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M.Jeschke - Handbook of Burns Volume 1 Acute Burn Care - 2013.pdf
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R. Girtler, B. Gustorff

Pathophysiology of pain after burn injuries

Pain after burn injuries has various reasons and sources. Burn depth and pain intensity are not always directly related. Thus, a solid fundamental knowledge of physiology and pathogenesis of burn injury pain is an important prerequisite for an adequate therapy and medication.

Nociceptive pain

Pain caused by stimulating the nociceptors play a vital role. Nociceptive pain has signal and warning functions for the human organism.

Nociceptors are situated at the free ends of the pain conducting nerve fibres. Depending on their localizations they are subdivided into somatic nociceptors (in the muscles, tendons, joints, bones and skin) and visceral nociceptors. A large proportion of them are sleeping nociceptors which can only be stimulated when inflammation occurs. Nociceptors are stimulated by chemical, thermal and mechanical noxa. This stimulation is conducted to the posterior horn of the spinal cord in the form of evoked potentials (translocation) via richly myelinated and fast conducting A -fibers (giving sharp and terebrant pain that can be easily located) and unmyelinated and slowly conducting C-fibres (giving permanent dull pain that cannot be easily located). In the spinal cord stimulating amino acids, as for example aspartate and glutamate, and substance P and calcitonine gene related peptide (CGRP) are set free, which causes the transmission to second order neurons (nociceptive-specific neurons and wide dynamic range neurons). The second order neurons conduct the pain impulses via the tractus spinothalamicus and the tractus spinoreticularis to the thalamus and then to cortical areas. The somatosensoric pain processing is carried out in the lateral thalamocortical pain processing system whereas the medial thalamocortical pain processing system shows connections to the limbic system. Here affective nociception occurs.

Nociception can be treated well with opioids and non-opioids.

Inflammation-related pain

Tissue trauma sets free inflammatory mediators (prostaglandines, cytokines, oxygen radicals. histamine, bradykinine and platelet activating factor). These cause a peripheral sensitization with decreasing the stimulus threshold of the nociceptors (primary hyperalgesia). Inflammation-related pain occurs within minutes and is reversible after wound healing.

Persistent and repeatedly occurring inflamma- tion-related pain might be caused by wound infection [1].

Continuous nociceptive inflow from the periphery sensitizes secondary nociceptive neurons in the posterior horn of the spinal cord (central sensitization). Their reaction to stimuli is an increased discharging frequency. Thus a higher number of glutamate, substance P and CGRP are set free, glutamate receptors are depolarized (AMPA receptors, NMDA receptors) and calcium flows into central pain neurons. Via expression of immediate early genes, the receptive areas in the periphery, with an increased sensitivity to pain in the neighboring unaffected area of the primary lesion, are enlarged [4].

Neuropathic pain

Neuropathic pain is caused by injury, deafferenciation, illness or functional disorders of the afferent nervous system. This kind of pain does not have any biological function and causes severe suffering and is often resistant to therapy.

The disorders in the afferent system can be accompanied by negative phenomena (hypesthesia and hypalgesia) and positive phenomena (paresthesia, dysesthesia, hyperalgesia, allodynia and burning, terebrant pain).

Neuropathic pain after burns can occur directly or after a period of time: on the one hand burns damage and partially destroy nerve structures. Some studies showed that neuropathic pain already occurred within 1 to 7 days after the burn injury happened [5]. On the other hand neuropathic sensation or pain can also occur a long time after the healing of a burn wound [6, 7]. It is assumed that this is due to an unsuccessful regeneration of the nerves with ingrowth of damaged neurons to neighboring healthy nerves or the formation of neurons.

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