Myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) is a debilitating disorder with suspected neuroinflammatory pathophysiology. However, previous diffusion tensor imaging (DTI) studies have reported inconsistent white matter abnormalities in ME/CFS, and specific white matter inflammatory changes remain poorly characterised. This study employed an advanced diffusion‐based neuroinflammation imaging (NII) model to investigate white matter neuroinflammation in ME/CFS. Diffusion MRI data from 67 ME/CFS patients (median age, 38; and 54 women) and 67 rigorously matched healthy controls (HCs) (median age 38; and 52 women) were analysed. Seven NII‐derived metrics were computed: hindered water ratio (NII‐HR), restricted fraction (NII‐RF), fibre fraction (NII‐FF), axial diffusivity (NII‐AD), radial diffusivity (NII‐RD), mean diffusivity (NII‐MD) and fractional anisotropy (NII‐FA). Conventional DTI metrics were also calculated. Tract‐based spatial statistics were used to perform voxel‐wise group comparisons, and multiple regression analysis was conducted to examine the relationship between NII/DTI metrics and clinical measures of mental health, physical health, sleep quality, disability, disease severity and disease duration. Compared to HCs, ME/CFS patients exhibited widespread white matter abnormalities, including significantly lower NII‐HR and NII‐RF, and significantly higher NII‐FF, NII‐AD, NII‐MD and NII‐FA across association, commissural and projection fibres. Additionally, some regions showed decreased NII‐AD and NII‐MD in ME/CFS. Lower NII‐RF, NII‐AD and NII‐MD in ME/CFS were significantly associated with worse mental health, while lower NII‐RF was also associated with a higher level of disability. Among ME/CFS patients, higher NII‐FF was associated with lower disease severity. Conventional DTI showed minimal group differences and no significant clinical associations. This study provides in vivo evidence of white matter neuroinflammation in ME/CFS, characterised by cerebral edema (reduced NII‐HR), cellular infiltration (reduced NII‐RF) and axonal reorganisation (increased NII‐FF). This suggests NII‐derived indices may serve as sensitive biomarkers for neuroinflammation in ME/CFS. Diffusion‐based neuroinflammation imaging (NII) reveals widespread white matter abnormalities in ME/CFS patients, undetected by conventional DTI. NII metrics associate with mental health, disability and disease severity, providing novel evidence of neuroinflammation and highlighting NII's potential as a biomarker in ME/CFS.

Neurological symptoms are central to Myalgic Encephalomyelitis/Chronic Fatigue Syndrome (ME/CFS), yet its underlying neurophysiological mechanisms remain elusive. We examined a neglected aspect of task-based functional MRI, focusing on how blood oxygenation level-dependent (BOLD) signals alter during cognitive tasks in ME/CFS. This prospective observational study utilised MRI scans on ME/CFS participants and healthy controls (HCs) with sedentary lifestyles (ACTRN12622001095752). Participants completed two blocks of a Symbol Digit Modalities Test, with 30 trials per block split into two sets. The fMRI signal changes between blocks and sets were compared within and between groups. Thirty-four ME/CFS participants (38 years +/- 10; 27 women) and 34 HCs (38 +/- 10; 27 women), were evaluated. In the second task block, ME/CFS participants exhibited increased activation in the right postcentral gyrus, contrasting with decreased activation in multiple regions in HCs. These results were further confirmed by significantly higher bilateral dynamic changes (2nd vs 1st set) in the motor, sensory and cognitive cortex in ME/CFS compared to HCs and significant correlations between those changes in the left primary motor cortex with fatigue severities. BOLD adaptation, potentially improving energy economy, was absent in ME/CFS, which may provide an underlying neurophysiological process in ME/CFS.

While post-infectious (PI-ME/CFS) and gradual onset (GO-ME/CFS) myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) manifest similar symptoms, it has long been suspected that different disease processes underlie them. However, the lack of biological evidence has left this question unanswered. In this study, how white matter microstructural changes in PI-ME/CFS and GO-ME/CFS patients were investigated. PI-ME/CFS and GO-ME/CFS patients were recruited based on consensus diagnoses made by two experienced clinicians and compared their diffusion MRI features with those of rigorously matched healthy controls (HCs) with sedentary lifestyles. PI-ME/CFS participants showed significantly higher axial diffusivity (AD) in several association and projection fibres compared to HCs. Higher AD in PI-ME/CFS was significantly related to worse physical health. In contrast, GO-ME/CFS participants exhibited significantly decreased AD in the corpus callosum. Lower AD in GO-ME/CFS was significantly associated with worse mental health in commissural and projection fibres. No significant group differences were found for fractional anisotropy, mean diffusivity, or radial diffusivity. Distinct patterns of AD alterations in PI-ME/CFS and GO-ME/CFS provide neurophysiological evidence of different disease processes and highlight the heterogeneities of ME/CFS.