Editorial : Chronic fatigue syndrome and depression 3 INTERPRETING DEPRESSION IN MEDICAL ILLNESS

Psychological and somatic factors contribute and interact across the spectrum of health and illness. Their relative influence varies from disorder to disorder and from case to case, and specific instances of disorder might in theory be plotted along a continuum to represent different weightings of psychological and somatic causation (Lipowski, 1986; Oken, 1987). We can criticize such a model for its implied dualism, and it is misleading to conceive of the psychological and somatic as distinct and mutually exclusive categories. Nevertheless, as levels of explanation, one may be more appropriate than the other, and it is thus legitimate to ask whether a given disorder should be understood primarily in somatic terms or whether a psychological model is more likely to advance our understanding. A disorder whose aetiology has been the subject of recent controversy is chronic fatigue syndrome (CFS), also commonly known as postviral fatigue syndrome and myalgic encephalotnyelitis. The issue of terminology is in itself controversial, and there may prove to be differences between these syndromes (Ramsay, 1988). Nevertheless, CFS will be the general term employed here; it is arguably the most appropriate at this stage of our understanding since it makes no assumptions about aetiology (Holmes et al. 1988; Lloyd et al. 1988). The central feature of the disorder is persistent and excessive fatiguability, and this may be accompanied by various other somatic and psychological symptoms, including acheing muscles and joints, headache, sore throat, painful lymph nodes, muscle weakness, sleep disturbance, mental fatigue, difficulty in concentrating, emotional lability and depression (David et al. 1988 a; Holmes el al. 1988). Its nature and causes are as yet undetermined, and the research to date suggests that it is an intriguingly complex disorder, with both biological and psychological features. On the one hand, there is accumulating evidence of biological abnormalities suggestive of viral infection and immunological dysfunction (see Archer, 1987; Bannister, 1988; David et al. 19886; Straus, 1988). However, there are inconsistencies between studies in the results obtained, no single defining factor has emerged, and the clinical significance of the abnormalities found has been questioned (Hellinger et al. 1988; Jacobson, 1988; Straus, 1988; Straus et al. 1988; Swartz, 1988). A detailed description of this body of research is beyond the scope of this paper. On the other hand, there is evidence of psychiatric disorder in a significant number of patients, with depression predominating. These findings might be taken to offer some indirect support for the hypothesis that CFS and depression are similar syndromes, with common aetiological factors. However, cross-sectional data are notoriously difficult to interpret, and there are various ways in which depression and psychological vulnerability in general could be implicated in CFS. Though depression will be the focus of this discussion, many of the arguments presented are relevant to other psychological categories, such as anxiety or somatization disorder; these have also been linked with CFS but to a lesser extent than depression.