The relationship between sensory thresholds and mechanical hyperalgesia in nerve injury

The paradoxical combination of sensory loss within the area where pain is felt together with pain evoked by non-noxious stimuli (allodynia) is a characteristic feature of neuropathic pain. This study examined the relationship between (mechanical and thermal) pain thresholds and dynamic and static hyperalgesia in 15 patients with traumatic nerve injury and brush-evoked pain. Sensory tests were carried out both in the allodynic skin area and in the unaffected contralateral mirror image skin. The sensory characteristics included: visual analogue scale (VAS) score of ongoing pain, detection and pain threshold to thermal and mechanical stimuli, and temporal summation to repetitive heat and pinprick stimuli. Temporal summation was evoked by pinprick stimuli at 2.0 Hz but not at 0.2 Hz in allodynic skin. No difference was observed in temporal summation to heat stimuli. There was a significant and inverse relation between heat and cold pain difference and mechanically evoked pain. Patients with heat hyperalgesia had a significantly higher VAS score of mechanical hyperalgesia than patients with heat hypoalgesia. There was no relationship between dynamic and static evoked hyperalgesia. These findings suggest a differential processing of repetitive thermal and mechanical stimuli in the central nervous system. Both dynamic and static mechanical hyperalgesia are maintained by activity in heat-sensitive nociceptors, but they are probably mediated by distinct mechanisms.