|This is the sensation of the continued presence of an amputated limb. It is most common after arm amputation and if the amputation is delayed after the initial injury. It is associated with tingling or pain, which is severe in 15% of cases. The perceived limb may be felt to be in an abnormal position. It is believed to be a state of central pain.
Mechanisms There may be many mechanisms underlying phantom limb pain. Damage to nerve endings is often important; subsequent erroneous regrowth can lead to abnormal and painful discharge of neurones in the stump, and may change the way that nerves from the amputated limb connect to neurones within the spinal cord. There is also evidence for altered nervous activity within the brain as a result of the loss of sensory input from the amputated limb.
Phantom limb pain is generable intractable and chronic; once it develops, it persists and is rarely improved by present medical treatments. Destructive surgical procedures are also of limited use. They can be effective for a few months, but pain always returns, and is frequently worse, and so surgery is generally only performed in patients with terminal illness.
Traditional treatment of phantom limb pain
Traditional treatments include phenytoin, carbamazepine, sympathetic nerve blocks, transcutaneous electrical nerve stimulation (TENS), dorsal column stimulation and cordotomy. Pre-emptive analgesia with epidurals has been claimed to prevent the development of phantom limb pain when instituted before surgical amputation.
Recently, some potentially valuable treatments have arisen, based on new ways of perceiving the origin of the pain itself.
Flor's group has shown that the development of phantom limb pain is correlated with changes in the way that peripheral areas of the body are represented in the sensory cortex. Although is not clear why this should lead to pain, this group devised experiments to reverse this cortical plasticity, to see whether pain sensations were also altered.
They found that use of an electrical prosthetic limb moved by signals from the patient's muscle reduced the pain if used for several hours per day. Brain imaging revealed that this effect was dependent on a reversion of the sensory cortex to its original state. A task involving repeated touching of the skin over the stump, to improve sensory discrimination there, also reduced phantom limb pain, possibly by replacing some of the sensory input to the brain that was lost following amputation.
Patrick Wall suggested that pain might be considered a 'need state', rather than simply a sensation. If this is the case, then the 'need' might involve movement to avoid or reduce pain.
There has been some evidence that stimulation of the motor cortex can reduce phantom limb pain. Ramachandran and Rogers-Ramachandran asked people with amputations of the arm and phantom limb pain to place their arms inside a mirror box so that they saw their remaining arm mirror-reversed to look like their amputated one. When they moved their remaining arm in the box they were 'fooled' into thinking that they were moving their amputated one, and their pain was reduced. Although this has proved less effective in some subsequent trials, it did suggest that phantom limb pain might reflect a loss of motor control to the limb, as well as loss of sensory input from it.
More recently the mirror box has been used with some success in pain that is not due to sensory loss. In fact, a box may not be required. In phantom limb pain due to a peripheral nerve injury, Giraux and Sirigu have shown that training patients to imagine their paralysed arms moving in relation to a moving arm on a screen in front of them can relieve phantom limb pain.
They suggest that these attempts to link the visual and motor systems might be helping patients to recreate a coherent body image, and so lessen pain as a result of reduced and disordered input. If this approach is successful, it may be that relatively simple treatments, such as patients imagining that they are swinging a golf club with their amputated limb, could have significant pain-relieving benefits.
These new approaches are all based on a shift in emphasis in phantom limb pain away from the site of damage – the stump – to the centre of pain processing: the brain. It appears that disordered inputs from the limb's sensory systems, combined with disrupted motor signal back to the limb, generate a mismatch between the brain's built-in map of the physical body and what is actually perceived. It would appear that this mismatch results in pain.
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