"Functional" or "psychosomatic" symptoms, e.g. a flu-like malaise, aches and pain and fatigue, are major features of major and in particular of melancholic depression.

BACKGROUND: Major depression is characterized by multifarious symptoms and symptoms clusters, such as the melancholic and anxiety symptom clusters. There is a strong comorbidity and a biological similarity between major depression and myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS).
OBJECTIVE: The aim of the present study was to examine "psychosomatic" symptoms reminiscent of ME/CFS in major depression.
METHODS: Toward this end, we examined the 12-item Fibromyalgia and Chronic Fatigue Syndrome Rating (FF) Scale and the Hamilton Depression Rating Scale (HDRS) in 103 major depressed patients by means of multivariate pattern recognition methods. results: Our findings support the existence of two factors, i.e. a fatigue and somatic (F&S) factor, i.e aches and pain, muscular tension, fatigue, concentration difficulties, failing memory, irritability, irritable bowel, headache, and a subjective experience of infection; and a depression factor, i.e. sadness, irritability, sleep disorders, autonomic symptoms, and a subjective experience of infection. Cluster analysis performed on the 12 FF items found two different clusters, which were separated by highly significant differences in the F&S items, the most significant being a subjective experience of infection, aches and pain, muscular tension, fatigue, concentration difficulties and failing memory. Multivariate analyses showed that the differences between both clusters were quantitatively, and not qualitatively, and reflected the severity of the F&S dimension. There was a strong association between the F&S symptoms and melancholia and chronic depression. Treatment resistant depression was characterized by higher scores on the depression factor score. There was a strong correlation between the HDRS score and the FF items, fatigue, a subjective experience of infection, and sadness. Our findings show that F&S symptoms are a major feature of depression and largely predict severity of illness, and chronic and melancholic depression.
CONCLUSIONS: It is concluded that the diagnostic criteria of depression and melancholia and rating scales to measure severity of illness should be modified to include the F&S symptom profile.