A ‘NORMAL’ PAIN RESPONSE
The most common question I am asked is whether I am working on physical or mental pain. I have given reasons for doubting the validity of that dualistic question. The second most common question is: 'Why is it that some people are resistant to pain?'. I understand the question and why it is asked. All of us fear pain and doubt our own ability to withstand it. That is to say, we fear the onset of private pain and doubt our ability to endure it with dignity in our public display. No one approaches death or danger without this fear predominating. At the back of our minds, we all have the image of some imagined stoic who bravely copes and says 'It only hurts when I laugh'.
I began to doubt the stability of this stereotype while I was still a medical student. I was assigned to take a blood sample from a massive sergeant major in the Guards with the Military Medal ribbon on his tunic. Unwisely, I sat him upright in a chair and knelt in front of him to insert a needle in an arm vein. As the blood entered the syringe, he fainted and collapsed on top of me. I learnt two lessons: that one should stand to one side of a vertical patient, and that bravery depends on the situation. We need, therefore, to explore what is stable and what is variable in us, and what is the nature of the variation.
Most of the machinery of the body operates in a stable range in all normal people at rest. That stability is the basis of the standard clinical examination. Measurements of blood pressure, blood sugar, body temperature and so on are standardized and are similar from Patagonia to Baffin Island, and in astronauts in orbit and in submarine crews on a deep dive. Similarly, the acuity of the senses is the same among normal people wherever they are tested. Vision and hearing thresholds are similar in all peoples. A special word is assigned to the sharp feeling of a pinprick in all languages. The threshold for feeling the temperature of water as it heats up from warm to painfully hot is about the same for everyone.
A very accurate test of pain threshold to electricity is to place electrodes on the skin of a volunteer and to slowly raise the strength of pulsed shocks. At a low level, the subject reports feeling an innocuous thump which turns to a sharp pricking sensation as the strength is raised. This is the pain threshold in the sense that it is the abrupt appearance of a new type of sensation which is mildly unpleasant but which, if augmented, would eventually become intolerable. Everyone who is free from disease has the same threshold for pain in this special sense. If someone has a raised threshold for this transition from dull to sharp, then they have a disorder such as a disease of their peripheral nerves as in diabetic neuropathy. So far so good. We all have the same pain threshold. The variability appears when we perceive pain above the threshold level and when we approach what we can tolerate.
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