Similar but different: Implications for the one versus many functional somatic syndromes discussion

The aim of this thesis was to investigate the validity and meaning of functional somatic syndrome (FSS) diagnoses, and to examine to which degree these diagnoses are able to identify separate groups of patients in the context of the lumper-splitter discussion. We approached this aim from different angles, taking into account the possible etiological pathways that may lead to the causation and persistence of FSS. We found that the case definitions of FSS overlap, and that patients with one FSS frequently meet diagnostic criteria of the other FSS. We reveal that the diagnostic overlap substantially increases when FSS are more serious and chronic in nature. Furthermore, psychiatric co-morbidity and reduced physical activity were characteristics of all FSS. However, we describe also remarkable differences in case definitions that might artificially decrease presumed overlap between FSS. We argue that the findings that FSS share non-symptom characteristics and that they respond to the same therapies, are not unique to FSS patients, and that it therefore is no reason to consider them similar. So, this thesis reveals that, although there is overlap in case definitions and psychiatric comorbidity, the differences between the FSS cannot be ignored. These results support recent suggestions that FSS have both specific and general characteristics. We therefore state that FSS may reflect the same underlying syndrome with different subtypes. This underlying syndrome should be more extensively investigated in the future to establish a valid and generally accepted diagnostic criteria across medical specialties.