Assessing the subarachnoid space anatomy on clinical imaging: utilizing normal and pathology to understand compartmentalization of the subarachnoid space.

backgroundThe goal of the study is to use CT imaging in patients with aSAH to evaluate the anatomic distribution of hemorrhage and compartmentalization of subarachnoid space to investigate potential in vivo visualization of recently discovered layer named subarachnoid lymphatic-like membrane (SLYM).

methodsWe conducted a retrospective cohort study of cases with aneurysmal SAH (aSAH) at our institution between January 2015 and June 2022. Subarachnoid hemorrhage distribution into superficial and deep subarachnoid spaces was classified based on proximity to the dural or pial surfaces, respectively, as seen on multiplanar CT head.

resultsA total of 97 patients with aSAH were included. Patients with lower modified Fisher score (MFS) of 1-2 were more likely to have SAH compartmentalizing in the "deep" pial-adjacent subarachnoid space. Patients with higher MFS of 3-4 were more likely to have SAH in both "superficial" and "deep" compartments along the brainstem. There is a significant association between the severity of aSAH - quantified by the MFS - and the distribution of the blood. Patients with higher MFS scores were roughly 7.6 times (p-value = 0.049) more likely to have hemorrhage at the "Superficial" juxta-dural subarachnoid compartment when compared to those with lower MFS scores.

conclusionThis study suggests an imaging correlate to the recently discovered "SLYM", potentially influencing aSAH compartmentalization, particularly in low-grade bleeds. While compartmentalization is limited in high grade cases, these findings warrant further investigation with advanced imaging techniques to validate this membrane's role and potential impact on CSF flow and aSAH pathophysiology.