Myalgic encephalomyelitis/chronic fatigue syndrome and fibromyalgia - overlap, differences, and emerging insights

BACKGROUND: Myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) and fibromyalgia (FM) are debilitating chronic illnesses with considerable symptom overlap. Their symptoms include profound fatigue, widespread pain, post-exertional deterioration, sleep disturbance, dysautonomia, and cognitive impairment. These illnesses frequently co-occur; ME/CFS often develops first, particularly after infection. This overlap creates a diagnostic grey area and contributes to severe reductions in quality of life. Despite these shared features, key distinctions remain essential. ME/CFS is characterised by post-exertional neuroimmune exhaustion (PENE)/post-exertional malaise (PEM), a hallmark of post-exertional worsening and multisystem involvement. Contrastingly, FM centres on chronic widespread pain and symptom variability. It is not characterised by PEM/PENE or the same extent of system dysfunction as ME/CFS. Both disorders lack a definitive biomarker as of 2025. Thus, diagnosis remains clinical and supported by objective tests where available. MAIN BODY: Although immune dysregulation is common in patients with each condition, distinct immune signatures have been observed. ME/CFS is characterised by fluctuating pro- and anti-inflammatory cytokine levels and a frequent reduction in natural killer (NK) cell function; this is consistent with immune exhaustion. Patients with FM exhibit elevated IL-6, IL-17 A, and IL-4 levels, and a broader immune imbalance linked to pain amplification rather than immune collapse. Viral infections do not directly cause either condition, but commonly act as triggers. Shared mechanisms, such as spinal reflex arc activation and microglial sensitisation, suggest a common pathway mediated by proprioceptor-induced microglial activation for chronic pain. ME/CFS causes autoimmunity-like processes, whereas evidence of autoimmune drivers for FM is limited. Gut microbiome studies have revealed reduced microbial diversity in patients with ME/CFS. Moreover, the two disorders are characterised by shared, yet distinct, microbial alterations. CONCLUSION: Given the chronic and debilitating nature of ME/CFS and FM, prevention and early intervention remain crucial, but understudied. Health education, workplace adaptations, and early diagnostic pathways may substantially reduce the disease burden. Many patients are outside formal healthcare systems. Therefore, digital tools such as symptom-tracking apps, biosensors, remote testing, and assistive technologies are becoming central to disease management and monitoring. These approaches support a transdiagnostic, patient-centred model capable of addressing both conditions and reaching populations that remain underserved.