Several studies have attempted to link fatigue to other markers of disease activity. These include general measures of nutritional status such as weight and albumin, and tumour-specific markers of disease. In general these studies show disappointingly few clear associations:Mendoza et al. (1999) showed a relationship between albumin levels and fatigue for both solid and haematological malignancy, but Monga et al. (1999) found no association between fatigue and haematocrit or body weight in patients with carcinoma of the prostate. Monga’s study also did not find any association between prostate-specific antigen and fatigue in this group. Similarly Stone et al. (1999) found no association between subjective fatigue and malnutrition.
Should we be surprised by the somewhat unspectacular associations between disease activity and fatigue? In a review of the literature Wessely et al. (1998) assessed the association between many different physical diseases and fatigue and in general found a consistent lack of association between disease severity and fatigue. This held for renal failure (Brunier and Graydon 1993), heart failure (Wilson et al. 1995), Parkinson’s disease (Friedman and Friedman 1993), and rheumatoid arthritis (Belza et al. 1993; Belza 1995).Whilst all these diseases are in themselves powerful risk factors for the development of disabling fatigue, there is little association between disease severity and severity of fatigue, or indeed other important symptoms.
There are probably a number of reasons for this failure to find an association between disease severity and symptom severity. Firstly, it may be an artefact of study design. Many studies selected homogeneous populations of patients among whom the disease status may not vary sufficiently to demonstrate the importance of severity as a risk factor for the symptom. Secondly, most studies are cross-sectional, and a true relationship between disease severity and fatigue may get lost in the ‘noise’ of interindividual differences. If longitudinal studies were used to follow patients across the course of their illnesses these might demonstrate rather more convincing associations. Thirdly, it may simply be that any association is overwhelmed by the importance of psychological and behavioural factors—the presence of depression, interindividual differences in terms of self-efficacy, the effects of behavioural change and deconditioning, and so on.
Should we be surprised by the somewhat unspectacular associations between disease activity and fatigue? In a review of the literature Wessely et al. (1998) assessed the association between many different physical diseases and fatigue and in general found a consistent lack of association between disease severity and fatigue. This held for renal failure (Brunier and Graydon 1993), heart failure (Wilson et al. 1995), Parkinson’s disease (Friedman and Friedman 1993), and rheumatoid arthritis (Belza et al. 1993; Belza 1995).Whilst all these diseases are in themselves powerful risk factors for the development of disabling fatigue, there is little association between disease severity and severity of fatigue, or indeed other important symptoms.
There are probably a number of reasons for this failure to find an association between disease severity and symptom severity. Firstly, it may be an artefact of study design. Many studies selected homogeneous populations of patients among whom the disease status may not vary sufficiently to demonstrate the importance of severity as a risk factor for the symptom. Secondly, most studies are cross-sectional, and a true relationship between disease severity and fatigue may get lost in the ‘noise’ of interindividual differences. If longitudinal studies were used to follow patients across the course of their illnesses these might demonstrate rather more convincing associations. Thirdly, it may simply be that any association is overwhelmed by the importance of psychological and behavioural factors—the presence of depression, interindividual differences in terms of self-efficacy, the effects of behavioural change and deconditioning, and so on.
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