Sensory determinants of thermal pain

It is still unclear whether the quality of painful thermal sensation is determined only by conduction in specific, dedicated nociceptive channels (i.e. C or Aδ nociceptors) or whether it is a result of integrated activity in both nociceptive and non-nociceptive systems. To evaluate this question, we conducted quantitative and qualitative somatosensory testing in spinal cord injury subjects who suffered from partial or complete loss of thermal sensibility. Testing was performed in skin areas, below the level of the lesion, which were either lacking any thermal sensibility, lacking only one thermal sensation (either heat or cold) or having normal thermal sensations. We found that, in areas lacking any thermal sensibility, warm and cold stimuli produced a sensation of pricking pain, which had no thermal quality and was detected at significantly higher thresholds than in normal controls (48.5 ± 1.8 and 9.7 ± 5.1°C for noxious heat- and noxious cold-induced pricking pain, respectively). Normal thermal pain sensations, consisting of normal perception of thermal quality and normal mean pain thresholds, were present both in normal skin areas (42.1 ± 1.9 and 27.6 ± 2.25°C for heat and cold pain, respectively) and in areas in which only one thermal modality remained intact, when tested for that modality. Thus, testing for heat pain in areas in which only warm sensation was intact, or cold pain when only cold was intact produced normal qualities and thresholds of pain (42.8 ± 3.4 and 24.4 ± 6.2°C for heat and cold pain, respectively). No spatial summation of pricking pain was observed, in contrast to the marked summation of heat pain in normal areas. In areas with only a single intact thermal modality, the quality of the perceived non-painful sensation was not determined by the thermal stimulus but by the intact modality (paradoxical sensation). Cold stimuli were perceived as warm in areas in which only warm sensation was preserved, and vice versa. A similar pattern was also seen for pain perception in areas with intact warm sensation. In these areas, both noxious heat and cold elicited a sensation of heat pain. No consistent pattern of heat-elicited pain was observed in areas in which only cold sensation was intact. These data suggest that the integrity of non-noxious thermal systems is essential for the normal perception of thermal pain, and that the subjective sensation of pain depends on the integration of information from nociceptive and non-nociceptive channels.