Patients in pain

Published on 03/03/2015 by admin

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Last modified 03/03/2015

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9 Patients in pain

Jayne Gallagher, Serge Nikolic

Introduction

Pain is a phenomenon with which we are all familiar. It is a part of everyday life and is a feature of various diseases. It most commonly accompanies an injury, where it serves its most important purpose; namely, to protect us, alert us and to make us remove ourselves from danger. The severity of pain, and its impact on an individual, ranges from a trivial occurrence such as a needle-prick injury to a sensation of such intensity that it induces thoughts of suicide.

Definition

The International Association for the Study of Pain proposed the following definition (1979): ‘Pain is an unpleasant sensory and emotional experience associated with actual or potential tissue damage or described in terms of damage’. It follows from this that pain is always a subjective sensation and is always unpleasant. This definition has two important implications: pain is not necessarily or always associated with ongoing tissue damage; it is a subjective experience and has an emotional as well as a sensory component.

Pain is notoriously difficult to describe. It is difficult to assess and even harder to quantify. Nonetheless, pain assessment is crucial in order to evaluate its impact on the sufferer and plan a treatment strategy.

Classification of pain

There are many ways in which pain can be classified in order to formulate an optimal treatment strategy. Despite this, classifying a particular pain state can be challenging because often the pain syndrome does not fall into a single category. The main reason for this is that the aetiology and pathogenesis of many pain syndromes is multifactorial.

Pain is commonly classified according to:

image aetiology and mechanism
image duration.

Aetiology and pathogenesis

image Physiological: an acute response to an injury.

image Inflammatory/nociceptive: pain generated and maintained by inflammatory mediators secondary to an ongoing disease process such as cancer.

image Neuropathic: pain arising from injury to or dysfunction of the central or peripheral nervous system.

image Psychosomatic: purely psychosomatic pain is rare. However, pain, especially chronic pain, almost invariably has an emotional and behavioural component.

Duration

image Acute: most commonly a physiological response to an injury. It resolves with the disappearance of a noxious stimulus or within the timeframe of a normal healing process.

image Chronic: it can either be associated with an ongoing pathological process, such as rheumatoid arthritis or malignancy, or be present for longer than is consistent with a normal healing time. Pain is arbitrarily described as chronic if it persists for longer than 3 months. Chronic pain is often associated with disability and a significant behavioural response. It is sometimes subdivided into pain associated with cancer and pain associated with non-malignant conditions.

Mechanisms of pain

At its simplest, pain is generated by a noxious stimulus that excites the central nervous system. This mechanism was first proposed by Descartes in the sixteenth century and conceptually still holds true, but it is crucial to appreciate that the final subjective experience of pain is shaped by various factors (Fig. 9.1).

image

Figure 9.1 Biopsychosocial model of pain.

A stimulus (which can be thermal, pressure, cold or chemical) excites nociceptors and is then transmitted to the spinal cord by two different classes of nerve fibre. Faster, myelinated Aδ fibres and smaller, slower, unmyelinated C fibres transmit the sensation to the dorsal horn of the spinal cord, where these primary afferent fibres synapse in lamina I, lamina II (substantia gelatinosa), lamina IV and some in lamina V. All of these afferent sensory fibres are excitatory. Second-order fibres are then carried in the spinothalamic and spinoreticular tracts to the thalamus, where they synapse. From the thalamus, third-order neurones project to the somatosensory cortex, anterior cingulate gyrus and the insular cortex, where they terminate. It is at this cortical level that a stimulus is perceived as pain.

However, it is known that the noxious sensory input may be modulated at several levels (including spinal cord and higher brain centres) by the nervous system, thereby altering the final pain experience. At the spinal level, the gate theory, proposed by Melzack and Wall in 1965, states that non-noxious stimulation of the large Aβ fibres inhibits the response to painful stimuli of neurones with wide dynamic range (WDR neurones, located primarily in lamina V), reducing the input of small fibres mediating the sensation of pain. A good example of this effect is ‘rubbing it better’. In addition, descending input from higher centres also modulates neural activity in the spinal cord, reducing or enhancing pain sensation. Such descending input is one of the mechanisms by which emotional and cognitive factors modulate pain perception. Much remains to be understood about the central pathophysiology of pain.

In recent years, pain management has increasingly adopted a biopsychosocial model. This has highlighted the need to take into account the interactions between biological, psychological and social factors leading to an individual’s pain experience (see Fig. 9.1).

The patient in pain

As with any other branch of medicine, careful and meticulous assessment of a patient in pain is important. Two questions should be considered when dealing with a patient in pain:

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