Pain
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Clinical Trial
Assessment of the reproducibility of intradermal administration of capsaicin as a model for inducing human pain.
The reproducibility and tolerability of intradermal (i.d.) administration of capsaicin as a method for eliciting human pain was assessed in healthy male volunteers (n = 12). The primary endpoints for assessing pain were spontaneous pain response and areas of allodynia, pinprick hyperalgesia and neurogenic inflammation. These were recorded before, immediately after, and at regular intervals following each of four doses (250 microg) of capsaicin (two per trial day). ⋯ A positive correlation was found between the area of allodynia and area of pinprick hyperalgesia (r(2) = 0.835). Overall, the model was well tolerated with no reports of adverse events. We conclude that the tolerability profile, and variability of i.d. capsaicin-induced pain is acceptable for pharmacological profiling of novel anti-nociceptive agents, with limited number of subjects.
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Facial expression of pain has rarely been researched in the context of facial expression of negative emotions with which it may occur. The main aim of the study was to investigate how pain expression resembled or differed from that of other negative emotions (fear, anger, sadness, surprise, disgust and embarrassment), using multidimensional scaling, a dimensional approach to understanding relationships among emotions. As possible misidentification of facial expressions by participants could distort those results, a judgement study as a categorical approach was conducted to examine the accuracy of identification of pain and negative emotion facial expressions. ⋯ Confidence in ratings approximated accuracy of identification. Multidimensional scaling revealed two dimensions: the first distinguished embarrassment from all other emotion expressions; the second separated pain, sadness and anger from fear, surprise and disgust. Possible explanations for these findings were sought in patterns of facial action units, and in the messages conveyed by the expressions according to Fridlund's Behavioural Ecology View.
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Randomized Controlled Trial Clinical Trial
Naloxone increases pain induced by topical capsaicin in healthy human volunteers.
Opioid receptors occur in locations of strategic importance within the central nervous system for modulation of pain. Is pain reduced by ongoing inhibition mediated by activation of these receptors? Experiments to date in which the opioid-receptor antagonist, naloxone, is administered during a painful event have yielded unclear results. Topically applied capsaicin can be used to induce tonic pain of moderate to severe intensity without tissue injury and is an ideal stimulus for studying acute pain modulation. ⋯ The naloxone induced a significant increase in pain compared both to baseline (P < 0.01) and placebo (P < 0.01). The peak effect, reached at 12-20 min after naloxone delivery, was 59% greater than placebo. This experiment suggests that acute pain is actively suppressed by endogenous opioid-receptor activation.
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Randomized Controlled Trial Clinical Trial
Blinding effectiveness and association of pretreatment expectations with pain improvement in a double-blind randomized controlled trial.
Patient, provider, and clinical investigator expectations concerning treatments are believed to play important roles in patient response. This study examined the association of patient and research nurse/physician pretreatment expectations of pain relief with actual pain relief, the accuracy of patient and research nurse guesses about patient medication assignment, and changes in research nurse and patient pain relief expectations over the course of a randomized double-blind trial of amitriptyline versus an active placebo for patients with chronic pain and spinal cord injuries (SCI). Patient expectations of pain relief with amitriptyline were associated significantly with actual pain decrease for patients in the amitriptyline, but not placebo, condition. ⋯ The research nurse's, but not the patients', expectations of pain relief with amitriptyline decreased significantly over the course of the study. These findings have implications for future randomized controlled trials. Fully double-blind conditions are very difficult to achieve, and it is informative to assess patient and research clinician expectations and guesses regarding medication assignment.
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Clinical Trial Controlled Clinical Trial
Enhanced temporal summation of second pain and its central modulation in fibromyalgia patients.
We have previously shown that fibromyalgia (FMS) patients have enhanced temporal summation (windup) and prolonged decay of heat-induced second pain in comparison to control subjects, consistent with central sensitization. It has been hypothesized that sensory abnormalities of FMS patients are related to deficient pain modulatory mechanisms. Therefore, we conducted several analyses to further characterize enhanced windup in FMS patients and to determine whether it can be centrally modulated by placebo, naloxone, or fentanyl. ⋯ However, naloxone injection had the same magnitudes of effect on first or second pain as that produced by placebo injection. Hypoalgesic effects of saline placebo and fentanyl on windup were at least as large in FMS as compared to NC subjects and therefore do not support the hypothesis that pain modulatory mechanisms are deficient in FMS. To the extent that temporal summation of second pain (windup) contributes to processes underlying hyperalgesia and persistent pain states, these results indirectly suggest that these processes can be centrally modulated in FMS patients by endogenous and exogenous analgesic manipulations.