Evidence of comorbidity among unexplained illness syndromes raises the possibility that all are variants of a single functional disorder, leading some to suggest that separate case definitions for chronic fatigue syndrome (CFS), fibromyalgia (FM), and multiple chemical sensitivity (MCS) may be unnecessary. Our objective was to determine whether discrete diagnostic labels provide useful information about physical functioning, symptom severity, and risk of psychiatric illness.
The sample consisted of 163 consecutive female referrals with CFS enrolled at a tertiary clinic. Each participant was retrospectively assigned to one of four groups: CFS only, CFS/FM, CFS/MCS, and CFS/FM/MCS. At enrollment, participants gave their history, underwent a physical examination and a standardized psychiatric interview (Diagnostic Interview Schedule), and answered self-report questionnaires.
Additional unexplained syndromes were prevalent: 37% met criteria for FM, and 33% met criteria for MCS. With the exception of FM-related pain and disability, there were few differences between the CFS only and CFS with comorbid illness groups. Patients with additional illness were more likely to have major depression and a higher risk of psychiatric morbidity compared with patients in the CFS only group (p <.01). Rates of lifetime depression increased from 27.4% in the CFS only group to 52.3% in the CFS/FM group, 45.2% in the CFS/MCS group, and 69.2% in the CFS/FM/MCS group.
The prevalence of comorbid illness in the present CFS sample and the failure to find widespread differences in symptom severity can be seen as support for the single syndrome hypothesis. On the other hand, the existence of discrete syndromes could not be ruled out because of reliable differences between CFS and CFS/FM. Increasing comorbidity was associated with a corresponding increase in risk of major depression.