Some aspects of temporal summation of second pain appear integrally related to patients' ongoing clinical pain. For example, enhanced ratings of aftersensations that occur after temporal summation are salient predictors of fibromyalgia patients' ratings of clinical pain, accounting for 27% of the variance in ratings of clinical pain (23). Tender point count and pain-related emotions accounted for about 22% of the remaining variance. The approach of using controlled stimuli to characterize the basis for allodynia and hyperalgesia as well as the severity of patients' ongoing "spontaneous" clinical pain is one that has been applied to several types of pain conditions, including fibromyalgia, IBS, and CRPS.
It is also important to recognize that characteristics of temporal summation of second pain just described have been observed in several different laboratories using different methods. Temporal summation of second pain occurs in response to repeated cutaneous electric shocks before and after blockade of myelinated axons (29). It occurs in response to repeated brief heat pulses of 51°C or repeated 53°C heat taps to the skin (6,30,31,36,37). Similar to heat pulses or taps, temporal summation occurs with repetitive stimulation of muscle nociceptive afferents and is greatly enhanced in fibromyalgia (23,34). Both muscle- and heat-induced temporal summation can be reduced by NMDA receptor antagonists such as dextromethorphan (34). However it is more evident that heat-induced temporal summation reflects secondary hyperalgesia in fibromyalgia patients because fibromyalgia patients do not report ongoing burning pain from the skin and they have areas of heat hyperalgesia that are remote from body areas reported as painful (22,23,34). Temporal summation also occurs with repetitive stimulation of visceral nociceptive afferents (38).
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