Stephan A Schug
One of the most puzzling pain syndromes is phantom limb pain. Limb (or other body part) amputation or severe deafferentation injury (e.g. brachial plexus avulsion) leads invariably to a sensation of the denervated body part, commonly called phantom sensation. In up to 80% of these cases, patients describe in addition noxious sensory phenomena in the body part, which fulfils then the definition of phantom pain. It must be distinguished from stump pain, which is pain exclusively at the site of the amputation,
The incidence of phantom pain is described variably between 2 and 97%. This variability seems to be the result of differences in definition, way of questioning the patient and a number of external factors. Many authors describe 80% as the most accurate estimate of its incidence. Factors increasing incidence of subsequent phantom pain are severity of preamputation pain and older age. Most of this pain will persist long-term and, contrary to previous opinion, only a limited percentage resolves spontaneously. However, for the phantom sensation telescoping of the limb (shrinking) and fading does occur.
The quality of phantom pain is quite variably and often described as burning, shooting, ischaemic or crushing. A common description is of agonising pain caused by the limb in a hyperextended or unnatural posture. It is mainly localized distally in the amputated limb and often closely resembles the pain before the amputation. It is of note, that phantom pain does not only develop after amputation of limbs, but has also been described after amputation of many body parts such as breast, genitalia, teeth, tongue and rectum.
The pathophysiology of phantom limb pain depends on peripheral and central factors. Peripheral factors may play a role, as stump manipulations, revisions, neuroma removal and nerve blocks can modify phantom pain. Muscle tension in the residual limb increases cramping and squeezing phantom pain. Furthermore, residual limb temperature is usual lower than the contralateral side and such low stump temperature is correlated to burning phantom pain. Another contributing peripheral factor is ectopic discharge, either from the stump neuroma or the respective dorsal root ganglion.
Central mechanisms may involve the spinal cord, the thalamus and the somatosensory cortex. On a spinal level, classical mechanisms of central hypersensitization might be involved in generation of phantom pain, although it is unclear, to what extent these contribute to the overall clinical picture. On a supraspinal level, Melzack proposed the concept of a neuromatrix to explain phantom pain. This neuromatrix is supposed to be genetically determined and then modulated by sensory experience, creating a so called neurosignature for each body part. In this theory, phantom sensations represent persistence of the neurosignature despite loss of the body part, while phantom pain means abnormal reorganisation of the matrix secondary to preexisting pain or the amputation process. More recent work, primarily by Flor and Birbaumer, has shown a physical basis for this theory. By neuromagnetic field analysis, they could show, that a somatosensory pain memory and altered homuncular structure in the somatosensory cortex may underlie phantom pain, while peripheral factors sustain this memory.
Prevention of phantom pain by perioperative regional anaesthesia has been attempted. Most trials and a meta-analysis have shown that preoperative, intraoperative and postoperative epidural anaesthesia/analgesia with local anaesthetics reduces the incidence of phantom pain significantly. The use of continuous peripheral nerve blockade seems to be ineffective. Pharmacological preventative interventions currently investigated include the use of NMDA antagonists and GABA agonists.
There are many treatment modalities described for phantom pain; most of them have efficacy rates indifferent from placebo.
Intravenous calcitonin is the only pharmacological intervention, which has been proven in a double-blind crossover study. Use of membrane stabilisers, tricyclics and opioids might identify responders, who could benefit from these empirical treatment options.
Pharmacological and behavioural treatments (including sympathectomies, beta blockers and biofeedback) that increase stump temperature reduce only the burning component of pain. Treatments that reduce muscle tensions reduce cramping phantom pain. TENS, acupuncture and physical therapy might induce sensory inflow from the stump area and reduce phantom pain.
The indications for invasive interventions are limited. Stump revisions should be limited to those few patients with stump pathology. All denervation procedures of the peripheral nervous system and cordotomies for phantom pain have been abandoned for ineffectiveness. Dorsal root entry-zone lesions have very limited efficacy for a limited time. Spinal cord stimulation has a ca. 50% efficacy rate in phantom limb pain.
Current interest focuses on treatments, which seek to make cortical reorganization regress towards normal. Such approaches might include use of myoelectric prostheses, training to discriminate electric stimuli applied to the stump and asynchronous stimulation of the mouth to separate cortical representation of merged regions as well as the use of mirror boxes.
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