In this chapter, it has become apparent that the causes of pain migrate as the condition develops. These sequential changes are particularly evident when a limb is amputated. The common picture of amputation is of a violent event in warfare or in accidents. However, amputation becomes more common in peaceful and ageing societies where it is necessary to remove a leg by surgery because the circulation has failed owing to disease of the blood vessels. The immediate pain of amputation is the consequence of the extensive tissue damage. We have described that sequence repeatedly in this chapter with the step-by-step reaction and recovery from damaged tissue. However, a novel painful element appears because amputation necessarily involves cutting the major nerves that supplied the limb.
An immediate and bizarre consequence is the appearance of the phantom limb as a clear, insistent sensation that convinces the patient that the limb is still present in every detail. This phantom limb is an invention of the brain, which has lost the normal steady sensory input from the limb. When the input fails, the cells that receive the input raise their excitability in an attempt to seek the missing input. The consequence is that the brain receives false signals as though the limb was still present. The sensation at the early stage is not painful but it rivets attention because the brain senses that there is something highly unusual happening.
As time goes by, the nerve fibres that have been cut attempt to grow back into the missing limb. The cut ends send out delicate sprouts which end up in a tangle because they have nowhere to go. These new fibres begin to generate spontaneous nerve impulses even though there is no stimulus. The patient feels tingling in the phantom. The young sprouts are unstable and respond easily to pressure. This produces painful tender spots on the stump. Because the sprouts try to grow out into the missing limb for the rest of the patient's life, these tender areas remain.
When the nerve was cut in the amputation, a massive injury barrage swept into the spinal cord. This generates the spreading tenderness even in undamaged tissue, just as we described for the twisted ankle. However, in addition, the outgrowing nerve sprouts find themselves in foreign territory quite different chemically from the areas in which they previously ended. The sprouts pick up these unusual chemicals and transport them to the spinal cord. The cells there get a second signal from these chemicals that something is seriously wrong, even after they have responded to the first signal by way of nerve impulses. The cells set about reorganizing in an attempt to react appropriately. They become more and more excitable, sometimes firing steadily, sometimes firing off in great coordinated explosions, and often overreacting to sensory signals arriving from the remaining uninjured part of the limb.
The patient feels the consequences of these cells attempting to restore normality. Some feel a steady ache in the phantom which can rise to a horrific sense of burning and a feeling that the missing foot or hand is in a permanent cramp. Some feel violent stabs of pain. The nature and intensity of these sensations vary from patient to patient. In everyone, gentle touching of the stump and remaining limb reveals patches of skin that are exquisitely sensitive and painful, with the sensation often radiating into the phantom. Unlike the painful signs of inflammation, which die down as the wound heals, these awful sensations of the amputee may continue with little modification for the rest of their lives.
Although ongoing and episodic pain with zones of painful tenderness are obvious in many amputees, they can occur in any disease in which nerves are damaged. Shingles (herpes zoster) is a condition that becomes more and more common with age. The acute stage is purely inflammatory. Chickenpox viruses, which have been hibernating in one of the dorsal root ganglia since the patient had chickenpox as a child, suddenly multiply in vast numbers. The viruses migrate along the sensory nerves to the skin supplied by the ganglion and produce a band of redness with swelling, pain and often a fever. All these signs of inflammation, with which we are now familiar, die down leaving scarred skin. However, some of the sensory nerves have been destroyed, and some patients develop all of the miseries suffered by amputees but limited to the area where the nerves are damaged. The skin aches and is exquisitely tender and there is a miserable deep ache. If this second phase follows the acute inflammation, the patient is likely to be in trouble for the rest of their life. Therapy is poor.
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Pain Relief
