Treating Phantom Pain

Untreated, phantom limb pain can become intractable and chronic; once it develops and persists the sensations are rarely improved by present medical treatments. Should the amputee feel that he or she is helpless to do anything about the pain it can grow at a seemingly uncontrollable rate. Destructive surgical procedures have been tried yet they prove to be of limited use. The procedures can be effective for a few months or more, but the phantom pains always return, frequently worse than before the surgery. These procedures that would cut the nerve endings in an attempt to alleviate the erroneous growth issues are therefore not advised and likely not covered by insurance.

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 peripheral areas of the body are represented in the sensory cortex. Even though it is not clear why this should lead to pain, they devised experiments to reverse this cortical plasticity to see whether the pain sensations were also altered.

The study 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 lost following amputation.

Visual tricks
In a recent publication, Patrick Wall suggested that pain might be considered a 'need state', like thirst, rather than simply a sensation. If that is so, then the 'need' might involve movement to avoid or reduce pain.

Evidence that stimulation of the motor cortex (the area that controls movement) can reduce phantom limb pain has been around for some time. Perhaps more surprising was a trial by Ramachandran and Rogers-Ramachandran in 1996. They 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 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 (brachial plexopathy), Giraux and Sirigu have shown that merely 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 recreate a coherent body image, and so reduce 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.

Finally, in experiments still being developed, we are constructing an arm in virtual reality which subjects with phantom limb pain will move themselves using motion capture techniques. Movement of their stump will be captured by a movement-tracking device, and used to project the movement of the reconstituted limb in virtual reality. We anticipate that this will lead to a sense of re-embodiment in the virtual arm and hence to a reduction of the pain.

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. For some reason, this mismatch results in pain.

Whichever of these new techniques proves effective – and simple enough to be used – the prospects for relief from pain are probably brighter than at any time since Weir Mitchell first coined the term phantom limb pain in 1872.