Characterization of chronic pain and somatosensory function in spinal cord injury subjects

The pathophysiology of the chronic pain following spinal cord injury (SCI) is unclear. In order to study it's underlying mechanism we characterized the neurological profile of SCI subjects with (SCIP) and without (SCINP) chronic pain. Characterization comprised of thermal threshold testing for warmth, cold and heat pain and tactile sensibility testing of touch, graphesthesia and identification of speed of movement of touch stimuli on the skin. In addition, spontaneously painful areas were mapped in SCIP and evoked pathological pain--allodynia, hyperpathia and wind-up pain evaluated for both groups. Both SCIP and SCINP showed similar reductions in both thermal and tactile sensations. In both groups thermal sensations were significantly more impaired than tactile sensations. Chronic pain was present only in skin areas below the lesion with impaired or absent temperature and heat-pain sensibilities. Conversely, all the thermally impaired skin areas in SCIP were painful while painfree areas in the same subjects were normal. In contrast, chronic pain could be found in skin areas without any impairment in tactile sensibilities. Allodynia could only be elicited in SCIP and a significantly higher incidence of pathologically evoked pain (i.e. hyperpathia and wind-up pain) was seen in the chronic pain areas compared to SCINP. We conclude that damage to the spinothalamic tract (STT) is a necessary condition for the occurrence of chronic pain following SCI. However, STT lesion is not a sufficient condition since it could also be found in SCINP. The abnormal evoked pain seen in SCIP is probably due to neuronal hyperexcitability in these subjects. The fact that apparently identical sensory impairments manifest as chronic pain and hyperexcitability in one subject but not in another implies that either genetic predisposition or subtle differences in the nature of spinal injury determine the emergence of chronic pain following SCI.