A review of the evidence for overlap among unexplained clinical conditions

Purpose: Unexplained clinical conditions share features, including symptoms (fatigue, pain), disability out of proportion to physical examination find ings, inconsistent demonstration of laboratory abnormalities, and an associ ation with "stress" and psychosocial factors. This literature review examin es the nature and extent of the overlap among these unexplained clinical co nditions and the limitations of previous research. Data Sources: English-language articles were identified by a search of the MEDLINE database from 1966 to January 2001 by using individual syndromes an d their hallmark symptoms as search terms. Study Selection: studies that assessed patients with at least one unexplain ed clinical condition and that included information on symptoms, overlap wi th other unexplained clinical conditions, or physiologic markers, Condition s examined were the chronic fatigue syndrome, fibromyalgia, the irritable b owel syndrome, multiple chemical sensitivity, temporomandibular disorder, t ension headache, interstitial cystitis, and the postconcussion syndrome. Da ta Extraction: information on authorship, patient and control groups, eligi bility criteria, case definitions, study methods, and major findings. Data Synthesis: Many similarities were apparent in case definition and symp toms, and the proportion of patients with one unexplained clinical conditio n meeting criteria for a second unexplained condition was striking, Tender points on physical examination and decreased pain threshold and tolerance w ere the most frequent and consistent objective findings. A major shortcomin g of all proposed explanatory models is their inability to account for the occurrence of unexplained clinical conditions in many affected patients. Conclusions: Overlap between unexplained clinical conditions is substantial . Most studies are limited by methodologic problems, such as case definitio n and the selection and recruitment of case-patients and controls.