Phantom Limb Pain
Our brains hold a representation of our body within them, a mental picture of the different parts of the body which is situated in our brain sensory cortex. The sensory cortex is programmed to look after particular parts of the body and this representation is known as the homunculus. Different parts of the body such as the lips and the hands are highly represented as body areas which need lots of brain control to understand what's going on. These areas of the body are much more important for managing our daily lives and are represented much more strongly in the region of the brain devoted to them.
An injured tissue releases inflammatory chemicals which both cause local pain nerves to fire off and wake up pain nerves which normally don't participate much. As the levels of nerve impulses increase up towards the spinal cord, the second stage nerves there become overexcited and begin to amplify the incoming messages, cranking up the pain levels transmitted up to the brain. This amplification of incoming nerve impulses is known as central sensitisation, occurring in every acute pain state but persisting in central pain states such as neuropathic pain, of which phantom pain is an example.
Pain needs to get through up to the brain and force its way into our conscious minds until we feel it, as our pains are always in our minds strictly speaking. Pain is never imaginary but our brains construct a virtual sensory reality so we can make sense of the world which includes touch, pain and visual realities. It is important to realise that the brain builds the pain experience that we endure and that this is not made up by our injured neck, disc prolapse or torn ligament.
Amputation causes a much greater disruption of the body's systems than appears obvious on the surface as it is not just the ligaments, muscles and bones which are surgically cut. The nerves which travel down the limb are also cut across and this has profound repercussions as the nervous system does not appreciate it when input from the body is cut off. When the constant streams of information coming in are cut off the nervous system reacts in abnormal ways which can cause unpleasant pain syndromes which are hard to treat.
The second stage nerves react badly to being deprived of their incoming streams of impulses, not by going off-line but by doing the opposite, by increasing their reactivity and responsiveness. Because the nerve has been cut and there are not messages coming through they can begin to fire off impulses for no reason, spontaneously. These overexcited nerves can produce a significant pain problem as while the leg does not exist any more the nerves which serve the leg areas are still present in the central nervous system. The brain's sensory areas responsible for the leg are still capable of manufacturing leg pain.
Pain which appears in an area of the body which is now absent is known as phantom pain and is a common side effect of amputation which develops in the weeks and months after the trauma. Phantom pain can be very unpleasant in nature, very deep and cold, or sharp and stabbing and so can be a particularly difficult pain to treat or to cope with. Neuropathic pain is the term for a pain like this which is generated internally by the central nervous system and not as normal pains which are secondary to tissue damage.
Neuropathic pain is difficult to treat with morphine type drugs such as fentanyl, codeine, morphine or tramadol and more common agents against this sort of pain are gabapentin, amitriptyline and pregabalin. Physical treatment agents such as transcutaneous electrical nerve stimulation may be used by attaching small electrodes to the skin and transmitting pain reducing signals to the central nervous system. Cognitive therapy may be necessary to help patients manage the problems of a long term pain.
Many patients with an amputation have a serious, long term problem in phantom pain, with severe pre-operative pain perhaps predisposing them to pain difficulties after operation. Involving a pain clinic with its multidisciplinary management from the team is a good approach.