Diagram representing fluid movements at the Virchow Robin space. The complex anatomical structure of the Virchow Robin space (VRS) allows a bidirectional fluid exchange between the VRS and both the brain extracellular space (ECS) and the subarachnoid CSF space (blue arrows). Glial (blue lines) and pial (yellow lines) cell membranes enclose the VRS and control fluid exchange. Note, that it is a matter of debate whether the VRS represents an open fluid fill space (see text for discussion). Both experimental and clinical evidence indicate the existence of a pathway along the basement membranes of capillaries, arterioles, and arteries for the drainage of ISF and solutes into the lymphatic system (red lines and green arrows). It is unclear, whether the subpial perivascular spaces around arteries and veins (light blue) serve as additional drainage pathways. Also, the proposed glymphatic pathway connecting the arterial and venous VRS with the venous perivascular space (black arrows) is still a matter of debate. A: artery, V: vein, C: capillary, VRS: Virchow Robin space, SAS: subarachnoid space.