Pain has physical and psychological dimensions, both of which may be measured; they form an important aspect of the diagnosis of painful disorders and are essential for the correct application of treatment and its assessment. Pain is a subjective experience but physiological changes that accompany it may be measured: they include changes in heart rate, muscle tension, skin conductivity and electrical and metabolic activity in the brain. These measures are most consistent in acute rather than chronic pain and they are used primarily in laboratory studies. Clinically, pain assessment includes a full history of the development, nature, intensity, location and duration of pain. In addition to clinical examination, self-report measures of pain are often used.
The use of words as descriptors of pain have permitted the development of graded descriptions of pain severity. For example, mild pain, moderate pain, severe pain and very severe pain, to which numerical values may be attached (1-4), may be graded on a numerical scale from 0 to 4 indicating the level of pain being experienced. In clinical practice, however, there is widespread use of a 0-10 scale, a visual analogue scale, which is easy to understand and use and is not affected by differences in language. Such measures are often repeated at intervals to gain information about the levels of pain throughout the day, after a given procedure or as a consequence of treatment. More sophisticated verbal measures use groups of words to describe the three dimensions of pain, namely its sensory component, the mood-related dimension and its evaluative aspect. This technique was devised by Melzack and others and is best seen in the Short-Form McGill Pain Questionnaire (5). The questionnaire requires the patient to be well acquainted with the words used. Often because of age, not having English as a first language or as a result of some form of mental impairment, the scale cannot be used. In its place it is possible to use a "faces scale" in which recognizable facial images representing a range of pain experiences from no pain to very severe pain are readily understood. Such scales are often used with children. In the case of patients with pain generated as a result of a lesion within the nervous system (neuropathic pain) specific measures have been devised to distinguish between that type of pain and pain arising outside the nervous system (6). In the assessment of a patient with neuropathic pain, the evaluation of sensory function is crucial and can be carried out at the bedside with simple equipment.
Another technique used in clinical assessment includes pain drawings, which allow the patient to mark the location of pain and its qualities using a code on a diagram of the body. A pain diary is used by patients to record levels of pain throughout the day, using a visual analogue scale. This reveals the pattern of pain severity in relation to drug therapy and activity levels. Finally, pain behaviour is often used to aid diagnosis. It is especially useful for determining the extent to which psychological factors influence pain. For example, a wide discrepancy between the behaviour exhibited in the clinic and what might be expected, given the nature of the disorder, is a valuable clue to a person's emotional state, ability to cope with pain and conscious or unconscious desire to communicate distress non-verbally to the clinician. Pain assessment should take account of the patient's sex and ethnic and cultural background, all of which tend to influence the clinical presentation.
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