VOL: 98, ISSUE: 38, PAGE NO: 41
Sharon Wood, MSc Pain Management, Dip Healthcare Research, RGN, IHBC Body Massage, is lecturer in nursing, University of Leeds
Sponsored by PFIZERIntroduction
Pain is the most commonly perceived symptom in adults. In the UK, 40.7% of patients present to their GP with a pain problem, and many are under-treated or suffer from incomplete pain relief (Mèntyselkè et al., 2001). Studies have shown that pain poses a major challenge in public health as the source of individual suffering and considerable healthcare costs. Its impact on society is expected to increase further, owing to a combination of changing attitudes and expectations, methods of medical management and social provision.
Pain is a complex phenomenon and can be classified in a number of ways. Throughout this series, it will be categorised broadly as acute and chronic pain.
Epidemiology of pain
Millions of people worldwide suffer from unnecessary pain following surgery as a result of ineffective acute pain management, and this is one of the most frequent reasons that people seek help from their GPs (Grichnik and Ferrante, 1991). Despite the advances in both the knowledge and management of pain, acute pain is often poorly managed. In 1990, the management of post-surgical pain in the UK was unsatisfactory (Royal College of Surgeons and Anaesthetists, 1990), yet, in 2001, half were still experiencing moderate to severe postoperative pain (Watt-Watson et al., 2001).
Sixty-one per cent of patients suffer from a variety of acutely painful conditions, and have reported severe to moderate levels of pain (McCaffery and Pasero, 1999). The use of patient-controlled analgesia systems or epidural analgesia has been shown to result in a reduction in pain, morbidity, mortality and an earlier discharge from hospital (Rigg et al., 2002).
As surgical management advances, patients are being discharged from hospital following surgery much earlier. However, 25-35% suffer from moderate to severe post-discharge pain, which impedes their resuming activities of daily living and may result in significant morbidity and even mortality (Wu et al., 2002). The consequence is a delay in recovery and return to work. Furthermore, it is one of the most common causes for readmission after day-case surgery (Chidambaram et al., 2001).
Chronic pain has a detrimental effect on physical health, employment, daily activities, psychological health and socio-economic wellbeing. It can result in excessive hospitalisation, unnecessary operations, over-medication and suicide (Smith et al., 2001). It affects 7-30% of the population and is one of the most common complaints seen by GPs (McHugh and Thoms, 2001).
Approximately 9% of primary care patients will develop distressing and disabling pain conditions (Gureje et al., 2001). A UK survey showed that 50% of people over the age of 70, and 20% of young adults (over 18 years) reported high degrees of pain or discomfort (Kind et al., 1998). Despite the fact that pain is widely experienced in the community, a category for pain is absent from the survey for disability by the UK Office of Population Censuses and Surveys (Kind et al., 1998). It is therefore difficult to estimate its social, economic and healthcare implications.
Think Point: What is the difference in the prevalence of acute and chronic pain in the UK? How might it be managed more effectively on a clinical level?
Disease process of pain
What is pain?
The word 'pain' is derived from the Latin 'poena' meaning a penalty or punishment. It is an abstract concept, which refers to a personal, private sensation of hurt. Pain acts as an important biological safety mechanism that warns us when something is wrong. It is a personal and subjective, multidimensional experience, and its intensity varies according to various psychological, physiological, social and cultural factors. Because pain is a private experience, it is impossible to know precisely what someone else's pain feels like. McCaffery's (1968) simple definition states that: 'Pain is whatever the experiencing person says it is and occurs whenever the experiencing person says it does'. This is a less precise definition than that composed by the International Association for the Study of Pain (1992) (Box 1).
Acute pain is a response to disease or injury, and therefore has a purpose in that it alerts an individual to a problem. It reduces over time and can be brief, lasting only a few minutes, or it may persist for several months (Box 1).
Acute pain has many dimensions and has been described as 'a constellation of unpleasant sensory, perceptual, emotional and mental experiences with associated autonomic, psychological and behavioural responses, provoked by injury, potential injury, or acute disease' (Grichnik and Ferrante, 1991).
Chronic pain is a more complex problem than acute pain, and becomes a continuous part of daily life (Box 1). It may involve both nociceptive and neuropathic elements (see Box 2 and text below), and its multidimensional nature involves the interaction of many factors, in different ways, and at different stages of the development of the pain. These include physiological, psychological, socio-economic, psychosocial, behavioural, motivational, affective, and environmental factors. Portenoy (1988) proposed a 'total pain' model for patients with cancer (Fig 1) but it can also be used for patients with chronic pain.
Think Point: List some of the factors that may influence the pain that patients experience.
Anatomy and physiology
A variety of theories have been developed to explain the complex phenomenon of pain. In 1965 Melzack and Wall proposed the gate control theory, which identifies the sensory, affective and evaluative dimensions of pain and integrates the physiological and psychological variables (International Association for the Study of Pain, 1992).
The pathophysiology of pain can be classified as 'nociceptive' and 'neuropathic' (Box 2) and is discussed here using these two classifications (McCaffery and Pasero, 1999).
The concept of nociception is used to describe how pain becomes a conscious sensation. It describes the specific response to noxious stimuli (stimuli that are damaging or potentially damaging to normal tissue).There are four basic processes involved in nociception (Figs 2 and 3).
1. Transduction of pain
Transduction begins at the periphery, where primary afferent neurones (nociceptors) that respond to noxious stimuli are distributed. Nociceptors are the free nerve endings of C fibres and A-delta fibres.
When these fibres are exposed to noxious stimuli, tissue damage, potential damage and/or inflammation occur. This causes the release of excitatory neurotransmitters which sensitise the nociceptors to the noxious stimuli, so initiating the nociceptors to transmit a pain impulse along the C and A-delta fibres. In order for the pain impulse to be transmitted, an exchange of sodium and potassium ions occurs at the cell membranes of the C and A-delta fibres, which results in an action potential and the transmission of a pain impulse. This completes the process of transduction.
2. Transmission of pain
The C fibres and A-delta fibres transmit the pain impulse from the site of transduction to the dorsal horn neurones in the spinal cord. (C fibres are unmyelinated and produce slow pain, which is poorly localised and dull and aching in nature. A-delta fibres are myelinated and produce fast pain, which is well localised, sharp and stinging in nature.) The pain impulse is then transmitted from the spinal cord via the spinothalamic tract to the brain stem and thalamus.
3. Perception of pain
The perception of pain is the end result of the neuronal activity of pain transmission; it is then that the pain becomes a conscious experience. A number of structures are involved in the perception of pain. These include the:
Reticular system: responsible for the autonomic and motor response to pain.
Somatosensory cortex: involved with the perception and interpretation of sensations. It identifies the intensity, type and location of the pain and relates the sensation to past experiences and cognitive activities.
Limbic system: responsible for the emotional and behavioural responses to pain.
4. Modulation of pain
The modulation of pain involves changing or inhibiting the pain impulses. The pathways involved in modulation are referred to as the descending pain system, as they involve fibres originating in the brain stem which descend to the spinal cord. These fibres release inhibitory neurotransmitters, for example, endogenous opioids, serotonin, gamma-aminobutyric acid (GABA), which inhibit the transmission of pain impulses, and therefore produce analgesia. This process, termed endogenous pain modulation, helps to explain the wide variations in the perception of pain in different people. Endogenous opioids (enkephalins and endorphins) are found throughout the central nervous system and prevent the release of some of the excitatory neurotransmitters, therefore inhibiting the transmission of pain impulses and producing analgesia.
Neuropathic pain is distinctly different from nociceptive pain. It is described as burning, dull, aching, tingling, electric shock-like or shooting. The exact mechanisms involved in the pathophysiology of neuropathic pain remain unclear. Chronic pain may result from current or past damage to the peripheral or central nervous system, and occur as a result of pathophysiological changes in the processing of information, which may become independent of the original painful event. In some cases, for example, amputation, this may have occurred in the peripheral nerves, but the mechanisms that underlie the neuropathic pain are generated in the central nervous system.
Some types of neuropathic pain may develop when the peripheral nervous system has become damaged, causing the pain fibres to transmit pain impulses repetitively. It may also be caused by a central mechanism in the spinal cord called 'wind-up'. Wind-up occurs when repeated, prolonged, noxious stimulation causes the dorsal horn neurones to transmit progressively increasing numbers of pain impulses. This results in patients feeling intense pain in response to a stimulus that dose not normally cause pain, for example, touch.
Signs and symptoms of pain Acute pain
Acute pain elicits sympathetic nervous system responses, causing physiological stress responses which are involved in promoting healing and fighting infection. In unrelieved acute pain, however, the stress response is prolonged and a variety of harmful effects may occur (Table 1). Patients experiencing acute pain often display visible signs of discomfort and physiological and behavioural changes. This is, however, not always the case and is influenced by a variety of factors including the suppression of pain behaviours (stoic response). Physiological signs are not always elevated, and medical conditions may alter the responses. The predominant psychological response in acute pain is anxiety.
Chronic pain differs from acute pain in that it does not elicit sympathetic nervous system responses. Patients with chronic pain may develop signs and symptoms that affect their quality of life such as fatigue, weight loss, sleep disturbances, loss of libido and constipation. They may experience feelings of despair and fear and may also feel that they have lost their autonomy. They are, as a consequence, psychologically more prone to depression and suicide than patients with acute pain. Patients with chronic pain often withdraw from social activities and may lose their jobs and alienate family and friends.
Think Point: A patient is admitted to hospital for pain management. He has both an acute and chronic element to his pain. Describe how he may express his pain and how the healthcare team may respond to his pain management needs.
This Part has explored some of the epidemiological factors that affect pain. The differences between acute and chronic pain have been discussed in terms of disease process, anatomy and physiology and associated signs and symptoms.
- Next week: Assessment and diagnosis of pain.