Hydrocephalus results from three possible impairments: (1) overproduction of CSF; (2) a defect in the absorption of CSF into the venous system; or (3) a blockage in the CSF outflow pathway. Any of these three causes may be either congenital or acquired. Congenital hydrocephalus usually arises during early gestation and can be due to hereditary or non-hereditary causes. One example of congenital hydrocephalus is Chiari malformations, which account for 40% of congenital cases, where an enlargement of the cerebellum leads to the blockage of the outlets from the fourth ventricle. Examples of acquired causes include trauma, tumor, cyst formation, infection, increased venous pressure, and most notably, intra-cerebral hemorrhage. Any obstruction causing occlusion of a CSF outflow path will produce ventricular enlargement (ventriculomegaly) proximal to the blockage site. Thus, a tumor or cyst growing into a CSF chamber or pressing on tissues surrounding an aqueduct can block narrow portions of the ventricular system mechanically. Likewise, circulating blood or a blood clot from hemorrhage can cause blockage mechanically or produce inflammation of the ventricular lining. Tumors can also cause an overproduction of CSF if the tumor is located in the choroid plexus, but this is rare. Hydrocephalus resulting from impaired reabsorption of CSF can also be due to conditions that cause a rise in venous pressure (thrombosis, occlusion of cerebral venous sinuses, severe congestive heart failure), meningitis or arachnoiditis. These latter two causes result from hemorrhage into the subarachnoid space, or poor flow through this area.
Two classifications of hydrocephalus exist: (1) communicating hydrocephalus, which occurs when there is a patent path for CSF to flow from the cerebral ventricles to the subarachnoid space through the foramina of Luschka and Magendie; and (2) non-communicating, or obstructive hydrocephalus, which occurs when the flow of CSF is blocked anywhere within the cerebral ventricles, preventing passage to the subarachnoid space through the fourth ventricle outlets. For example, blockage of the cerebral aqueduct, or aqueductal stenosis, accounts for about 20% of all infantile hydrocephalus cases, while Dandy Walker malformation (enlargement of the fourth ventricle occluding the foramina of the fourth ventricle) accounts for less than 5% of all cases of hydrocephalus (Menkes, 2000). Non-communicating hydrocephalus is the most common class of hydrocephalus, and may cause all four ventricles to become enlarged, depending on the location of the obstruction.