Disco Duro

Following the Gate Control Theory, pain is now widely considered to be a multidimensional phenomenon. Karoly (1985) has argued that the Gate Control Theory permits pain to be viewed as a multidimensional experience involving sensation, emotion, cognitive evaluation, and behavioural response. As such, the potential for measuring aspects of pain is to some extent unlimited, potentially involving physiological measures, sensory, affective, behavioural, cognitive and lifestyle impact

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measures (see Williams and Erskine, 1995 for a review o f some o f the available measures).

Melzack and Wall (1996) themselves state that:

“Pain in man comprises two components - behaviour and conscious experience - which can both be measured with appropriate tools” (p. 124).

Whilst this distinction appears reasonable to some extent, it cannot be considered a clear-cut one. What might count as the beginning of behaviour and the end

o f conscious experience is debatable and implies a mind-body dualism that certain researchers attempt to transcend (e.g. Turner, 1992).

In addition to a sensory and affective component, Melzack and Wall argue that there is a third ‘evaluative’ dimension to pain, and the McGill Pain Questionnaire (MPQ; Melzack, 1975) has been developed to measure these three facets of the subjective pain experience. This questionnaire contains groups of pain descriptors of increasing intensity, which fall into three broad categories: those reflecting the sensory qualities o f pain (e.g. stabbing, sharp); those reflecting the affective qualities o f pain (e.g. fearful, cruel); and those reflecting the evaluative response to pain (e.g. miserable, troublesome).

Whilst this three-way distinction makes theoretical sense, in factor-analytic studies o f people’s word selection from the MPQ, the sensory- affective distinction has been supported more than the idea of a third distinct evaluative dimension. Although this sensory-affective distinction may be supported, it is hypothesized to reflect pre- conscious dimensions o f pain reflecting the fact that pain is an inherently unpleasant sensation. However, it is unclear to what extent people’s responses on the McGill reflect more o f a sensory-evaluative distinction, in that people select the sensory words to describe their pain (a unitary phenomenon, e.g. throbbing, scalding) and the affective words to describe their evaluation of that pain, i.e. how it feels to experience pain (e.g. distressing, horrible etc.).

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1.7.1 M easuring subjective pain intensity.

The models o f both Melzack and Wall, and Leventhal and Everhart propose that pain comprises numerous dimensions but that it is consciously experienced as a unitary phenomenon. It is frequently this experience o f pain that is measured in research, and usually the intensity o f this experience which is o f interest. Whilst pain measurements can include verbal and numeric self-rating scales, behavioural observations, and measures of physiological response, because pain is primarily considered to be a subjective experience, self-report scales are considered to be the most valid (Katz and Melzack, 1999).

There are a variety o f measures that can be used to measure subjective pain intensity, ranging from numeric rating scales to verbal rating scales. The numerical rating scales are typically 11 or 101 point scales anchored at two extremes of pain intensity. The most commonly used verbal rating scale (included on the McGill Pain Questionnaire) consists of five descriptors: mild, discomforting, distressing, horrible, excruciating.

Jensen, Karoly and Braver (1986) compared six different methods for measuring subjective pain intensity, including both Visual Analogue Scales and Verbal Rating Scales. They concluded that in terms of both utility and validity all six methods were comparable. More recently, Jensen and McFarland (1993) conducted a study into the reliability and validity of pain intensity measurements in chronic pain patients and concluded that multiple pain measures should be used in studies being conducted on pain treatment, where changes in pain scores are important, but argued that:

“the coefficients obtained (a stability coefficient o f 0.63 and a validity coefficient of 0.74) indicate that a single (pain) measure may be useful in ‘basic research’ (i.e., examining the relationship between pain intensity ratings and other variables among large groups o f patients)” (p. 200).

The measurement o f pain is, however, far from straight forward, and some o f the difficulties inherent in its measurement have been discussed recently by Williams, Davies and Chadury (2000). In a study examining the use o f visual rating and numerical

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rating scales, Williams et al. found considerable variability in how pain patients interpreted and responded to the task o f providing ratings o f their pain. For example, in providing ratings of pain, patients often placed a primary emphasis on the extent to which the pain upset their ability to do things, although there was lack of consistency both within and between patients in how they set about providing a rating o f pain. On the basis of their findings Williams et al. argue that pain ratings cannot be seen as a simple mapping o f an internal stimulus to a rating scale but rather as “an attempt to construct meaning” (p. 457) as the patient interprets the end points of the scale in relation to a range o f factors which included understandings o f their own pain and other people’s perception of the pain rating they were about to give. Williams et al. have also questioned the psychometric properties of pain scales such as the VAS as their findings suggest that the VAS may not be used by patients as though it is a uniformly linear scale. However, whilst their paper points to potential sources o f error in pain measurement, no clear solutions to this problem are provided, although a call for further research in this area is made. Clearly caution must be used in interpreting findings based on VAS assessments o f pain intensity, although, with the exception of Study 3 (Chapter 5), pain measures are only used in this thesis to confirm the group status o f the participants (i.e. whether they are pain patients or non-pain controls) and so this issue will not be discussed further in the thesis.

7.7,2 Memory and measurement

A key issue in the measurement o f pain and pain-related constructs, is the role o f memory and its affect on measurement accuracy. This issue has been reviewed in detail elsewhere (Erskine, Morley and Pearce, 1990). In brief, findings are mixed as to the role o f pain intensity and mood on recall accuracy o f previous pain experiences. For example, it has been demonstrated that chronic pain sufferers frequently over-estimate their pain when asked to recall it (e.g. Linton and Melin, 1982). However, other

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evidence suggests that, rather than a general tendency to overestimate previous pain, it is present pain intensity levels that affects recall accuracy in chronic pain sufferers. For example, Eich, Reeves, Jaeger and Graff-Radford (1985) showed that high present pain intensity was associated with overestimation o f pain whereas low present pain intensity was associated with underestimation o f pain. There is also mixed evidence as to the role o f mood on recall accuracy. Although anxiety has been linked to poor recall o f pain intensity in dental patients (Kent, 1989), Linton (1991) demonstrated that there was no evidence o f a relationship between depression and accuracy o f pain intensity recall in chronic pain sufferers.

Erskine et al. (1990) point to a number o f factors which may affect recall accuracy, and these include whether the pain is intermittent or continuous, the particular mood state being experienced at the time (anxiety or depression), and the aspect o f pain experience being examined, pain intensity or pain-distress. What seems clear is that recall for pain intensity is frequently inaccurate, and hence reliance on memory in pain measurement should be reduced and present pain intensity measures used wherever possible.