Pathways of cerebrospinal fluid outflow: a deeper understanding of resorption

Cerebrospinal fluid (CSF) absorption has long been held to predominantly entail drainage into the venous outflow system via the intracranial arachnoid granulations. Newer data suggest pathways involving spinal arachnoid granulations and lymphatic channels may also make substantial contributions to CSF outflow. The putative major CSF outflow pathways and their proportionate contribution to CSF absorption were reviewed in this article. CSF is absorbed and drained in bulk not just through cerebral arachnoid granulations (CAG) but also through spinal arachnoid granulations (SAG) and a lymphatic pathway involving egress through cranial and spinal nerve sheaths. The proportions of CSF that efflux through each of these major pathways have yet to be determined with any certainty in humans, though existing evidence (the majority of which is derived from animal studies) suggests that lymphatic pathways may account for up to 50 % of CSF outflow-presumably leaving the CAG and SAG to process the balance. Knowledge of the CSF pathways holds implications for our ability to understand, prognose, and even treat diseases related to CSF circulation and so is a matter of considerable relevance to neuroradiology and neurology.