Overview of the CSF dual outflow system.

It was firmly established in the mid-twentieth century that the arachnoid villi represented an open pathway between the subarachnoid space and the dural venous sinuses. Intracellular and extracellular pathways within the villous structure provided the conduit for cerebrospinal fluid (CSF) and particulate matter. The importance of the lymphatic system was established by the demonstration of CSF tracers entering the nasal lymphatic system via the perineural subarachnoid space enveloping the olfactory nerve rootlets. It appears that because of the late development of the arachnoid villus (AV) system, the lymphatic outflow system is the more dominant one in the young animal, but in the mature animal the importance of both systems appears equal. In general, the lymphatic system in lower animals appears dominant, but in the case of primates, this may not be the case. The global outflow system has a definite opening pressure of ca. 50-70 mm of water, and the balance between production of CSF and absorption occurs at a resting pressure of ca. 115 mm water. The bicompartmental CSF outflow curves obtained from hydrocephalic patients support the presence of a dual outflow system utilized in normal CSF drainage.