Pain
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We assessed cortical excitability and intracortical modulation systematically, by transcranial magnetic stimulation (TMS) of the motor cortex, in patients with fibromyalgia. In total 46 female patients with fibromyalgia and 21 normal female subjects, matched for age, were included in this study. TMS was applied to the hand motor area of both hemispheres and motor evoked potentials (MEPs) were recorded for the first interosseous muscle of the contralateral hand. ⋯ These neurophysiological alterations were not linked to medication, as similar changes were observed in patients with or without psychotropic treatment. In conclusion, fibromyalgia is associated with deficits in intracortical modulation involving both GABAergic and glutamatergic mechanisms, possibly related to certain aspects of the pathophysiology of this chronic pain syndrome. Our data add to the growing body of evidence for objective and quantifiable changes in brain function in fibromyalgia.
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Analgesics currently available for the treatment of pain following ophthalmic surgery or injury are limited by transient effectiveness and undesirable or adverse side effects. The cornea is primarily innervated by small-diameter C-fiber sensory neurons expressing TRPV1 (transient receptor potential channel, subfamily V, member 1), a sodium/calcium cation channel expressed abundantly by nociceptive neurons and consequently a target for pain control. Resiniferatoxin (RTX), a potent TRPV1 agonist, produces transient analgesia when injected peripherally by inactivating TRPV1-expressing nerve terminals through excessive calcium influx. ⋯ Importantly, RTX analgesia (a) did not impair epithelial wound healing, (b) left the blink reflex intact and (c) occurred without detectable histological damage to the cornea. Immunohistochemistry showed that loss of CGRP immunoreactivity, a surrogate marker for TRPV1-expressing fibers, extended at least to the corneal-scleral boundary and displayed a progressive return, coincident with the return of capsaicin sensitivity. These data suggest that RTX may be a safe and effective treatment for post-operative or post-injury ophthalmic pain.
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Non-invasive, movement-based models were used to investigate muscle pain. In rats, the masseter muscle was rapidly stretched or electrically stimulated during forced lengthening to produce eccentric muscle contractions (EC). Both EC and stretching disrupted scattered myofibers and produced intramuscular plasma extravasation. ⋯ In contrast, stretching significantly increased the number of P2X(3) muscle afferent neurons for 12d. The sustained, elevated P2X(3) expression evoked by EC and stretching may enhance nociceptor responsiveness to ATP released during subsequent myofiber damage. Movement-based actions such as EC and muscle stretching produce unique tissue responses and modulate neuropeptide and nociceptive receptor expression in a manner particularly relevant to repeated muscle damage.
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Alexithymia, the inability to identify or label emotions, has been shown to be associated with pain in patients with a number of chronic pain conditions. We sought to: (1) replicate this association in samples of persons with chronic pain secondary to neuromuscular disease, (2) extend this finding to other important pain-related measures, and (3) to determine whether relationships among alexithymia and study variables existed after controlling for negative affect. One hundred and twenty-nine individuals with muscular dystrophy and chronic pain were administered measures of alexithymia (Toronto Alexithymia Scale, TAS-20), pain intensity (0-10 NRS), pain interference (Brief Pain Inventory Interference scale), mental health (SF-36 Mental Health scale; as a proxy measure of negative affect) and vitality (SF-36 Vitality scale). ⋯ Although the strengths of these associations were reduced when mental health was used as a control, the associations between the Difficulty Identifying Feelings scale and vitality, and the Externally Oriented Thinking and Total TAS scales and pain intensity remained statistically significant. The findings replicate and extend previous findings concerning the associations between alexithymia and important pain-related variables in a sample of persons with chronic pain and neuromuscular disease. Future research is needed to determine the extent to which the associations are due to (1) a possible causal effect of alexithymia on patient functioning that is mediated via its effects on negative affect or (2) the possibility that alexithymia/outcome relationships reflect response bias caused by general negative affectivity.