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Symptoms and types

Allodynia, meaning "other pain", is an exaggerated response to otherwise non-noxious stimuli and can be either static or mechanical. Allodynia is not referred pain, but can occur in other areas than the one stimulated (it is also dysesthetic). Allodynia is also different from hyperalgesia, even though both conditions are characterized by intense pain.

An example of allodynia is when a person perceives light pressure or the movement of clothes over the skin as painful, whereas a healthy individual will not feel pain.

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[1].
  • Thermal (hot or cold) allodynia – pain from normally mild skin temperatures in the affected area


Allodynia is a clinical feature of many pain conditions, such as migraine, postherpetic neuralgia, fibromyalgia, and neuropathies. It can also be caused by a variety of other conditions, for example, lowering the barometric pressure[2].


Pathophysiology at the cellular level

The cell types involved in nociception and mechanical sensation are the cells responsible for allodynia. In healthy individuals, nociceptors sense information about pain and temperature at the skin and transmit it to the spinal cord. The cell bodies of these neurons lie in dorsal root ganglia, important structures located on both sides of the spinal cord. The axons then pass through the 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 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 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[3].

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[4][5]. A different study reports the appearance of descending fibers at the injury site[6]. 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 (see Pathophysiology at the molecular level)[7]. The same effect is achieved in the spinal cord by the recruitment of immune system cells such as monocytes/macrophages and T lymphocytes[8].

Pathophysiology at the molecular level

There is a strong body of evidence that the so called sensitization/hyperexcitability 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 chemokines and other molecules important in the inflammatory response.

A very important molecule in the thalamus appears to be 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[9]. In any case, CCL21 binds to CCR7 and CXCR3 receptors on microglia in the thalamus[10]. The physiologic response to the binding is probably the production of prostaglandin E2 (PGE2) by cyclooxygenase 2 (COX-2)[11]. Activated microglia making PGE2 can then sensitize nociceptive neurons as manifested by their lowered threshold to pain[12].

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[13]. TNF-alpha then binds to the TNF receptors expressed on nociceptors, activating the 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)[14]. This mechanism also explains the perpetuation of sensitization and thus allodynia. TNF-alpha might also increase the number of AMPA receptors, and decrease the numbers of GABA receptors on the membrane of nociceptors, both of which could change the nociceptors in a way that allows for their easier activation[15]. A still other outcome of the increased TNF-alpha is the release of PGE2, with a mechanism and effect similar to the ones in the thalamus[16].


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 enkephalins, endorphins, and 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[17].

Unfortunately, these endogenous mechanisms are often damaged and unfunctional in people suffering from allodynia, so the application of pharmaceuticals is needed.

Man-made compounds

A lot of compounds have been found that alleviate the pain from allodynia; they can be both synthetic and plant-derived. Some of them are specific for a certain type of allodynia, while others are not discriminative. Here is a list with the types of allodynia and the pharmaceuticals used for treating them:[18]

  • Static mechanical allodynia - Na-channel blockers, opioids
    • Lidocaine (IV)
    • Alfentanyl (IV)
    • Adenosine (IV)
    • Ketamine (IV)
    • Glycine antagonist
    • Venlafaxine

The list of compounds that can be used to treat allodynia is even longer than this. For example, many non-steroidal anti-inflammatory drugs, such as naproxen, can inhibit COX-1and/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[19].

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 neurotransmitters or the enzymes that remove neurotransmitters not bound to receptors.

YM-230888 is a compound that can bind to one of the metabotropic glutamate receptors, 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[20].

Endocannabinoids 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[21].


  1. ^ LoPinto, C.; Young, W.B., and Ashkenazi, A. (2006). "Comparison of dynamic (brush) and static (pressure) mechanical allodynia in migrane". Cephalalgia 26: 852-856.
  2. ^ Sato, J.; Morimae, H., Seino, Y., Kobayashi, T., Suzuki, N., and Mizumura, K. (1999). "Lowering barometric pressure aggravates mechanical allodynia and hyperalgesia in a rat model of neuropathic pain". Neuroscience letters 266: 21-24.
  3. ^ Purves et al, ed.: "Nueroscience, 4th ed.", pages 231-250. Sinauer Associates, Inc., 2008
  4. ^ Wasner, G.; Naleschinski, D., and Baron, R. (2007). "A role for peripheral afferents in the pathophysiology and treatment of at-level neuropathic pain in spinal cord injury? A case report". Pain 131: 219-225.
  5. ^ Yezierski, R.P.; Liu, S., Ruenes, G.L., Kajander, K.J., and Brewer, K.L. (1998). "Excitotoxis spinal cord injury: behavioral and morphological characteristics of a central pain model". Pain 75: 141-155.
  6. ^ Kalous, A.; Osborne, P.B., and Keast, J.R. (2007). "Acute and chronic changes in dorsal horn innervation by primary afferents and descending supraspinal pathways after spinal cord injury". The Journal of Comparative Neurology 504: 238-253.
  7. ^ Zhao, P.; Waxman, S.G., and Hains, B.C. (2007). "Modulation of thalamic nociceptive processing after spinal cord injury through remote activation of thalamic microglia by cycteine-cycteine chemokine ligand 21". The Journal of Neuroscience 27: 8893-8902.
  8. ^ Wei, X.H.; Zang, Y., Wu, C.Y., Xu, J.T., Xin, W.J., and Liu, X.G. (2007). "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". Experimental Neurology 205: 471-484.
  9. ^ Zhao, P.; Waxman, S.G., and Hains, B.C. (2007). "Modulation of thalamic nociceptive processing after spinal cord injury through remote activation of thalamic microglia by cycteine-cycteine chemokine ligand 21". The Journal of Neuroscience 27: 8893-8902.
  10. ^ Dijkstra, I.M.; de Haas, A.H., Brouwer, N., Boddeke, H.W.G.M., and Biber, K. (2006). "Challenge with innate and protein antigens induces CCR7 expression by microglia in vitro and in vivo". Glia 54: 861-872.
  11. ^ Alique, M.; Herrero, J.F., and Lucio-Cazana, F.J. (2007). "All-trans retinoic acid induces COX-2 and prostaglandin E2 synthesis in SH-SY5Y human neuroblastima cells: involvement of retinoic acid receptors and extracellular-regulated kinase 1/2". Journal of Neuroinflammation 4: 1-9.
  12. ^ Rukwied, R.; Chizh, B.A., Lorenz, U., Obreja, O., Margarit, S., Schley, M., and Schmelz, M. (2007). "Potentiation of nociceptive responses to low pH injections in humans by prostaglandin E2". The Journal of Pain 8: 443-451.
  13. ^ Hasko, G.; Pacher, P., Deitch, E.A., and Vizi, E.S. (2007). "Shaping of monocyte and macrophage function by adenosine receptors". Pharmacology & Therapeutics 113: 264-275.
  14. ^ Wei, X.H.; Zang, Y., Wu, C.Y., Xu, J.T., Xin, W.J., and Liu, X.G. (2007). "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". Experimental Neurology 205: 471-484.
  15. ^ Stellwagen, D.; Beattie, E.C., Seo, J.Y., and Malenka, R.C. (2005). "Differential regulation of AMPA receptor and GABA receptor trafficking by tumor necrosis factor-alpha". The Journal of Neuroscience 25: 3219-3228.
  16. ^ Coutaux, A.; Adam, F., Willer, J.C., and Le Bars, D. (2005). "Hyperalgesia and allodynia: peripheral mechanisms". Joint Bone Spine 72: 359-371.
  17. ^ Purves et al, ed.: "Nueroscience, 4th ed.", page 248-250. Sinauer Associates, Inc., 2008
  18. ^ Granot, R.; Day, R.O., Cohen, M.L., Murnion, B., and Garrick, R. (2007). "Targeted pharmacotherapy of evoked phenomena in neuropathic pain: A review of the current evidence". Pain Medicine 8: 48-64.
  19. ^ Jakubowski, M.; Levy, D., Kainz, V., Zhang, X.C., Kosaras, B., and Burstein, R. (2007). "Sensitization of central trigeminovascular neurons: Blockade by intravenous naproxen infusion". Neuroscience 148: 573-583.
  20. ^ Kohara, A.; Nagakura, Y., Kiso, T., Toya, T., Watabiki, T., Tamura, S., Shikata, Y., Itahana, H., and Okada, M. (2007). "Antinociceptive profile of a selective metabotropic glutamate receptor I antagonist YM-230888 in chronc pain rodent models". European Journal of Pharmacology 571: 8-16.
  21. ^ Mitchell, V.; Greenwood, R., Jayamanne, A., and Vaughan, C.W. (2007). "Actions of the endocannabinoid transport inhibitor AM404 in neuropathic and inflammatory pain models". Clinical and Experimental Pharmacology and Physiology 34: 1186-1180.
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Allodynia". A list of authors is available in Wikipedia.
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