title: A review of the evidence for overlap among unexplained clinical conditions.
abstract: OBJECTIVE
  Unexplained clinical conditions share features, including symptoms (fatigue, pain), disability out of proportion to physical examination findings, inconsistent demonstration of laboratory abnormalities, and an association with "stress" and psychosocial factors. This literature review examines the nature and extent of the overlap among these unexplained clinical conditions and the limitations of previous research.
  
  
  METHODS
  English-language articles were identified by a search of the MEDLINE database from 1966 to January 2001 by using individual syndromes and their hallmark symptoms as search terms.
  
  
  METHODS
  Studies that assessed patients with at least one unexplained clinical condition and that included information on symptoms, overlap with other unexplained clinical conditions, or physiologic markers. Conditions examined were the chronic fatigue syndrome, fibromyalgia, the irritable bowel syndrome, multiple chemical sensitivity, temporomandibular disorder, tension headache, interstitial cystitis, and the postconcussion syndrome.
  
  
  METHODS
  Information on authorship, patient and control groups, eligibility criteria, case definitions, study methods, and major findings.
  
  
  RESULTS
  Many similarities were apparent in case definition and symptoms, and the proportion of patients with one unexplained clinical condition meeting criteria for a second unexplained condition was striking. Tender points on physical examination and decreased pain threshold and tolerance were the most frequent and consistent objective findings. A major shortcoming 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 definition and the selection and recruitment of case-patients and controls.
authors.affiliation: Department of Medicine, Division of Internal Medicine, Harborview Medical Center, 325 Ninth Avenue, Box 359780, Seattle, WA 98104, USA. laaron@u.washington.edu
authors.forename: L A
authors.initials: LA
authors.lastname: Aaron
authors.forename: D
authors.initials: D
authors.lastname: Buchwald
mesh_terms: D001924:Brain Concussion / Q000175:diagnosis / Q000209:etiology; D015984:Causality; D003109:Colonic Diseases, Functional / Q000175:diagnosis / Q000209:etiology; D018856:Cystitis, Interstitial / Q000175:diagnosis / Q000209:etiology; D003933:Diagnosis; D015673:Fatigue Syndrome, Chronic / Q000175:diagnosis / Q000209:etiology; D005356:Fibromyalgia / Q000175:diagnosis / Q000209:etiology; D006261:Headache / Q000175:diagnosis / Q000209:etiology; D006801:Humans; D018777:Multiple Chemical Sensitivity / Q000175:diagnosis / Q000209:etiology; D012107:Research Design; D013577:Syndrome; D013706:Temporomandibular Joint Dysfunction Syndrome / Q000175:diagnosis / Q000209:etiology; D009626:Terminology as Topic