Neuropathic Pain

The brain's sensory cortex, which receives and interprets incoming information, maintains a representation of the body physically within itself. The homunculus is the name given to the diagram obtained when each part of the body is plotted against its place on the sensory cortex, with more important areas of the body being illustrated as larger areas of the brain. Various areas, such as the hands and the lips, take up much more brain area due to their importance in normal function, and it is these most important areas to control which need greater sensory awareness and greater processing power to work out responses.

When we suffer an injury the pain comes directly from that part, streaming in from the highly irritated nerve ends and the normally silent nerves woken up by the chemical soup of the injury. As the barrage of impulses comes in to the spinal cord it meets the second stage nerves which will take the messages on into the central nervous system. These second stage nerves become highly excited by the incoming torrents of impulses and amplify the signal significantly, passing on much higher pain levels to the higher brain centres.

We don't feel pain until it reaches the higher brain centres and intrudes upon our consciousness. In a sense, all our pain is in our minds, as it does not exist unless it gets up to our conscious brain. Our pain is not imaginary, our brains are constantly creating a virtual reality for us to understand the world, a virtual visual reality, a virtual touch reality and also a virtual pain reality when it's appropriate. This concept is important in that it is the brain which constructs our pain reality and not the broken ankle, the slipped disc or the burnt hand.

When a limb in amputated it is obvious the muscles, ligaments and bones are all cut, but what is less clear, and much more important for the future, is that the nerves travelling down the part are also cut through. Cutting the part of the nervous system off from the centre means a sudden loss of incoming signals from the amputated part, with serious side effects for the individual. When the nervous system is deprived of its incoming information the consequences can be unpleasant.

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.

Drug treatment of phantom pain is difficult as the morphine chemicals such as morphine, fentanyl, tramadol and codeine are often not very effective. The nerve treatment agents such as amitriptyline, gabapentin and pregabalin are used against neuropathic pain with some effectiveness. Other treatments include transcutaneous electrical nerve stimulation (TENS), an electrode based stimulation treatment which can be self-managed. Cognitive therapy may also be useful to start to cope with what can be a long term problem.

Phantom pain can be an intractable, serious problem for anyone with an amputation, and having significant pain before the amputation may make the likelihood of phantom pain greater. A multidisciplinary approach involving a pain clinic is most likely to be helpful.