Because cerebrospinal fluid is secreted and reabsorbed continuously, the fluid pressure in the ventricles remains relatively constant. However, infection, a tumor, or a blood clot can interfere with the fluid's circulation, increasing pressure within the ventricles (intracranial pressure). This can collapse cerebral blood vessels, retarding blood flow. Brain tissues may be injured by being forced against the skull.

A lumbar puncture (spinal tap) can measure cerebrospinal fluid pressure. A physician inserts a fine, hollow needle into the subarachnoid space between the third and fourth or between the fourth and fifth lumbar vertebrae--below the end of the spinal cord. An instrument called a manometer measures the pressure of the fluid, which is usually about 130 millimeters of water (10 millimeters of mercury). At the same time, samples of cerebrospinal fluid may be withdrawn and tested for the presence of abnormal constituents. Red blood cells, for example, may indicate a hemorrhage in the central nervous system.

A temporary drain inserted into the subarachnoid space between the fourth and fifth lumbar vertebrae can relieve excess pressure. In a fetus or infant whose cranial sutures have not yet united, increasing intracranial pressure (ICP) may cause an enlargement of the cranium called hydrocephalus, or water on the brain. A shunt to relieve hydrocephalus drains fluid away from the cranial cavity and into the digestive tract, where it is either reabsorbed into the blood or excreted.

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