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Editorial II: What use is pain?

2005; Elsevier BV; Volume: 94; Issue: 2 Linguagem: Inglês

10.1093/bja/aei017

1471-6771

Tim Nash,

Pediatric Pain Management Techniques

Ask any medical student, trainee anaesthetist, or patient 'what use is pain?' and they will tell you it is protective, or it is a warning. The view that it is protective has puzzled me for some considerable time. As anaesthetists, we spend entire careers trying to control pain, enabling early mobilization to reduce the complications from surgery. In this evidence-based era, the claim that pain is protective needs to be substantiated. Even the IASP definition of pain does not address this issue, considering pain to be an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage.1Merskey H Bogduk N Classification of Chronic Pain. IASP Press, Seattle1994: 201Google Scholar In no way does it suggest that pain has a use or is protective. It appears that at best it informs that something biologically harmful is happening to our bodies.2Melzack R The Puzzle of Pain. Penguin Education, Harmondsworth1977: 15-16Google Scholar So where has the view that pain is protective come from? The development of thought around pain until modern times has been well described.3Rey R History of Pain. Edition La Decouverte, Paris1993Google Scholar Plato considered pain was felt when 'atoms' suffered alteration, and pleasure when they were restored to their original state. Similarly, Timaeus considered those parts that underwent violent alterations and were restored gradually and with difficulty to their original condition produced the greatest pains. Hippocrates considered pain purely a clue to disease, that is a symptom. The Platonic view was developed by the Epicureans, who viewed supreme pleasure as a total absence of every type of pain. Aristotle thought the heart to be the centre of all sensibility, and the soul to be immortal, and pain was part of suffering. He preceded Christianity by over 300 yr. Christian thinking persisted into mediaeval times, with pain then considered to be a divine gift or a sacrificial offering, bringing faithful believers closer to Christ. Celsus reiterated the Hippocratic view in 100 AD, considering that pain only announced specific disorders and provided a prognosis. Galen was the first to suggest that pain had a use. He appreciated that there was a network of nerves leading to the brain. He considered that 'Nature indeed has had a triple end in view in the distribution of nerves: she wished to give sensibility to organs of perfection, movement to organs of locomotion, and to all the others the faculty of recognizing the experience of injury. The third aim of nature in the distribution of nerves is the perception of that which can cause harm' (De Usus Partium). If living things did not have this, he reasoned, they would soon die. He gave as an example pain in the intestines, which he considered signified noxious substances piling up, corroding, ulcerating, and rotting the intestines. The pain spurs us to get rid of these noxious substances before they can do damage. As Christianity was in its ascendancy at that time and he refused to admit that the soul was immortal, this view did not develop further. Christianity had total control over scientific thought, as centres of learning were the monasteries or Church institutions. Thus, further scientific debate was stifled until the reformation. Sydenham in the 17th century reckoned that pain caused reflex movement for retraction and flight, and brought a supra-threshold stimulus to consciousness for the benefit of the patient and the doctor. This was a step beyond Hippocrates, who had only seen pain as a symptom, a clue to disease, and now suggested that pain was there to aid doctors in diagnosis. Even in the 18th century, when science had broken free of the influence of the church, only a few considered pain useful. Leriche in 1939 considered pain had no use at all, stating: 'Defence reaction? Fortunate warning? But as a matter of fact, the majority of diseases, even the most serious, attack us without warning. When pain develops …. It is too late…. The pain has only made more distressing and more sad a situation already lost…. In fact, pain is always a baleful gift, which reduces the subject of it, and makes him more ill than he would be without it.' The notion that pain has a distinct use, and especially that it is protective and a warning, developed in the last century.4Melzack R Wall P The Challenge of Pain. Penguin Books, Harmondsworth1982: 15-19Google Scholar5Nagasako EM Oaklander AL Dworkin RH Congenital insensitivity to pain: an update.Pain. 2003; 101: 213-219Abstract Full Text Full Text PDF PubMed Scopus (208) Google Scholar6Sternbach RA Pain: A Psychophysiological Analysis. Academic Press, New York1968: 95-115Crossref Google Scholar, 7Sternbach RA Congenital insensitivity to pain: a critique.Psychol Bull. 1963; 60: 252-264Crossref Scopus (18) Google Scholar Indeed, I do not recall it being prevalent when I was a student in the 1960s. Sternbach6Sternbach RA Pain: A Psychophysiological Analysis. Academic Press, New York1968: 95-115Crossref Google Scholar, 7Sternbach RA Congenital insensitivity to pain: a critique.Psychol Bull. 1963; 60: 252-264Crossref Scopus (18) Google Scholar is quoted in the much read 'Puzzle of Pain' as stating that those with congenital insensitivity to pain, without the ability to feel pain, provide convincing testimony on the value of pain. He had thoroughly reviewed all the reported cases of insensitivity to pain at that time. He considered that true congenital insensitivity to pain should have insensitivity to pain present since birth, and not acquired secondarily to trauma or disease; that this insensitivity to pain should be general, and to a variety of potentially noxious stimuli over the entire body with no or only slight involvement of other sensory modalities; and with no general mental or physical retardation, that is normal in every other respect. He states clearly in the first sentence of his abstract that no available reported case of apparent insensitivity to pain met strict requirements for the syndrome. The 17 'probable' cases that he was able to identify were neurologically and behaviourally heterogeneous. He does state that the ability of this group to survive was seriously impaired. He also states that there is reason to suspect that general cutaneous sensitivity and visceral sensitivity are always impaired to some degree in these patients, which becomes apparent when careful testing for absolute and difference thresholds prevents the use of alternative cues. Indeed, he points out that the same peripheral nerve fibres respond to light touch, pressure, temperature, and pain. Further, many children with congenital insensitivity to pain went on to develop sensory radicular neuropathies. The one case he reported that most nearly fitted his criteria for true congenital insensitivity to pain was a 22-yr-old student. She had all senses except pain, but he points out that there was no sneezing or coughing or gag reflex. She developed Charcot's joints in her spine and became paraplegic. She was described as being in the habit of using her other sense modalities to check for otherwise unsuspected cuts, burns, fractures, etc. She died from bronchopneumonia and amyloidosis. The absent cough reflex may have been important here in relation to the bronchopneumonia. Amyloidosis could well have been associated with the development of neuropathy, and can be secondary to an inflammatory arthritis. Charcot's joints result from painless neuropathic joints, not purely painless joints, and probably as a result of neurovascular, not neurotraumatic change.8Weatherall DJ Ledingham JGG Warrell DA The Oxford Textbook of Medicine. 3rd edn. Oxford University Press, Oxford1996: 2979Google Scholar Diabetic tarsal neuropathic joints may indeed be painful. It is clear that this patient not only had an absence of pain, but also a degree of sensory neuropathy as well that was responsible for her lack of gag, cough and sneeze reflex as well as her Charcot's joints. This patient is quoted as an example of early death as a direct result of insensitivity to pain! Other cases are quoted that are also taken as evidence of 'testimony on the value of pain'. One common factor runs through the discussion. Sternbach considered all patients were potentially saved or salvageable by medical intervention, such as the lady with appendicitis who only reported a tight feeling, and her doctor suspected appendicitis and admitted her to hospital. A further instance is quoted of pre-eclampsia in another patient who did not experience headache, and was saved by the quick thinking of her doctor, who diagnosed her condition on other aspects of her presentation. It is stated that she would otherwise have died. Pre-eclampsia occurs not uncommonly without headache, and is only at risk of becoming fatal if it progresses to eclampsia, which of course it may do without treatment. There is one case reported of a burn from a bathroom heater in a 7 yr old, who felt no pain and was 'branded' by the gratings.4Melzack R Wall P The Challenge of Pain. Penguin Books, Harmondsworth1982: 15-19Google Scholar Those who have experienced full-thickness burns are aware of their painlessness, at least initially. Sternbach7Sternbach RA Congenital insensitivity to pain: a critique.Psychol Bull. 1963; 60: 252-264Crossref Scopus (18) Google Scholar points out that those cases that came closest to fitting his criteria for true congenital insensitivity to pain were neurologically and behaviourally heterogeneous with variations in their neural deficits. Basing the premise that pain is protective on these cases is therefore clearly flawed. I do not believe that we can substantiate the existence, development, and evolution of pain across all sensate beings in this way, purely to enable the development of medicine and medical technology, or aid doctors in reaching a diagnosis. Pain surely had no use prior to our ability to do an appendicectomy. Sternbach does consider that the ability to survive is seriously impaired and depends in large measure on the patient's ability to utilize other sensory cues of actual or potential tissue damage in these cases. If they survive childhood, they have learned to rely on other cues. He was able to show some slight impairment of difference thresholds for temperature in a family he studied when cues such as wetness of test tubes were avoided. Yet he concludes that the concept that a sense of pain is necessary for survival seems generally to be true, and relates it in particular to avoidance of injury rather than disease. However, he has already pointed out that the ability to be aware of injury also relies on the ability to use other, presumably sensory, cues. He also considers that the pain of angina, the headache of hypertension, and toothache are examples of stimuli that can cause a person to modify his/her ongoing behaviour in ways that minimize the danger. Is it reasonable to argue this, when in fact these responses are probably more directed at limiting the pain? We have no evidence that this change in behaviour increases the likelihood of survival. Current knowledge of congenital insensitivity to pain has been well summarized, with a suggested full system of classification.5Nagasako EM Oaklander AL Dworkin RH Congenital insensitivity to pain: an update.Pain. 2003; 101: 213-219Abstract Full Text Full Text PDF PubMed Scopus (208) Google Scholar The condition is now described as Hereditary Sensory and Autonomic Neuropathy, together with another group who have congenital indifference to pain, where pain is felt but the affective response is deficient. Nagasako and colleagues state that the observation that these people often die in childhood because they fail to notice injuries and illnesses has been viewed as compelling evidence that the ability to perceive pain has great survival value. However, their classification of the five types of Hereditary Sensory and Autonomic Neuropathy (HSAN types I–V), differentiated by clinical presentation, types of nerve fibre loss, and genetics, show a clear loss of various nerve fibres and sensory modalities in each group. HSAN type III appears to be the group most at risk of early demise. Half of the patients so categorized, otherwise known as familial dysautonomia or Riley-Day syndrome, die before the age of 30 yr. They suffer widespread autonomic dysfunction, loss of pain and temperature sensation, and have difficulties feeding in infancy and incidents of elevated body temperature. There is a severe loss of unmyelinated fibres, and a total absence of large-diameter myelinated neurones. Can we therefore ascribe their reduced longevity purely to their inability to feel pain, when clearly there is such gross deficiency of sensory neurones? If pain had any important use, then surely we should experience it with every injury. But pain is not always felt immediately after injury. Beecher9Melzack R Wall P The Challenge of Pain. Penguin Books, Harmondsworth1982: 15Google Scholar found 65% of severely wounded soldiers and 20% of civilians undergoing major surgery had little or no pain for hours or days after the injury. Indeed, 37% of injured patients attending an emergency clinic felt no pain for many minutes or even hours after the injury.10Melzack R Wall PD Ty TC Acute pain in an emergency clinic: latency of onset and descriptor patterns related to different injuries.Pain. 1982; 14: 33-43Abstract Full Text PDF PubMed Scopus (159) Google Scholar Clearly, it is not a reliable informant. Again, what other sense do we have to which we have ascribed its own modulating system? Does sight, sound, touch, taste, or smell have a modulating system to equal the pain gate? Pain is not even an essential part of the withdrawal reflex, which happens even before the pain is felt.11Hervey GR The functions of pain.in: Holden AV Winlow W The Neurobiology of Pain. Manchester University Press, Manchester1984: 399-403Google Scholar Pain normally produces strong aversive responses. However, dogs can be trained to seek painful electric shocks that normally produce strong aversive behaviour, when they receive a reward of food after each shock.12Pavlov IP Lectures on Conditioned Reflexes. International Publishers, New York1928: 27-30Google Scholar It is clear that pain is involved in learning and memory, normally producing aversion but if the reward is good enough, it may lead to the animal seeking the pain source to obtain the reward.13Iversen LL The chemistry of the brain.Sci Am. 1979; 241: 134-149Crossref PubMed Scopus (35) Google Scholar If our evidence that pain has a use, and is protective and a warning, is based on knowledge about congenital or hereditary insensitivity to pain, then we must accept that the evidence does not support this view, or is severely flawed. These cases, especially those where longevity is affected, clearly have major loss of nerve fibres, and more generalized neuropathies.5Nagasako EM Oaklander AL Dworkin RH Congenital insensitivity to pain: an update.Pain. 2003; 101: 213-219Abstract Full Text Full Text PDF PubMed Scopus (208) Google Scholar The cases originally described by Sternbach are also sadly lacking in a clear link between insensitivity to pain without other neurological abnormality and the cause of death, or indeed the certainty that pain sensation would have led to a different outcome.