Articles: neuralgia.
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Curr. Opin. Neurobiol. · Feb 2016
ReviewAfferent hyperexcitability in neuropathic pain and the inconvenient truth about its degeneracy.
Neuropathic pain, which arises from damage to the nervous system, is a major unmet clinical challenge. Reversing the neuronal hyperexcitability induced by nerve damage is a logical treatment strategy but has proven frustratingly difficult. Here, we propose a novel explanation for that difficulty. ⋯ Despite offering multiple drug targets, this scenario is problematic: if multiple sufficient changes are triggered by nerve injury, then no single change is necessary for hyperexcitability. This so-called degeneracy compromises therapeutic interventions because drug effects on any one ion channel can be circumvented by changes occurring in other ion channels. Overcoming degeneracy demands a more integrative approach to drug discovery.
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Among many mechanisms implicated in the development of neuropathic pain after nerve damage is a profound dysfunction of GABAergic inhibitory controls, manifested by ongoing pain, mechanical hypersensitivity, and thermal hyperalgesia. In some respects, neuropathic pain can be considered a "disease" of the nervous system, with features in common with trauma-induced seizures. Indeed, first-line management involves anticonvulsant therapy. ⋯ In related studies, we demonstrated that medial ganglionic eminence cell transplants are also effective in a chronic neuropathic itch model in which there is a significant loss of dorsal horn inhibitory interneurons. Most importantly, in contrast to systemic or intrathecal pharmacological therapies, adverse side effects are minimized when the inhibitory control, namely, γ-aminobutyric acid release, occurs in a spinal cord circuit. These studies suggest that therapy targeted at repairing the GABAergic dysfunction is a viable and novel alternative to the management of neuropathic pain and itch, particularly those that are or become refractory to traditional pharmacotherapy.
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Neuropathic pain (NP), a common form of human pain, often poorly responds to analgesic medications. In this review the authors discuss the pathophysiology and conventional treatment of neuropathic pain and provide evidenced-based statements on the efficacy of botulinum neurotoxins (BoNTs) in this form of pain. The level of efficacy for BoNT treatment in each category of NP is defined according to the published guidelines of the American Academy of Neurology. ⋯ It is probably effective (level B) in posttraumatic neuralgia and painful diabetic neuropathy. The data on complex regional pain syndrome, carpal tunnel syndrome, occipital neuralgia, and phantom limb pain are preliminary and await conduction of randomized, blinded clinical trials. Much remains to be learned about the most-effective dosage and technique of injection, optimum dilutions, and differences among BoNTs in the treatment of neuropathic pain.
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Over the last decade, the apparent increase in placebo responses in randomized controlled trials (RCTs) of neuropathic pain have complicated and potentially limited development and availability of new effective pain medication. Placebo analgesia and nocebo hyperalgesia effects are well described in nociceptive and idiopathic pain conditions, but less is known about the magnitude and mechanisms of placebo and nocebo effects in neuropathic pain. In neuropathic pain, placebo treatments have primarily been used as control conditions for active agents under investigation in RCTs and these placebo responses are typically not controlled for the natural history of pain and other confounding factors. ⋯ Large placebo analgesia but no nocebo hyperalgesic effects have been found, and the underlying mechanisms are beginning to be elucidated. Here we review placebo and nocebo effects and the underlying mechanisms in neuropathic pain and compare them with those of nociceptive and idiopathic pain. This allows for a novel discussion on how knowledge of psychological, neurobiological, and genetic factors underlying well-controlled placebo effects may help improve the information that can be obtained from and potentially restore the utility of RCTs.
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Brain responses to nociception are well identified. The same is not true for allodynic pain, a strong painful sensation in response to touch or innocuous cold stimuli that may be experienced by patients with neuropathic pain. Brain (or spinal cord) reorganization that may explain this paradoxical perception still remains largely unknown. ⋯ Both thalamic function and structure have been reported to be abnormal or impaired in neuropathic pain conditions including in the basal state, possibly explaining the spontaneous component of neuropathic pain. A further indication as to how the brain can create neuropathic pain response in SII and insular cortices stems from examples of diseases, including single-case reports in whom a focal brain lesion leads to central pain disappearance. Additional studies are required to certify the contribution of these areas to the disease processes, to disentangle abnormalities respectively related to pain and to deafferentation, and, in the future, to guide targeting of stimulation studies.