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Introduction Cerebrospinal fluid (CSF) loss in spontaneous intracranial hypotension (SIH) may be associated with changes in glymphatic flow. We used advanced diffusion-weighted MRI (dMRI) to analyze brain interstitial fluid shifts in patients with CSF loss, before and after closure of a spinal CSF leak. We hypothesized that patients with SIH have an altered relative free water distribution in the brain tissue which is modified by the closure of the spinal CSF leak.Aim of Study The aim was to investigate whether SIH patients have an altered distribution of interstitial free water in the brain compared to healthy controls and how this changes after leak closure.Method The free water component (V-CSF) of the whole brain gray and white matter, corona radiata, amygdala, hippocampi and parahippocampal gyrus was extracted from dMRI scans as well as from 23 age-matched healthy controls ( figure 1). Baseline gray matter and hippocampal V-CSF were correlated with the Bern SIH score.Results We included 40 patients with SIH and pre-therapeutic imaging, and of those 23 with additional post-therapeutic imaging. After leak closure, there was a significant increase in V-CSF within the global gray matter (p=0.029), corona radiata (p<0.01), hippocampi (p=0.001), and parahippocampal gyrus (p=0.009). Compared to healthy controls, there was a decreased free water fraction before leak closure in the hippocampi (p=0.019) and posterior corona radiata (p=0.011) ( figure 1). We found a significant negative correlation between the initial Bern SIH score, and gray matter (p=0.0091) and also hippocampal V-CSF (p=0.014).Abstract A105 Figure 1Segmentations and regional free-water fraction (V-CSF) in spontaneous intracranial hypotension (SIH) patients before (Pre) and after (Post) therapy versus healthy controls (NC). A: SPM12-based gray matter segmentation (left panel) and atlas based segmentations of the anterior (green), superior (light blue) and posterior (dark blue) corona radiata (right panel). B-C: After leak closure, there was a significant increase in V-CSF in the gray matter and all regions of the corona radiata (PCR: posterior corona radiata; anterior and superior corona radiata not shown). E: Freesurfer-& Atlas-based segmentations of mesial temporal structures (yellow: orbitofrontal cortex, orange: Amygdala, red: Hippocampus, green: parahippocampal gyrus). F-G: There was a significant increase in V-CSF in the hippocampi and parahippocampal gyrus after treatment. C, F: Compared to normal controls (NC), there also was a significant decrease in V-CSF in the hippocampi and posterior corona radiataConclusion Brain interstitial fluid shifts may reflect altered glymphatic flow in patients with CSF loss.Conflict of Interest No