Although hydrocephalus is a multifactorial disorder, the processes res
ponsible for neurologic impairment can be classified into primary and
secondary mechanisms. Primary mechanisms include mechanical compressio
n and stretching of brain parenchyma, ischemia and anoxia, cerebral ed
ema, and blood brain barrier dysfunction. These processes lead to seco
ndary mechanisms, which include cytologic and cytoarchitectural altera
tions of neurons, reduced size and numbers of cerebral microvessels, a
xonal degeneration and demyelination, and so on. Shunting studies sugg
est that neuronal cell death may not play a major role until severe st
ages of hydrocephalus and that some impairments in connectivity can be
reversed. Relatively early shunting may alleviate many of the patholo
gic features of hydrocephalus, but residual impairments in neurotransm
itter levels and dependence on anaerobic respiration leave the treated
hydrocephalic brain vulnerable to subsequent insults.