterotopic noxious conditioning stimulation was used to assess this conditioned pain modulation effect. A significant autonomic response to pain was present for skin conductance and two heart rate variability parameters in all experimental groups. There was an interaction effect of the skin conductance response to pain, but no difference in HRV responses to pain in any of the groups. In patients with acute WAD, no significant correlations were present between pain, pressure pain thresholds, pain inhibition and any of the autonomic parameters. Results of this study refutes autonomic dysfunction in response to pain in both acute and chronic WAD. The dysfunctional conditioned pain modulation as typically seen in patients with acute and chronic WAD, appears unrelated to autonomic responses to pain.