Changes to the nervous system after injury and surgery

Neuropathic pain has many etiologies and mechanisms.24 Nerve injury, such as transection, stretching, or constriction, will obviously cause structural and functional changes. However, it is important to appreciate that injury to other tissues changes the pain system as well.25 A good example is sunburn, a thermal injury to the skin. To explain the pain after sunburn simply in terms of "damage to the skin'' is an illogical and inadequate explanation; thermal injury to the skin initiates a cascade of changes, including the release of inflammatory mediators from the damaged cells. These change the nociceptors by reducing their thresholds and increasing excitability, both at the periphery and in the spinal cord. This hyperexcitability results in allodynia and hyper-algesia. It is this sensitization in the sensory nerves to the skin (caused by the damage) that causes the ongoing pain, not the damage to the skin itself. In the same way, the injuries involved in surgery can also cause changes, both peripheral and central, resulting in sensitization and hyperalgesia. Many postsurgical pain syndromes are in fact allodynic and hyperalgesic syndromes.

Neuronal plasticity after injury occurs not only at the periphery and the spinal cord, but also in the brain. Pons et al.26 first described remapping of the sensory cortex after deafferentation. Cortical remapping is now known to occur in humans after limb amputation27 and may be evident soon after injury.28 It has been shown to change with time.29 Plasticity can also occur in the thalamus.30 In an interesting case described by Halligan et al.,31 a patient developed the sensation of a third arm following a stroke, suggesting that this may be a two-way process -injury to the brain can cause misperceptions at the periphery. The changes in the brain after amputation contribute to the reason why amputees have phantom sensations and pain.

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