Chronic neuropathic pain is a particularly troublesome secondary consequence of spinal cord injury (SCI) which affects approximately 40% of persons with SCI. Despite the high prevalence of SCI-related neuropathic pain and its impact on rehabilitation efforts, current treatments for this type of pain are often ineffective. The development of more successful interventions for central neuropathic pain is limited by the fact that the mechanisms responsible for this type of pain are not clearly understood, and definitive diagnostic and outcome tools have yet to be established. Quantitative sensory testing (QST), which allows for noninvasive evaluation of somatosensory processing mechanisms in human subjects, has recently been advocated as a potentially critical tool for the investigation of neuropathic pain, including aiding in the validation of ratings of ongoing pain, in the evaluation of treatment efficacy, and in the successful translation of research from animal models to clinical treatment paradigms. The goal of the proposed pilot project is to examine the utility of specific QST measures, which are aimed at assessing the integrity of thermal and pain integrative sensory processing, for the assessment of neuropathic pain characteristics in persons with SCI. Because the methods to be employed in this pilot study have been reported in a very limited number of publications, the rationale for conducting this pilot project is to provide initial support for the hypotheses outlined below, which will guide the design of a full research protocol to thoroughly validate these QST techniques for neuropathic pain in SCI. Objective 1: Examine the relationship between the degree of spatial integration of thermal noxious stimulation and the presence of neuropathic pain in persons with SCI. Stimulus-response relationships will be measured by asking subjects to rate the intensity of pain produced by thermal stimuli of varying temperatures delivered using one thermal probe (9cm2 area) and those delivered using two thermal probes side-by-side (18cm2 total area). Spatial summation will be assessed by comparing the differences in pain intensity ratings for the small stimulus area (one probe) and the large stimulus area (two probes) across subjects. We hypothesize that greater differences in pain ratings between the two stimulus sizes (one probe vs. two probes) will occur in participants with SCI and neuropathic pain (i.e., greater spatial summation) compared to participants with SCI who do not have pain, suggesting enhanced integration of noxious inputs across space in persons who have neuropathic pain. Objective 2: Compare perceptual responses to the "thermal grill" stimulus between persons with SCI and neuropathic pain and persons with SCI who do not have pain. A thermal grill stimulus will be used to assess thermal and pain integration processes. The thermal grill, which is typically composed of a number of interlaced warm (40oC) and cool (20oC) stimulus bars presented simultaneously, produces a paradoxical "burning pain" sensation when presented to the skin of a person with intact somatosensory function. This experience of pain due to the thermal grill has been suggested to be abnormal in persons with neuropathic pain. In the proposed pilot study thermal grill-produced pain thresholds will be obtained in persons with SCI and neuropathic pain and in persons with SCI who do not have neuropathic pain. It is hypothesized that the thermal grill threshold for participants with neuropathic pain will be significantly higher than for participants without neuropathic pain, suggesting that the normal integration of information from warm and cool sensory pathways is disrupted in those with neuropathic pain. Results from the proposed pilot studies will yield insight regarding the specific somatosensory system mechanisms that may be related to the experience of neuropathic pain, and will provide support for future studies aimed at fully validating the use of these QST techniques as potential diagnostic and outcome measures for neuropathic pain in SCI.
Chronic neuropathic pain affects approximately 40% of persons with spinal cord injury (SCI), is often refractory to treatment, and significantly limits physical and psychological function after injury. The VA's network of medical centers, which provides health care for approximately 17% of people with SCI in the US, is well-suited for conducting clinical trials of new pain treatments for neuropathic pain. Currently, however, the tools necessary to evaluate the efficacy of neuropathic pain treatments are lacking. The proposed pilot studies will assess the utility of quantitative sensory testing for the measurement of neuropathic pain in persons with SCI. Results from this project will lead to the development of specific sensory testing techniques for the accurate and reliable measurement of the impact of treatment interventions on neuropathic pain. This will, in turn, help rehabilitation health care practitioners be better equipped to manage SCI-related neuropathic pain, a problem which limits functional recovery for a significant proportion of our veteran population.