Pain
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Tibia fracture in rats results in chronic vascular and nociceptive changes in the injured limb resembling complex regional pain syndrome (CRPS) and up-regulates expression of interleukin 1β (IL-1β), interleukin IL-6 (IL-6), tumor necrosis factor-α (TNF-α), and nerve growth factor-β (NGF-β) in the hindpaw skin. When fractured rats are treated with cytokine or NGF inhibitors nociceptive sensitization is blocked. Because there is no leukocyte infiltration in the hindpaw skin we postulated that resident skin cells produce the inflammatory mediators causing nociceptive sensitization after fracture. ⋯ Local injections of IL-6 and TNF-α induced hindpaw mechanical allodynia lasting for several days and modest increases in temperature and edema. These data indicate that activated keratinocytes proliferate and express IL-1β, IL-6, TNF-α, and NGF-β after fracture and that excess amounts of inflammatory mediators in the skin cause sustained nociceptive sensitization. This is the first study demonstrating in vivo keratinocyte expression of IL-6, TNF-α and NGF-β in a CRPS model and we postulate that the keratinocyte is the primary cellular source for the inflammatory signals mediating cutaneous nociceptive sensitization in early CRPS.
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Randomized Controlled Trial Multicenter Study
A multicenter, placebo-controlled, double-blind, multiple-crossover study of Fentanyl Pectin Nasal Spray (FPNS) in the treatment of breakthrough cancer pain.
This randomized, double-blind, crossover study assessed the efficacy and tolerability of a new rapid onset nasal fentanyl formulation (Fentanyl Pectin Nasal Spray; FPNS) for breakthrough cancer pain (BTCP). Eighty-three of 114 patients experiencing one to four BTCP episodes/day while taking ≥60 mg/day of oral morphine or equivalent successfully identified an effective dose of FPNS during a titration phase and entered a double-blind phase in which 10 BTCP episodes were treated with this effective dose (7) or placebo (3). Compared with placebo, FPNS significantly improved mean summed pain intensity difference (SPID) from 10 min (P<0.05) until 60 min (P<0.0001), including the primary endpoint at 30 min (P<0.0001). ⋯ Approximately 70% of patients were satisfied or very satisfied with the convenience and ease of use of FPNS. Only 5.3% of patients withdrew from treatment due to adverse events, no significant nasal effects were reported, and 87% of patients elected to continue open-label treatment post-study. In this short-term study, FPNS was safe, well tolerated, and rapidly efficacious for BTCP.
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Despite growing interest in the placebo effect, the neural correlates of conditioned analgesia are still incompletely understood. We investigated herein on brain activity during the conditioning and post-conditioning phases of a placebo experimental paradigm, using event-related fMRI in 31 healthy volunteers. Brief laser heat stimuli delivered to one foot (either right or left) were preceded by different visual cues, signalling either painful stimuli alone, or painful stimuli accompanied by a (sham) analgesic procedure. ⋯ Specifically, a large focus in the right prefrontal cortex showed activity related to analgesia, irrespective of the expected side of stimulation. Analgesia was also related to decreased activity, detectable immediately following noxious stimulation, in parietal, insular and cingulate pain-related clusters. Our findings of dynamic changes in prefrontal areas during placebo conditioning, and of direct placebo effects on cortical nociceptive processing, add new insights into the neural bases of conditioned placebo analgesia.
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Quantitative sensory testing (QST) is commonly used to evaluate peripheral sensory function in neuropathic conditions. QST measures vary in repeated measurements of normal subjects but it is not known whether QST can reflect small changes in epidermal nerve fiber density (ENFd). This study evaluated QST measures (touch, mechanical pain, heat pain and innocuous cold sensations) for differences between genders and over time using ENFd as an objective-independent measure. ⋯ Variation in measurements over time was large in a fraction of normal subjects. We conclude that most QST measures detect relatively large differences in epidermal innervation (12.2 ENFs/mm), but response to mechanical pain was the only sensory modality tested with the sensitivity to detect small changes in innervation (4.18 ENFs/mm). Since some individuals had large unsystematic variations, unexpected test results should therefore alert clinicians to test additional locations.
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Focused attention typically enhances neural nociceptive responses, reflected electroencephalographically as increased amplitude of pain-evoked event-related potentials (ERPs). Additionally, pain-evoked ERPs are attenuated by hypertension and baroreceptor activity, through as yet unclear mechanisms. There is indirect evidence that these two effects may interact, suggesting that baroreceptor-related modulation of nociception is more than a low-level gating phenomenon. ⋯ Cued stimuli were associated with larger P2 amplitude, but this effect was abolished for stimuli presented during baroreceptor activation. No cardiac timing effect was observed for un-cued stimuli. Taken together, these findings suggest a close integration of cognitive-affective aspects of expectancy and baroreceptor influences on pain, and as such may cast further light on mechanisms underlying mental and physiological contributions to clinical pain.