Fig. 9

A schematic diagram of one interpretation of the differences in CSF flow in normal subjects and those with communicating hydrocephalus. In normal subjects, CSF is secreted by the choroid plexuses into the lateral (L), IIIrd and IVth ventricles. Some fluid is also secreted into the parenchyma, presumably by the blood–brain barrier. The magnitude of the net flow through the cerebral aqueduct is close to the sum of the secretions into the lateral and IIIrd ventricles. Fluid passes through the IVth ventricle and cisterna magna (CM) on its way to routes of outflow from the brain, i.e. via arachnoid villi, nerve tracts through the cribriform plate, and both perivascular and perineural pathways. At least some of these routes allow exit of fluid from the parenchyma without it ever mixing with CSF in the large cavities. In communicating hydrocephalus there is some deficit in the normal route of outflow indicated by the red X. The observation of reversed net flow through the cerebral aqueduct implies that another source of fluid enters CSF at some point, shown in the diagram as the cisterna magna, and, when combined with the secretion from the choroid plexus in the IVth ventricle, it equals the flow through the aqueduct. Some other pathway allowing fluid to escape from the ventricles must exist perhaps emerging from the IIIrd ventricle as shown. One possibility is flow through swollen periventricular parenchyma eventually reaching an exit route, perhaps either perivascular or perineural. Fluid exit via absorption across the blood–brain barrier is unlikely because this would require substantial alteration of barrier properties (see Sect. 5.1 with the caveats in sections 3.2 and 2.7 in [15]