pain

pain The International Association for The Study of Pain has provided the following definition of pain, which is used world-wide amongst scientists and clinicians interested in pain.

Pain is ‘an unpleasant sensory and emotional experience associated with actual or potential damage, or described in terms of such damage. Pain is always subjective. Each individual learns the application of the word through experiences relating to injury in early life’.

Noxious stimulation of a part of the body gives rise to electrical activity in the nervous system, extending from the periphery to the brain. Receptors and pathways dedicated to the nerve impulses giving rise to pain are described as components of somatic sensation and of visceral sensation. That activity is modulated within the central nervous system, both within the dorsal horns of the grey matter of the spinal cord and at higher levels. In this manner the input to the brain generated by noxious stimulation peripherally may be enhanced, diminished, or even, under certain circumstances, abolished — for example, in the heat of battle or a game of football. Thus, although noxious stimulation occurs, pain may not be felt at the time; such a mechanism clearly has value for survival of the individual in certain cases.

Our understanding of the physiology of pain control owes a great deal to the work of Melzack and Wall of some thirty-five years ago. Respectively a psychologist/physiologist and neurophysiologist, they proposed the gate-control theory of pain, which brought together previous work on the role of the nervous system in the generation of pain. They stated that within the dorsal horn of the spinal cord there are transmission cells (‘Trans cell’ in the figure) and that, as a result of tissue damage and stimulation, nerve impulses pass to those cells, which project further nerve impulses to the brain, where pain is experienced. The level of activity of the transmission cells is controlled by small adjacent cells which either excite or inhibit them. In turn the level of activity of the smaller cells is determined by the extent to which they are stimulated by nerve impulses from the body or the brain. Large diameter nerve fibres (beta fibres), which are stimulated by touch, excite the small inhibitory cells (white circles in the figure) adjacent to the transmission cells. In contrast, tissue injury excites other (A delta and C) nerve fibres. The former are large diameter fibres which conduct rapidly and the latter are small diameter fibres which conduct slowly. Both stimulate the transmission cell and small excitatory cells (black circles in the figure). Therefore in an acute injury, for example when the thumb is struck by a hammer, the A delta and C fibre activity exceeds the activity in beta fibres and pain is felt. When the injured part is rubbed vigorously the pain lessens and it does so because rubbing the skin stimulates beta fibres to the point where their level of stimulation of the small inhibitory cells exceeds that of the stimulation by the A delta and C fibres of the small excitatory cells. As a result, the activity of the transmission cell is reduced or ceases. This mechanism is involved when clinicians use transcutaneous electrical nerve stimulation (TENS) to relieve pain. Neurons descending from the brain may also excite or inhibit activity of the transmission cells within the spinal cord by influencing the small adjacent excitatory and inhibitory cells. For example, in states of emotional calmness, inhibition of transmission cell activity occurs, and less pain is experienced than in states of anxiety, when the activity of the transmission cells is increased by stimulation of the small excitatory cells.

In some situations pain may be felt when part of the body is missing, for example after the amputation of a limb or breast. Such ‘phantom pains’ are located in the absent part at a site where pain may have been felt before the part was lost. How then can pain, which is at times chronic and excruciating, be experienced in a limb that does not exist as a physical reality? The answer lies in the way the brain functions. Activity in areas of the brain concerned with sensory activity in the missing limb continues despite the absence of the limb, and gives rise to a phantom. If in addition central pain processes are active, phantom pain is experienced in the phantom limb. Such pain may be eliminated by stimulation of the sensory cerebral cortex but not by the division of nerves or the spinal cord. This supports the view that, although most people believe that pain actually exists at a site in the body that hurts, it is in fact a part of consciousness and the result of brain activity.

Until recently it was thought that the sensory and emotional elements of pain experience were linked solely to specific areas of the brain, namely the sensory and the emotional cortex, respectively. However, recent work using non-invasive brain imaging techniques — for example positron emission scanning — has revealed this model to be too simple. It is true that within the brain there is a degree of functional specialization for pain, but this is only part of the story. For example, damage to one half of the cerebral cortex does not necessarily abolish pain sensations from the opposite side of the body, and damage to areas of the brain associated with emotion does not necessarily remove the emotional component of pain. The reason for these apparent anomalies seems to lie in the fact that pain is generated within a widely distributed system or neuronal network. In this way, the brain detects tissue injury even when there is considerable damage to the nervous system. The brain functions as an active system, which filters, selects, and integrates sensory input against the background of lifelong experiences, both physical and emotional, which are preserved in the systems devoted to memory. One brain output from this process is pain.

Pain therefore occurs only in the conscious individual, and it is essential for survival. A small but unfortunate number of people are born without the capacity to feel pain. As a result they suffer horrific injuries in childhood and die young as a result of accidents or undiagnosed disorders, which in normal people give rise to pain.

In everyday life pain is recognized in two forms, namely acute pain and chronic pain. The former has a protective function. It alerts us to damage to the body, it increases our level of arousal, it directs our attention to the cause of the pain, and generates behaviour that leads to an escape from it. The chief emotion associated with acute pain is anxiety, and this subsides when pain is relieved and the cause is understood. In contrast, chronic pain does not appear to the sufferer to have any purpose and indeed has negative qualities. It gives rise to feelings of anxiety and at times of depression. The behaviours generated include withdrawal from social activities and a search for relief. The latter may well lead the sufferer to move from one doctor to another and to non-medical practitioners in the hope of pain relief. At times that process itself may generate more physical suffering through unnecessary investigation and the end result is pain, despair, and depression.

Both acute and chronic forms of pain are familiar, but in addition pain occurs in two other, quite different situations. It may occur as a symptom in a depressive illness. In other words it is not, as is commonly thought in such situations, that depression has developed because pain is being experienced but, in fact, the pain is part of a primary depressive illness. Up to half of those who develop depressive illnesses experience physical symptoms unrelated to any obvious underlying pathology, and of those symptoms pain is the most common. The failure of doctors to appreciate this fact does occasionally lead to a prolonged search for a physical cause for pain because its presence overshadows other features of a depressive illness.

Pain occurs in individual's experiencing anxiety, or emotional tension. For example, tension headaches are very common. The presence of anxiety in a pain sufferer tends both to increase the severity of pain experienced and to reduce the individual's tolerance or ability to cope with it.

Pain may occur in the absence of an obvious physical cause and where the features of a mental illness are not detectable. Individuals with this type of pain may have had a trivial injury but the level of pain and disability with which they present is out of all proportion to the severity of that injury. In addition, the behaviour shown by the sufferer reveals considerable dependence upon others, loss of willingness to take responsibility for themselves, their home, and their work, and a preoccupation with a search for a ‘cure’ for the pain, which they regard firmly as physical in origin.

Consideration of pain problems in which an underlying physical cause is either minimal or absent highlights the fact that when trying to understand pain it is necessary not only to consider its sensory aspects, but also its emotional ones. Indeed it has been said that to ignore the emotional aspects of pain is to look at only one part of the problem, and probably not the most important part at that. The definition of pain given earlier reinforces this point.

As a consequence of the need to encompass the physical, psychological, and social aspects of pain experience, clinicians and pain researchers have developed what is known as the biopsychosocial model of pain. It is based upon what we know about the generation and control of pain within the nervous system, and also its psychological aspects and the social factors that influence the thinking of individuals about pain and their behaviour. This approach to pain has lead to the development of powerful psychological tools for pain management, which come under the broad heading of cognitive–behavioural theory and practice.

Consideration of socio-cultural and learning factors reveals that learning about pain takes place within a definite social context, and the way each of us behaves when in pain reflects that fact. At a national level it is customary in general for those who are from Northern European countries to regard complaints about pain, especially amongst men, as a weakness of character. In contrast, in Southern European countries to complain about pain is regarded as beneficial to the sufferer. These are very broad generalizations but do have some basis in fact. An important psychological mechanism by which we learn the behaviours we exhibit when in pain is defined as operant learning. It is a process by which overt behavioural responses to a stimulus are significantly influenced by their consequences, including the responses of others to them.

Operant learning is well illustrated by the effects of a simple injection upon a child. The sight of the needle and the pain experienced is an ‘unconditioned stimulus’ and as a response to it the child cries. On the next occasion the child cries at the sight of the syringe and needle, which have become ‘the conditioned stimulus’. If crying leads to the abandonment of the injection the child has developed a ‘conditioned escape response’. Seeing another child crying before an injection which is then not given leads to another type of learning — ‘an observational learning model’.

In some individuals such mechanisms lead to the development of pain behaviours that have a negative effect upon their lives — for example, the excessive use of rest to relieve pain, or the abuse of powerful narcotic-related drugs may actually lead to increasing chronicity of pain and disability. To counter such developments psychologists have developed techniques based upon operant conditioning, which are designed to reverse maladaptive pain behaviours and to replace them by adaptive behaviours. In other words, their techniques involve the use of learning of behaviour designed to lead to coping with pain and everyday life rather than withdrawing from them. Put in simple terms, ‘good behaviour is rewarded and bad behaviour is punished’.

Operant conditioning has been criticized on the grounds that it does not take sufficient account of mental activity. In other words, individuals have thoughts about pain and attitudes towards it. They draw on memories of past experience when in pain, and this leads to thinking and behaviour, which is the result of those experiences. Such thoughts and attitudes, or cognitions, as they are called, cannot be ignored when a clinician is evaluating a person in pain and planning their treatment. For this reason, a purely behavioural approach has been replaced by a cognitive–behavioural approach to pain analysis and management. The main cognitive elements that have been identified include beliefs about pain and its causes, beliefs about the extent to which the individual feels he or she has control over pain, and the extent to which individuals believe that they are able to function despite pain. Therefore, self-efficiency is a significant factor in determining ability to cope.

People in pain often develop what are described by psychologists as ‘cognitive errors’. For example, they may indulge in what is known as ‘catastrophizing’. In other words they develop an unnecessarily negative view of their condition and its likely outcome. In such a state they tend to focus to a extent upon the negative features of their disorder. It has been demonstrated that negative qualities of thought, and catastrophizing in particular, are consistently linked to the development of depression in chronic pain disorders. The manipulation of coping mechanisms is of great significance when considering the management of pain and especially of chronic pain. We are all familiar with coping strategies, some of which are regarded as active — for example, indulging in active and distracting behaviour, whereas others are passive — for example, taking rest or medicines. If the strategy used maximizes function in the presence of pain and reduces anxiety, then it is said to be adaptive. On the other hand, if the strategies used involve too much rest, too great a dependence on medication or on others, or conversely too much activity which provokes excessive pain, they are maladaptive. Cognitive therapies involve changing thoughts and attitudes about pain with a view to changing self-management in the direction of adaptive behaviour: a change which often leads to a lessening of pain.

Michael R. Bond

Bibliography

Gatchell, R. J. and Turk, D. C. (ed.) (1996). Psychological approaches to pain management. The Guilford Press, New York and London.
Main, J. C. and and Spanswick, C. C. (2000) Pain management: an interdisciplinary approach. Churchill Livingstone, Edinburgh & London.
Wall, P. (1999). Pain; the science of suffering. Weidenfeld and Nicolson, London.

See also analgesia; central nervous system; endorphins; opiates and opioid drugs; somatic sensation; visceral sensation.