MECHANISMS OF MYALGIC ENCEPHALITIS/CHRONIC FATIGUE SYNDROME (ME/CFS) AND LONG COVID WITH PREDOMINANT FATIGUE: A MAGNETIC RESONANCE SPECTROSCOPY STUDY OF THE BRAIN AND MUSCLE AT 7 TESLA

Abstract Background Myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) is a chronic and debilitating condition, which core symptoms are fatigue not caused by exertion and not alleviated by rest, post- exertional malaise, decline in functioning, and cognitive symptoms, often described as 'brain fog’ (1). Similar symptoms are also present in 'long-COVID’, a sequela of the acute infection with the SARS-CoV- 2 virus (2). While underlying pathogenetic processes are still largely unknown impaired mitochondrial function and energy metabolism in both conditions, and clotting problems in long-COVID, have been suggested. Aims & Objectives the aim was to investigate differences in the biochemistry of the brain and muscle between patients with ME/CFS, long-COVID and healthy volunteers (HV), using the state-of-art ultra- high-field proton magnetic resonance spectroscopy (H1-MRS) at 7 Tesla (7T). In particular, we aimed to assess metabolites involved in energy processing (creatine and lactate). Additionally, we aimed to explore the relationship between MRS findings with the cognitive function. Method: 24 CFS/ME patients, 25 long-COVID patients with predominant fatigue, and 24 healthy volunteers underwent H1-MRS scanning of the brain and calf muscle at 7 Tesla. Voxels were positioned in the pregenual and dorsal ACC (pgACC and dACC, respectively). Participants completed the Stroop Color and Word Test, testing ability to inhibit cognitive interference, related to the ACC function. Other cognitive functions tested included verbal memory and learning, working memory, verbal fluency and executive function. Results Compared to HV, participants with ME/CFS had increased levels of lactate in both pgACC (ME/CFS 1.52 mM, HV 1.22; p=0.003) and dACC (ME/CFS 1.45 mM, HV 1.40 mM; p=0.005), while participants with long-COVID had decreased concentrations of total choline in dACC as compared to healthy individuals (long-COVID 2.25 mM, HV 2.77 mM; p=0.0002). In the ME/CFS group, we observed a negative correlation between verbal fluency and both pgACC and dACC lactate concentrations (pgACCr=-0.614, p=0.01; dACC r=-0,623, p=0.008), while in the long-COVID group we observed a correlation between total choline concentrations and executive function (r=-0.592, p=0.005). There were no significant between-group differences in terms of muscle metabolites. Discussion & Conclusions In ME/CFS, increased levels of lactate suggest dysfunction in energy metabolism, with lactate accumulation suggesting ill mitochondrial health and a shift towards anaerobic metabolism (3). Low levels of total choline in long-COVID patients are interesting in the context of the recently reported association between blood clots and 'brain fog’ in long COVID (4), and reports of neuroprotective effects of choline in animal models, preventing disseminated intravascular coagulation (5). Importantly, differences in findings between ME/CFS and long COVID suggest that the underlying neurobiological mechanisms, while leading to similar clinical presentations with fatigue and brain fog, may differ. This has implications for future research, suggesting that patients with ME/CFS and those with fatigue in the course of long COVID should not be studied as a single group, at least until the mechanisms are better understood, and that different treatments may be needed despite similarity of symptoms. References 1) Prins, J.B., van der Meer, J.W., Bleijenberg, G. (2006) 'Chronic fatigue syndrome.’ Lancet, 367, pp.346–355. 2) Kelly, J.D., Curteis, T., Rawal, A., et al. (2023) 'SARS-CoV-2 post-acute sequelae in previously hospitalised patients: systematic literature review and meta-analysis.’ Eur Respir Rev, 32, pp.220254. 3) Brooks, G.A. (2018) 'The Science and Translation of Lactate Shuttle Theory.’ Cell Metab, 27, pp.757- 785. 4) Taquet, M., Skorniewska, Z., Hampshire, A., et al. (2023) 'Acute blood biomarker profiles predict cognitive deficits 6 and 12 months after COVID-19 hospitalization.’ Nat Med, 29, pp.2498-2508 5)Blusztajn, J.K., Slack, B.E. and Mellott, T.J. (2017) 'Neuroprotective actions of dietary choline.’ Nutrients, 9, p.815.