PAIN AND THE WHOLE BODY: MUSCLE RESPONSES
Just about every high-school biology text contains a diagram where a finger touches a saucepan and is rapidly withdrawn. It is used to 'explain' pain as the method of avoiding injury run by a reflex mechanism consisting of sensory afferents which make motor nerves withdraw the hand. I despise that diagram for its triviality. I would estimate that we spend a few seconds in an entire lifetime successfully withdrawing from a threatening stimulus. Unfortunately, we are destined to spend days and months in pain during our lifetime, none of which is explained by that silly diagram. It is true that we and all creatures rapidly withdraw from a threatening stimulus by using a simple input-output loop of nerve fibres. Even the eye-blink is a special example of such a reflex. The brain is not involved in the existence of this reflex but is very much involved in controlling its size. In a paraplegic with the spinal cord completely cut across, a gentle tweak to a toe provokes a particularly violent withdrawal, even though the person feels nothing and cannot willingly withdraw the leg.
Let us pause here to consider the fact that, very, very rarely, children are born who grow up with no sensation of pain. This is called congenital analgesia, and these children are completely normal in other respects. We are not considering here those poor children who are born with very severe, general central nervous system defects and seem not to notice a pinch. The otherwise normal children with congenital analgesia have been the subject of intense study because they are so fascinating and test all our ideas about the meaning and usefulness of our normal ability to perceive pain.
One such case was studied over a period of twenty years up to the time she was a student at McGill University in Montreal. A strong pinch to the foot failed to produce a withdrawal or to provoke pain. When pinched and asked what she felt, she replied in a calm way that 'it feels like a very strong pressure and I know that if you pinch much harder you will injure my foot'. All her other body sensations - touch, pressure, warm, cold and movement - appeared completely normal. How had she grown up without the massive protection supposedly provided by the withdrawal reflex? She had continuous monitoring by her doctor father, mother and siblings, who were all aware of her problem. Gross damage such as a cut, burn or fracture does not need pain to be rapidly detected by the victim. Appendicitis had been diagnosed in her by the signs of fever, inflammation and gut motility, even though she had no pain. Unusual accidents do occur in such people in novel situations. For example, as a child in the deep Canadian winter, she climbed up to look out of the window and knelt on a hot radiator. One could still see line scars on her knees as an adult.
How then do such children learn to avoid harmful stimuli? They learn very rapidly from the alarm of others, from their teaching and from shame. The alarm of others even affects animals. Sheep and pigs explore novel objects with their wet noses. When a thin electrical wire is stretched around a field, a few animals briefly nudge the wire, after which the entire flock keep their distance from the wire. There is no point in smacking a child with congenital analgesia to reinforce orders with pain, but they normally learn to detect anger in adults. Twins with congenital analgesia were studied in detail by specialist child psychologists who discovered that personal and social development appeared completely normal throughout their childhood. It is true that these children have to be warned not to push their luck in the more violent games which children adore. I examined a teenage woman with congenital analgesia who was a champion trampolinist. I was relieved that her enthusiasm for this sport was fading.
The Canadian student with congenital analgesia died at the age of 22 from osteomyelitis. Why? The history of these cases is that the analgesia begins to fade as they grow older. They begin to report deep pains, such as headaches, toothaches and period pains, and later to feel superficial pain. For some this spontaneous recovery comes too late. Why? We have already described the two phases of pain after twisting an ankle. The first sharp, brief pain does not occur in those with congenital analgesia. However, anyone knows that they have twisted an ankle by clues which are not painful and which we use to learn to avoid the situation. Furthermore, when we stumble without pain, we are alerted, we are scared and we learn.
What is far more important is the second phase of pain, in which the pain is deep and spreading. In this phase, far from jerking the foot away, we hold it still, guard it and limp in order not to put pressure on it. All of us have minor accidents several times a year, often so minor that we may forget them but, during the recovery time, we guard the damaged area, protect it and move it as little as possible. That motor behaviour which is the opposite of the sudden brief withdrawal is crucial for recovery because the area of damage cannot complete the inflammatory and recovery processes if it is moving and under pressure.
With congenital analgesia, this recovery phase with guarding after minor injury does not occur. The consequence is that the surfaces of joints and ligaments never fully recover. Furthermore, the joint is in particularly bad shape to counteract the next trivial injury. Strangely, a severe injury such as a fracture does not have such severe consequences because the damaged limb is put in plaster and held stationary until healing occurs. The consequence of repetitive minor injuries with congenital analgesia is that joints, particularly ankles, knees and wrists, become demolished. The dead and damaged tissue becomes a target in which bacteria can flourish and eat their way through the bone into the marrow. This explosive invasion, called osteomyelitis, is still extremely difficult to treat, even with antibiotics, because it is difficult for the medicine to penetrate into this hidden horror. This was the cause of death for the Canadian student and others.
We understand from the experience of children with congenital analgesia that pain has a protective role, not so much in its very acute phase associated with movement, but in the prolonged phase associated with stillness. Even in the acute phase, the muscle reaction is not isolated to simply withdrawing the injured part. You cannot withdraw your foot from a sharp stone unless you stiffen the other leg. If you did not, it would be a disaster because you would collapse back onto the source of your pain. These widespread reactions become much more obvious in the second phase. Joints are splinted by the highly unusual, steady, simultaneous contractions of all the muscles that can move the joint. This contributes to the stiffness which plagues people with arthritis. It is most obvious in the ubiquitous back pains in which the long muscles on both sides of the spine are in steady contraction in their attempt to stop the vertebrae moving. You have only to look at the stiff, twisted postures in a crowd of people walking in the street to diagnose those of our fellow citizens with back pain. Orthopaedic surgeons make 90 per cent of their diagnoses by watching their patients walk into the examination room. Dogs are a wonderful example of the widespread readjustment of muscles produced by a small injury to one foot. They switch effortlessly to a three-legged gait with one leg steadily flexed. This requires an instant reorganization of all the leg and body muscles. And so it does with us.
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