A pilot study of sensory responses to the thermal grill in persons with spinal cord injury and neuropathic pain

Chronic neuropathic pain (NP) occurs in approximately 40% of persons with spinal cord injury (SCI), but little is known about the mechanisms underlying this condition, and effective treatments are lacking. Previous investigations of differences in somatosensory function between persons with SCI and NP (SCI-NP) and persons with SCI without NP (SCI-noNP), using tactile, thermal, and pain thresholds, have produced equivocal results. We conducted a pilot study to measure responses to a complex thermosensory stimulus, the “thermal grill” (TG), in SCI-NP and SCI-noNP subjects. The TG illusion refers to the painful burning sensation that is typically elicited when warm (∼40°C) and cool (∼20°C) stimuli are presented in close proximity, despite the fact that presentation of these stimuli alone does not evoke pain. The paradoxical pain sensation evoked by the TG relies on central integration of thermal afferent information, which has been suggested to be disrupted in persons with central NP. Therefore, we hypothesized that SCI-NP subjects would show greater disruption of thermal integration, as measured by TG thresholds, compared to SCI-noNP subjects. Persons with chronic SCI were consented and completed a detailed pain history to establish group membership: SCI-NP or SCI-noNP. Thermal grill pain thresholds (TGPT, and independent cold pain and hot pain thresholds (CPT, HPT) were measured on the skin in the dermatome at the neurologically-determined level of injury for each subject. Severity of NP-related symptoms was assessed using the Neuropathic Pain Scale (NPS). TGPT relative to CPT, and TGPT relative to HPT, were both significantly different (p<0.05) between SCI-NP (n=8) and SCI-noNP (n=5) groups. Severity of NP-related symptoms (NPS10 score) was significantly correlated with TGPTs (r=0.579, p<0.05). The results from our pilot study provide preliminary evidence that mechanisms underlying the central integration of thermal information differ between persons with SCI and NP and those without NP.