Intracranial venous pressures, hydrocephalus and effects of cerebrospinal fluid shunts.

Data concerning venous anatomy, interstitial fluid pressure and cerebral blood flow indicate that obstruction of cerebral venous outflow (as a whole or involving the deep venous system alone) is the essential cause of hydrocephalus. Choroidal and ventricular veins both belong to the deep system. Choroidal venous pressure determines cerebrospinal fluid pressure; pressure in the ventricular veins determines interstitial fluid pressure in the paraventricular white matter. A decrease in deep cerebral blood flow causes paraventricular atrophy. CSFP is higher than interstitial fluid pressure. normally and in venous obstruction. Thus, CSFP prevents venocongestive edema (but not inflammatory edema) of the brain. Collateral venous pathways are described. Venous obstruction causes hydrocephalus only when it leads to insufficient blood flow. Cerebrospinal fluid shunting causes increased CBF as essential therapeutic effect in hydrocephalus, but also causes venocongestive brain edema, which explains the decrease in ventricle size and the side effects of shunting.