The fluid within the ventricles of the brain, bathing its surface and that of the spinal cord. Normally a clear, colourless fluid, its pressure when an individual is lying on one side is 50 to 150 mm water.

Analysis of the CSF provides useful information in various conditions and is invaluable in the diagnosis of acute and chronic inflammatory diseases of the nervous system. Bacterial MENINGITIS results in a large increase in the number of polymorphonuclear LEUCOCYTES, while a marked lymphocytosis is also seen in viral meningitis and ENCEPHALITIS, tuberculous meningitis and neurosyphilis. The total protein content is raised in many neurological diseases, being particularly high with NEUROFIBROMATOSIS and GUILLAIN-BARRÉ SYNDROME, while the immunoglobulin G fraction is raised in MULTIPLE SCLEROSIS, neurosyphilis, and connective-tissue disorders. The glucose content is raised in diabetes (see DIABETES MELLITUS), but may be very low in bacterial meningitis, when appropriately stained smears or cultures often define the infecting organism. The CSF can also be used to measure immune proteins produced in response to infection, helping diagnosis in cases where the organism is not grown in the laboratory culture.