The brain and spinal cord form the central nervous system (CNS), and they are surrounded and protected by the bones of the skull and the vertebral column (back bone). Between the brain and skull are three other protective coverings called the meninges, which are special membranes that completely surround the brain and spinal cord.

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Within the brain are the ventricles. Ventricles can be thought of as chambers filled with fluid. There are four ventricles in all: two lateral ventricles, the third ventricle, and the fourth ventricle. The ventricles are connected by narrow passageways.

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Cerebrospinal Fluid (CSF) flows through the four ventricles and then flows between the meninges in an area called the subarachnoid space. CSF cushions the brain and spinal cord against forceful blows, distributes important substances, and carries away waste products.

Under normal conditions, a delicate balance exists between the amount of CSF produced and the rate at which it is absorbed. Our bodies produce approximately one pint of CSF every day, continuously replacing it as it is absorbed.

Hydrocephalus develops when this balance is altered and is characterized by an abnormal accumulation of CSF within the ventricles. This accumulation of CSF increases the pressure in the brain causing the ventricles to enlarge and the brain to be pressed against the skull.

CSF is primarily produced within the lateral the third ventricles by delicate tufts of specialized tissue called the choroid plexus. In some cases, hydrocephalus can develop when the choroid plexus produces too much CSF. This can happen when there is a tumor on the choroid plexus, for example.

CSF flows from the lateral ventricles through two narrow passageways into the third ventricle. From the third ventricle, it flows down another long passageway known as the aqueduct of Sylvius into the fourth ventricle. From the fourth ventricle, it passes through three small openings called foramina and into the subarachnoid space surrounding the brain and the spinal cord. If the flow of CSF at any of these points is blocked, hydrocephalus can develop. This is often referred to as non-communicating hydrocephalus.

It has traditionally been thought that CSF is absorbed through tiny, specialized cell clusters called arachnoid villi near the top and midline of the brain. The CSF then passes through the arachnoid villi into the superior sagittal sinus, a large vein, and is absorbed into the bloodstream. Once in the bloodstream, it is carried away and filtered by the kidneys and liver in the same way as other bodily fluids. However, more recent research has shown that CSF is also absorbed through other pathways as well. When CSF absorption is blocked or reduced, hydrocephalus can develop. This is often referred to as communicating hydrocephalus because there is no obvious blockage within the ventricular system.

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