The Splitting vs. Lumping Problem
ME/CFS sits at the center of a cluster of contested diagnoses whose boundaries are actively debated. Fibromyalgia, irritable bowel syndrome, post-treatment Lyme disease, complex regional pain syndrome, Gulf War illness, mast cell activation syndrome, and hypermobile Ehlers-Danlos syndrome each share substantial symptom overlap with ME/CFS, and with each other. The core question — whether these represent distinct diseases (“splitting”) or manifestations of a common underlying process (“lumping”) — has clinical, research, and policy consequences.
The classic lumping argument was articulated by Wessely et al. (1999): the overlap between these syndromes — in case definition, demographics, non-symptom associations, and treatment response — outweighs their differences, arguing for dimensional rather than categorical classification (Wessely et al., 1999). The splitting counterargument has two forms. The clinical form notes that each condition has at least one distinguishing feature: PEM in ME/CFS, widespread pain in fibromyalgia, confirmed prior infection in PTLDS, documented trigger event in CRPS and GWI (Murovska et al. 2026). The mechanistic form argues that these distinguishing features reflect different rates of particular pathophysiological nodes — shared biology but condition-specific weighting — rather than different diseases (Shoenfeld et al., 2020).
The sociological dimension matters for patients navigating the healthcare system: how a condition is classified determines which specialist sees the patient, what treatments are offered, whether insurance covers care, and how seriously symptoms are taken (Barker et al., 2024). Historical precedent is instructive: conditions that start as “contested” (multiple sclerosis before MRI, rheumatoid arthritis before RF testing, epilepsy before EEG) eventually acquire a biological definition and exit the contested category entirely. ME/CFS remains in the contested zone, and this chapter examines how its closest neighbors shape — and are shaped by — the same classification conflicts.
Consequence: The splitting-vs-lumping debate is not an academic taxonomy exercise — it determines whether a patient receives a rheumatology referral or a psychiatry referral, whether exercise is prescribed or contraindicated, whether disability claims are approved, and whether research funding targets distinct diseases or a shared post-infectious syndrome cluster.