Eva Kosek and Anna Januszewska
The aim was to investigate the mechanisms of pain referral in patients with whiplash associated disorder. Pain was induced in 12 controls and 12 patients with whiplash associated disorder by intramuscular electrical stimulation in the infraspinatus muscle and the ipsilateral upper arm. Conditioning stimulation amounted to a pain intensity rated as 2/10 and 4/10. During conditioning stimulation in the infraspinatus muscle, sensitivity to test stimuli was assessed in the referred pain area (i.e., upper arm) and vice versa. Test stimuli consisted of intramuscular electrical stimulation corresponding to innocuous perception threshold, electrical pain threshold, and pain intensities rated as 2/10, 4/10 and 6/10, respectively. Compared to controls, patients with whiplash associated disorder had increased pain sensitivity and indicated larger areas of referred pain. During conditioning stimulation in the infraspinatus muscle all subjects reported referred pain in the upper arm and innocuous perception thresholds and electrical pain thresholds decreased. Conditioning stimulation in the upper arm did not affect sensitivity to test stimuli in the infraspinatus muscle. In conclusion this study found that patients with whiplash associated disorder had increased sensitivity to painful stimulation, reported larger areas of referred pain during the same subjectively painful conditioning stimulation, including proximal pain referral which was never seen in controls, indicating aberrant processing of nociceptive input. The perceptual integration of nociceptive stimuli during simultaneous stimulation did not differ between groups suggesting that divergence of nociceptive input from the focal pain area leading to excitation of neurones with projected fields in the referred pain area most likely explains referred pain in both groups alike.
European Journal of Pain, 2008, 12(5), 650-660