Regardless of whether the mechanical allodynia is A-beta or high threshold, it often has characteristics similar to pains evoked by unmyelinated C nociceptive afferents described above for second pain (6). Thus, repeated brief mechanical stimulation of allodynic patients often evokes slow temporal summation of burning pain, as shown in Figure 1 (right panel). For some CRPS patients, slow temporal summation of burning pain occurs when gentle mechanical stimuli or electrical stimulation of A-beta afferents are applied at rates of once per three seconds. For other patients, slow temporal summation occurs only with more intense but normally nonpainful mechanical stimuli. Still other patients do not exhibit slow temporal summation with these types of repetitive mechanical stimuli. Both mechanical allodynia and slow temporal summation of allodynia are completely or nearly completely reversed by anesthetic blockade of sympathetic ganglia in some CRPS patients, indicating that these sensory abnormalities can sometimes be dynamically maintained by sympathetic efferent activity, presumably activity that induces continuous input over nociceptive afferents. Slow temporal summation of mechanical allodynia, particularly that induced by stimulation of A-beta afferents, is abnormal because such types of stimuli neither evoke pain in pain-free subjects nor in CRPS patients when such stimuli are delivered to homologous contralateral pain-free zones. In fact, A-beta afferent stimulation even at extremely high frequencies does not evoke pain in normal human subjects (39). Therefore, A-beta mechanical allodynia and abnormal slow temporal summation of mechanical allodynia may represent an exaggeration and/or abnormal triggering of physiological mechanisms that already exist in normal pain-free individuals. Such mechanisms can be demonstrated in the latter by temporal summation of experimentally induced second pain, as described earlier. Thus, under some pathological conditions after nerve injury or nerve dysfunction, A-beta input may somehow gain access to and trigger the same temporal summation mechanisms normally activated by C afferent stimulation. In other pathological conditions, sensitized nociceptors themselves are likely to be the direct proximal cause of the slow temporal summation of mechanical allodynia.
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