Phantom limb pain – pain appearing to come from where an amputated limb used to be – is often excruciating and almost impossible to treat. New approaches, based on a better understanding of the brain’s role in pain, may be opening the way to new treatments.
After amputation of a limb, an amputee continues to have an awareness of it and to experience sensations from it. These phantom limb sensations are also present in children born without a limb, suggesting that perception of our limbs is ‘hard-wired’ into our brain and that sensations from the limbs become mapped onto these brain networks as we develop.
If phantom limb sensations are normal then so too, alas, is phantom limb pain. This occurs in a majority of those who lose their limbs. In fact, limbs do not need to be lost; it also occurs in conditions in which the brain is disconnected from the body, such as peripheral nerve injuries and after spinal cord injury, when an area becomes insentient (and usually paralyzed).
The pain is described in various ways: burning, aching, ‘as if the hand is being crushed in a vice,’ etc. Such words, however, cannot fully encompass the experience of living with such a pain. In those with chronic pain after spinal cord injury it is frequently the pain rather than the paralysis that interferes with work and social life. One woman has said that paralysis does not stop life, but pain may.
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 neurons in the stump, and may change the way that nerves from the amputated limb connect to neurons 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.
Unfortunately, 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, frequently worse, and so surgery is only performed in patients with terminal illness.
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. Although is not clear why this should lead to pain, they 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 lost following amputation.