Fibromyalgia Syndrome (FMS), Myalgic Encephalomyelitis/Chronic Fatigue Syndrome (ME/CFS) and Long COVID (LC) are similar multisymptom clinical syndromes but with difference in dominant symptoms in each individual. There is existing and emerging literature on possible functional alterations of the central nervous system in these conditions. This review aims to synthesise and appraise the literature on resting-state quantitative EEG (qEEG) in FMS, ME/CFS and LC, drawing on previous research on FMS and ME/CFS to help understand neuropathophysiology of the new condition LC. A systematic search of MEDLINE, Embase, CINHAL, PsycINFO and Web of Science databases for articles published between December 1994 and September 2023 was performed. Out of the initial 2510 studies identified, 17 articles were retrieved that met all the predetermined selection criteria, particularly of assessing qEEG changes in one of the three conditions compared to healthy controls. All studies scored moderate to high quality on the Newcastle-Ottawa scale. There was a general trend for decreased low-frequency EEG band activity (delta, theta, and alpha) and increased high-frequency EEG beta activity in FMS, differing to that found in ME/CFS. The limited LC studies included in this review focused mainly on cognitive impairments and showed mixed findings not consistent with patterns observed in FMS and ME/CFS. Our findings suggest different patterns of qEEG brainwave activity in FMS and ME/CFS. Further research is required to explore whether there are phenotypes within LC that have EEG signatures similar to FMS or ME/CFS. This could inform identification of reliable diagnostic markers and possible targets for neuromodulation therapies tailored to each clinical syndrome.

To explore the neuropsychological profile and the integrity of the olfactory network in patients with COVID-19-related persistent olfactory dysfunction (OD).
Patients with persistent COVID-19-related OD underwent olfactory assessment with Sniffin\' Sticks and neuropsychological evaluation. Additionally, both patients and a control group underwent brain MRI, including T1-weighted and resting-state functional MRI (rs-fMRI) sequences on a 3 T scanner. Morphometrical properties were evaluated in olfaction-associated regions; the rs-fMRI data were analysed using graph theory at the whole-brain level and within a standard parcellation of the olfactory functional network. All the MR-derived quantities were compared between the two groups and their correlation with clinical scores in patients were explored.
We included 23 patients (mean age 37 ± 14 years, 12 females) with persistent (mean duration 11 ± 5 months, range 2-19 months) COVID-19-related OD (mean score 23.63 ± 5.32/48, hyposmia cut-off: 30.75) and 26 sex- and age-matched healthy controls. Applying population-derived cut-off values, the two cognitive domains mainly impaired were visuospatial memory and executive functions (17 % and 13 % of patients). Brain MRI did not show gross morphological abnormalities. The lateral orbital cortex, hippocampus, and amygdala volumes exhibited a reduction trend in patients, not significant after the correction for multiple comparisons. The olfactory bulb volumes did not differ between patients and controls. Graph analysis of the functional olfactory network showed altered global and local properties in the patients\' group (n = 19, 4 excluded due to artifacts) compared to controls. Specifically, we detected a reduction in the global modularity coefficient, positively correlated with hyposmia severity, and an increase of the degree and strength of the right thalamus functional connections, negatively correlated with short-term verbal memory scores.
Patients with persistent COVID-19-related OD showed an altered olfactory network connectivity correlated with hyposmia severity and neuropsychological performance. No significant morphological alterations were found in patients compared with controls.

Background: Persistent coronavirus disease 2019 (COVID-19) symptoms are increasingly well-reported in cohort studies and case series. Given the spread of the pandemic, number of individuals suffering from persistent symptoms, termed \'long COVID\', are significant. However, type and prevalence of symptoms are not well reported using systematic literature reviews. Objectives: In this scoping review of the literature, we aggregated type and prevalence of symptoms in people with long COVID. Eligibility Criteria: Original investigations concerning the name and prevalence of symptoms were considered in participants ≥4-weeks post-infection. Sources of Evidence: Four electronic databases [Medline, Web of Science, Scopus, and the Cochrane Central Register of Controlled Trials (CENTRAL)] were searched. Methods: A scoping review was conducted using the Arksey and O\'Malley framework. Review selection and characterisation was performed by three independent reviewers using pretested forms. Results: Authors reviewed 2,711 titles and abstracts for inclusion with 152 selected for full-text review. 102 articles were subsequently removed as this did not meet inclusion criteria. Thus, fifty studies were analysed, 34 of which were described as cohort studies or prospective cohort studies, 14 were described as cross-sectional studies, one was described as a case control study, and one was described as a retrospective observational study. In total, >100 symptoms were identified and there was considerable heterogeneity in symptom prevalence and setting of study. Ten studies reported cardiovascular symptoms, four examined pulmonary symptoms, 25 reported respiratory symptoms, 24 reported pain-related symptoms, 21 reported fatigue, 16 reported general infection symptoms, 10 reported symptoms of psychological disorders, nine reported cognitive impairment, 31 reported a sensory impairment, seven reported a dermatological complaint, 11 reported a functional impairment, and 18 reported a symptom which did not fit into any of the above categories. Conclusion: Most studies report symptoms analogous to those apparent in acute COVID-19 infection (i.e., sensory impairment and respiratory symptoms). Yet, our data suggest a larger spectrum of symptoms, evidenced by >100 reported symptoms. Symptom prevalence varied significantly and was not explained by data collection approaches, study design or other methodological approaches, and may be related to unknown cohort-specific factors.