Allodynia 3339030 215495359 2008-05-28T13:25:44Z DOI bot 6652755 Citation maintenance. You can [[WP:DOI|use this bot]] yourself! Please [[User:DOI_bot/bugs|report any bugs]]. {{Infobox_Disease | Name = {{PAGENAME}} | Image = | Caption = | DiseasesDB = 30788 | ICD10 = | ICD9 = | ICDO = | OMIM = | MedlinePlus = | eMedicineSubj = | eMedicineTopic = | MeshID = }} '''Allodynia''', meaning "other pain", is a [[pain]]ful response to a usually non-painful [[Stimulus_%28physiology%29|stimulus]] and can be either static or mechanical. Allodynia differs from ''[[referred pain]]'', but can occur in areas other than the one stimulated. It is [[Dysesthesia|dysesthetic]]. Allodynia is different from ''[[hyperalgesia]]'', an extreme reaction to a stimulus which is normally painful. ==Types== There are different kinds or types of allodynia: *Mechanical allodynia (also known as tactile allodynia) **Static mechanical allodynia – pain in response to light touch/pressure **Dynamic mechanical allodynia – pain in response to brushing<ref name="pmid16776701">{{cite journal |author=LoPinto C, Young WB, Ashkenazi A |title=Comparison of dynamic (brush) and static (pressure) mechanical allodynia in migraine |journal=Cephalalgia |volume=26 |issue=7 |pages=852–6 |year=2006 |pmid=16776701 |doi=10.1111/j.1468-2982.2006.01121.x}}</ref> *Thermal (hot or cold) allodynia – pain from normally mild skin temperatures in the affected area ==Causes== Allodynia is a clinical feature of many painful conditions, such as [[neuropathies]], [[postherpetic neuralgia]], [[fibromyalgia]], and [[migraine]]. ==Pathophysiology== ===Pathophysiology at the cellular level=== The cell types involved in [[nociception]] and [[Mechanoreceptors| mechanical sensation]] are the cells responsible for allodynia. In healthy individuals, [[nociceptor]]s sense information about pain and temperature at the skin and transmit it to the spinal cord. The [[Soma| cell bodies]] of these neurons lie in [[Dorsal root ganglion| dorsal root ganglia]], important structures located on both sides of the spinal cord. The [[Axon| axons]] then pass through the [[Posterior horn| dorsal horn]] to make connections with secondary neurons. The secondary neurons cross over to the other (contralateral) side of the spinal cord and reach nuclei of the [[thalamus]]. From there, the information is carried through one or more neurons to the [[Postcentral gyrus| somatosensory cortex]] of the brain. Mechanoreceptors follow the same general pathway. However, they do not cross over at the level of the spinal cord, but at the thalamus instead. In addition, they are grouped in tracts that are spatially distinct from the nociceptive tracts. Despite this anatomical separation, mechanoreceptors can influence the output of nociceptors by making connections with the same [[Interneuron| interneurons]], the activation of which can reduce or completely eliminate the sensation of pain. Another way to modulate the transmission of pain information is via descending fibers from the brain. These fibers act through different interneurons to block the transmission of information from the nociceptrors to secondary neurons.<ref name="isbn0-87893-725-0">{{cite book |author=Fitzpatrick, David; Purves, Dale; Augustine, George |title=Neuroscience |publisher=Sinauer |location=Sunderland, Mass |year=2004 |pages=231-250 |isbn=0-87893-725-0 |oclc= |doi=}}</ref> Both of these mechanisms for pain modulation have been implicated in the pathology of allodynia. Several studies suggest that injury to the spinal cord might lead to loss and re-organization of the nociceptrors, mechanoreceptors and interneurons, leading to the transmission of pain information by mechanoreceptors<ref name="pmid17509762">{{cite journal |author=Wasner G, Naleschinski D, Baron R |title=A role for peripheral afferents in the pathophysiology and treatment of at-level neuropathic pain in spinal cord injury? A case report |journal=Pain |volume=131 |issue=1-2 |pages=219–25 |year=2007 |pmid=17509762 |doi=10.1016/j.pain.2007.03.005}}</ref><ref name="pmid9539683">{{cite journal |author=Yezierski RP, Liu S, Ruenes GL, Kajander KJ, Brewer KL |title=Excitotoxic spinal cord injury: behavioral and morphological characteristics of a central pain model |journal=Pain |volume=75 |issue=1 |pages=141–55 |year=1998 |pmid=9539683 |doi=}}</ref> A different study reports the appearance of descending fibers at the injury site.<ref name="pmid17640046">{{cite journal |author=Kalous A, Osborne PB, Keast JR |title=Acute and chronic changes in dorsal horn innervation by primary afferents and descending supraspinal pathways after spinal cord injury |journal=J. Comp. Neurol. |volume=504 |issue=3 |pages=238–53 |year=2007 |pmid=17640046 |doi=10.1002/cne.21412}}</ref> All of these changes ultimately affect the circuitry inside the spinal cord, and the altered balance of signals probably leads to the intense sensation of pain associated with allodynia. Different cell types have also been linked to allodynia. For example, there are reports that [[microglia]] in the thalamus might contribute to allodynia by changing the properties of the secondary nociceptors.<ref name="pmid17699671">{{cite journal |author=Zhao P, Waxman SG, Hains BC |title=Modulation of thalamic nociceptive processing after spinal cord injury through remote activation of thalamic microglia by cysteine cysteine chemokine ligand 21 |journal=J. Neurosci. |volume=27 |issue=33 |pages=8893–902 |year=2007 |pmid=17699671 |doi=10.1523/JNEUROSCI.2209-07.2007}}</ref> The same effect is achieved in the spinal cord by the recruitment of [[immune system]] cells such as [[monocyte]]s/[[macrophage]]s and [[T cell|T lymphocytes]].<ref name="pmid17459378">{{cite journal |author=Wei XH, Zang Y, Wu CY, Xu JT, Xin WJ, Liu XG |title=Peri-sciatic administration of recombinant rat TNF-alpha induces mechanical allodynia via upregulation of TNF-alpha in dorsal root ganglia and in spinal dorsal horn: the role of NF-kappa B pathway |journal=Exp. Neurol. |volume=205 |issue=2 |pages=471–84 |year=2007 |pmid=17459378 |doi=10.1016/j.expneurol.2007.03.012}}</ref> ===Pathophysiology at the molecular level=== There is a strong body of evidence that the so called [[sensitization]] of the central nervous system contributes to the appearance of allodynia. Sensitization refers to the increased response of neurons following repetitive stimulation. In addition to repeated activity, the increased levels of certain compounds lead to sensitization, as well. The work of many researchers has led to the elucidation of pathways that can result in neuronal sensitization both in the thalamus and dorsal horns. Both pathways depend on the production of [[chemokine]]s and other molecules important in the [[Inflammation|inflammatory]] response. A very important molecule in the thalamus appears to be [[CCL21|cysteine-cysteine chemokine ligand 21]] (CCL21). The concentration of this chemokine is increased in the [[ventral posterolateral nucleus]] of the thalamus where secondary nociceptive neurons make connections with other neurons. The source of CCL21 is not exactly known, but two possibilities exist. First, it might be made in primary nociceptive neurons and transported up to the thalamus. Most likely, neurons intrinsic to the ventral posterolateral nucleus make at least some of it.<ref name="pmid17699671"/> In any case, CCL21 binds to [[C-C chemokine receptor type 7]] and Chemokine receptor [[CXCR3]] [[Sensory receptor|receptors]] on [[microglia]] in the thalamus.<ref name="pmid16977602">{{cite journal |author=Dijkstra IM, de Haas AH, Brouwer N, Boddeke HW, Biber K |title=Challenge with innate and protein antigens induces CCR7 expression by microglia in vitro and in vivo |journal=Glia |volume=54 |issue=8 |pages=861–72 |year=2006 |pmid=16977602 |doi=10.1002/glia.20426}}</ref> The physiologic response to the binding is probably the production of [[prostaglandin]] E<sub>2</sub> (PGE<sub>2</sub>) by [[cyclooxygenase]] 2 (COX-2).<ref name="pmid17204142">{{cite journal |author=Alique M, Herrero JF, Lucio-Cazana FJ |title=All-trans retinoic acid induces COX-2 and prostaglandin E2 synthesis in SH-SY5Y human neuroblastoma cells: involvement of retinoic acid receptors and extracellular-regulated kinase 1/2 |journal=J Neuroinflammation |volume=4 |issue= |pages=1 |year=2007 |pmid=17204142 |doi=10.1186/1742-2094-4-1}}</ref> Activated microglia making PGE<sub>2</sub> can then sensitize nociceptive neurons as manifested by their lowered threshold to pain.<ref name="pmid17337250">{{cite journal |author=Rukwied R, Chizh BA, Lorenz U, ''et al'' |title=Potentiation of nociceptive responses to low pH injections in humans by prostaglandin E2 |journal=J Pain |volume=8 |issue=5 |pages=443–51 |year=2007 |pmid=17337250 |doi=10.1016/j.jpain.2006.12.004}}</ref> The mechanism responsible for sensitization of the central nervous system at the level of the spinal cord is different from the one in the thalamus. [[Tumor necrosis factor-alpha]] (TNF-alpha) and its receptor are the molecules that seem to be responsible for the sensitization of neurons in the dorsal horns of the spinal cord. Macrophages and lymphocytes infiltrate the spinal cord, for example, because of injury, and release TNF-alpha and other pro-inflammatory molecules.<ref name="pmid17056121">{{cite journal |author=Haskó G, Pacher P, Deitch EA, Vizi ES |title=Shaping of monocyte and macrophage function by adenosine receptors |journal=Pharmacol. Ther. |volume=113 |issue=2 |pages=264–75 |year=2007 |pmid=17056121 |doi=10.1016/j.pharmthera.2006.08.003}}</ref> TNF-alpha then binds to the TNF receptors expressed on nociceptors, activating the [[Mitogen-activated protein kinase| MAPK]]/[[NF-kappa B]] pathways. This leads to the production of more TNF-alpha, its release, and binding to the receptors on the cells that released it ([[autocrine signalling]]).<ref name="pmid17459378"/> This mechanism also explains the perpetuation of sensitization and thus allodynia. TNF-alpha might also increase the number of [[AMPA receptor]]s, and decrease the numbers of [[GABA receptor]]s on the membrane of nociceptors, both of which could change the nociceptors in a way that allows for their easier activation.<ref name="pmid15788779">{{cite journal |author=Stellwagen D, Beattie EC, Seo JY, Malenka RC |title=Differential regulation of AMPA receptor and GABA receptor trafficking by tumor necrosis factor-alpha |journal=J. Neurosci. |volume=25 |issue=12 |pages=3219–28 |year=2005 |pmid=15788779 |doi=10.1523/JNEUROSCI.4486-04.2005}}</ref> Another outcome of the increased TNF-alpha is the release of PGE<sub>2</sub>, with a mechanism and effect similar to the ones in the thalamus.<ref name="pmid16214069">{{cite journal |author=Coutaux A, Adam F, Willer JC, Le Bars D |title=Hyperalgesia and allodynia: peripheral mechanisms |journal=Joint Bone Spine |volume=72 |issue=5 |pages=359–71 |year=2005 |pmid=16214069 |doi=10.1016/j.jbspin.2004.01.010}}</ref> ==Treatment== ===Endogenous body mechanisms for reducing pain=== As already mentioned, there are descending neurons that modulate the perception of pain. Many of these neurons originate in nuclei in the [[brainstem]] and pass through the [[periaqueductal gray]] (PAG) area of the [[midbrain]]. The body possesses an additional mechanism to control pain: the release of endogenous opioids, especially at the level of the PAG. There are neurons that release [[enkephalin| enkephalins]], [[endorphin| endorphins]], and [[dynorphin| dynorphins]] at the PAG, and in this way modulate its ability to modulate pain perception. Other neurons can release their endogenous opioids at the source of the pain, as well. If this occurs, the transmission of pain information from the nociceptors to the secondary neurons is blocked, and no pain is felt<ref name="isbn0-87893-725-0"/> Unfortunately, these endogenous mechanisms are often damaged and unfunctional in people suffering from allodynia, so the application of pharmaceuticals is needed. ===Drugs=== Numerous compounds alleviate the pain from allodynia. Some are specific for certain types of allodynia while others are general. They include:<ref name="pmid17244104">{{cite journal |author=Granot R, Day RO, Cohen ML, Murnion B, Garrick R |title=Targeted pharmacotherapy of evoked phenomena in neuropathic pain: a review of the current evidence |journal=Pain Med |volume=8 |issue=1 |pages=48–64 |year=2007 |pmid=17244104 |doi=10.1111/j.1526-4637.2007.00156.x}}</ref> ;Dynamic mechanical allodynia - compounds targeting different ion channels; opioids *[[Mexiletine]] *[[Lidocaine]] (IV/topical) *[[Tramadol]] *[[Morphine]] (IV) *[[Alfentanil]] (IV) *[[Ketamine]] (IV) *Methylprednisone ([[intrathecal]]) *[[Adenosine]] *[[Glycine]] antagonist *[[Desipramine]] *[[Venlafaxine]] ;Static mechanical allodynia - Na-channel blockers, opioids * Lidocaine (IV) *Alfentanyl (IV) *Adenosine (IV) *Ketamine (IV) *Glycine antagonist *Venlafaxine ;Cold allodynia *[[Lamotrigine]] *Lidocaine (IV) The list of compounds that can be used to treat allodynia is even longer than this. For example, many [[non-steroidal anti-inflammatory drug]]s, such as [[naproxen]], can inhibit COX-1 and/or COX-2, thus preventing the sensitization of the central nervous system. Another effect of naproxen is the reduction of the responsiveness of mechano- and thermoreceptors to stimuli.<ref name="pmid17651900">{{cite journal |author=Jakubowski M, Levy D, Kainz V, Zhang XC, Kosaras B, Burstein R |title=Sensitization of central trigeminovascular neurons: blockade by intravenous naproxen infusion |journal=Neuroscience |volume=148 |issue=2 |pages=573–83 |year=2007 |pmid=17651900 |doi=10.1016/j.neuroscience.2007.04.064}}</ref> Other compounds act on molecules important for the transmission of an [[action potential]] from one neuron to another. Examples of these include interfering with receptors for [[neurotransmitter]]s or the enzymes that remove neurotransmitters not bound to receptors. YM-230888 is a compound that can bind to one of the [[metabotropic receptor]]s, preventing glutamate from binding and the transmission of the signal. This compound can also overcome the blood-brain barrier, allowing it to be useful in the thalamus.<ref name="pmid17597604">{{cite journal |author=Kohara A, Nagakura Y, Kiso T, ''et al'' |title=Antinociceptive profile of a selective metabotropic glutamate receptor 1 antagonist YM-230888 in chronic pain rodent models |journal=Eur. J. Pharmacol. |volume=571 |issue=1 |pages=8–16 |year=2007 |pmid=17597604 |doi=10.1016/j.ejphar.2007.05.030}}</ref> [[Endocannabinoid]]s are molecules that can relieve pain by modulating nociceptive neurons. When [[anandamide]], an endocannabinoid, is released, pain sensation is reduced. Anandamide is later transported back to the neurons releasing it using transporter enzymes on the plasma membrane, eventually disinhibiting pain perception. However, this re-uptake can be blocked by AM404, elongating the duration of pain inhibition.<ref name="pmid17880375">{{cite journal |author=Mitchell VA, Greenwood R, Jayamanne A, Vaughan CW |title=Actions of the endocannabinoid transport inhibitor AM404 in neuropathic and inflammatory pain models |journal=Clin. Exp. Pharmacol. Physiol. |volume=34 |issue=11 |pages=1186–90 |year=2007 |pmid=17880375 |doi=10.1111/j.1440-1681.2007.04692.x}}</ref> ==References== {{reflist|2}} {{pain}} [[Category:Pain]] [[da:Allodyni]] [[de:Allodynie]] [[fr:Allodynie]] [[pl:Allodynia]]