Hyperalgesia is defined as increased report of pain in response to a high-intensity stimulus that is normally painful (Merskey 1986). An operational definition is a leftward shift of the stimulus-response function so that somatic sensory stimuli are painful at a lower stimulus intensity than usual (Meyer et al. 1994). Hyperalgesia must be distinguished from allodynia, which is a sensation of pain evoked by a stimulus that normally is never painful. In distinguishing hyperalgesia from allodynia, "it is important to recognize that allodynia involves a change in the quality of the sensation" (e.g., pain versus nonpain, or a change in a modality or aspect of pain), whereas hyperalgesia is a change in the quantity or intensity of a sensation (Merskey 1986).
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