Effects of experimental and clinical noxious counterirritants on pain perception.

It is commonly accepted that application of a sustained noxious stimulus frequently suppresses the perception of pain. In this investigation, we have determined whether painful forearm ischemia suppresses tooth pain resulting from an acute irreversible pulpitis. We have also determined whether the physiological responses to toothache alter the perception of pain evoked by experimental procedures. Ten male subjects experiencing a painful toothache (group TA) and 7 age-matched pain-free male subjects (group PF) participated in these studies. During session 1, heat pain threshold and tolerance values were determined for both groups. The times to ischemic pain onset and ischemic pain tolerance were determined for both groups using the submaximal effort tourniquet procedure. The effect of the tourniquet procedure on the intensity, unpleasantness, and spatial distribution of toothache was also assessed. Session 2 was conducted on 7 TA and 7 PF subjects 1 week later and was conducted like session 1 with the exception that group TA was not experiencing tooth pain during this session. Measures of thermal pain perception and forearm ischemic pain perception were not altered by the occurrence of toothache. In contrast, sustained noxious forearm ischemia produced a marked reduction in the intensity, unpleasantness and spatial distribution of pulpal pain. These effects on pulpal pain remained for at least 5 min after removal of the tourniquet while the arm was pain free. These findings suggest that a noxious conditioning stimulus does not universally inhibit pain perception but instead depends on unidentified interactions between the noxious test and conditioning stimuli.