MECHANISMS OF HYDROCEPHALUS
Hydrocephalus refers to abnormal dilation of the ventricular system in association with narrowing of the subarachnoid spaces. Elevation of the intraventricular pressure may or may not be present. Hydrocephalus is not synonymous with ventriculomegaly. Most forms of hydrocephalus are due to an imbalance between the formation and absorption of cerebrospinal fluid, that is, inadequate passage of cerebrospinal fluid (CSF) from its point of production within the ventricular system to its point of absorption into the systemic circulation. The excess intraventricular CSF that results from this imbalance in turn causes an increase in intracranial pressure. The degree and pattern of enlargement of the CSF pathways and the amount of damage to central nervous system (CNS) structures depend on both the severity and the pathogenesis of the hydrocephalus.1–3
An important site of CSF absorption is into the sagittal sinus via the arachnoid villi, that is, the pacchionian granulations. Absorption of CSF at the pacchionian granulations is passive. The valve system of the arachnoid villi has an opening pressure of approximately 5 mm of mercury. Above this opening pressure, the rate of fluid absorption is directly proportional to the intracranial pressure. Equilibrium between CSF production and absorption occurs at approximately 10 mm of mercury (14 cm water). Capillaries throughout the CNS also contribute substantially to CSF absorption. In particular, there is relatively free bidirectional mixing of CSF with fluid in the interstitium of the brain.4
Approximately 60% of CSF production is via the choroid plexus, with the remainder by other mechanisms such as parenchymal capillaries and the ependyma. The rate of CSF production in the brain is relatively constant in most individuals; overproduction of CSF is not the cause of hydrocephalus in the great majority of cases (choroid plexus papilloma is an important exception). Acute hydrocephalus is generally due to pathology that blocks the flow of CSF or results in impaired resorption of fluid. Typically, this is related to blockage of flow in the ventricular system, in the basal cisternae, or in the subarachnoid space along the cerebral convexities. Diminished absorption can also result from abnormalities of the arachnoid villi. Ventriculomegaly due to mechanical obstruction of CSF drainage pathways causes brain expansion, compression of cortical veins, secondary venous congestion, impaired brain capillary absorption of CSF, and elevated intracranial pressure. This sequence of events may exacerbate hydrocephalus and cause symptomatic reduction in cerebral oxygenation.
Acute obstructive hydrocephalus causes elevation of intraventricular pressure, ventricular enlargement, and brain expansion. Compression of cortical veins by the expanding brain leads to intracranial venous congestion and elevation of the intracranial pressure that counteracts ventricular expansion. As the intracranial pressure increases in the patient with hydrocephalus, there is increased CSF absorption through the arachnoid membrane and the stroma of the choroid plexus. Fluid may also egress through the extracellular spaces of the cortical mantle, that is, transependymal CSF flow that appears as periventricular edema on CT and MR ...