Microstructural differences have previously been detected in comparisons of COVID-19 patients with controls, particularly in the insula, cuneus, inferior temporal and anterior cingulate regions.

Here we report diffusion magnetic resonance imaging (3 T Siemens MRI) findings from 20 participants (mean age: 45.3, 55% female), both immediately after recovery and at a 3-month follow-up. 

Fractional anisotropy (FA), mean diffusivity (MD), mode of diffusivity (MO), free water fraction (F), tissue-specific FA (FAt) and tissue-specific MD (MDt) were obtained using DTI data with b=700 and 1400 (DIPY free-water model). Regions of interest in the grey matter and white matter were delineated using FreeSurfer. To assess differences between baseline and follow-up, a paired t-test, the Wilcoxon Test and Friedman Test were performed, corrected for false-discovery rate of 0.05. Effect size (Cohen’s d) was also computed (d>0.45 deemed large effect).

All three tests revealed decreased F in the hippocampus and decreased MD in the parahippocampal region of the WM at follow-up. In the GM, F was increased in the medial orbitofrontal region. In the WM, MD was increased in the paracentral region and MDt was increased in the parahippocampal and lateral orbitofrontal regions.

These results suggest that microstructural abnormalities persist following recovery. Increased extracellular fluid (i.e. F and MD) in the frontal lobe suggest spreading of COVID-19 impact, while decreased F and MD in the hippocampal region suggest debris accumulation as part of the inflammatory process. None of the regions affected in sub-acute COVID-19 appear to fully recover within three months.