Nørregaard, J, Bülow, P M, Prescott, E et al. · Scandinavian journal of rheumatology · 1993 · DOI
This study followed 91 fibromyalgia patients for about 4 years to see if their muscle pain was caused by other medical conditions and how their symptoms changed over time. Most patients reported their pain got worse, and while many experienced severe fatigue, very few actually met the diagnostic criteria for chronic fatigue syndrome. The good news was that muscle strength remained stable and most cases of fibromyalgia were not caused by an underlying disease.
This study is important because it clarifies the relationship between fibromyalgia and ME/CFS, showing they are distinct conditions despite symptom overlap—particularly fatigue. Understanding that most fibromyalgia cases are not secondary to other diseases helps researchers and clinicians focus on primary pathophysiological mechanisms. For ME/CFS patients, this work highlights the need for condition-specific diagnostic criteria and recognition that fatigue-dominant presentations may represent different disease entities.
This study does not establish that ME/CFS and fibromyalgia never co-occur or are mutually exclusive; it only shows most fibromyalgia patients do not meet CFS criteria. The study does not identify the underlying causes of fibromyalgia or explain the mechanisms linking the two conditions. Additionally, 1993 diagnostic criteria for CFS differ from current definitions, so the actual overlap rate may differ with modern case definitions.
About the PEM badge: “PEM required” means post-exertional malaise was an explicit required diagnostic criterion for participant inclusion in this study — not that PEM was studied, observed, or discussed. Studies using criteria that do not require PEM (e.g. Fukuda, Oxford) are tagged “PEM not required”. How the atlas works →
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