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The most common method that doctors use to measure cerebrospinal fluid (CSF) pressure is the lumbar puncture, a called a spinal tap, a procedure during which a physician inserts a needle between two of the patient's lumbar vertebrae and into the space surrounding the spinal cord. The physician attaches the needle to a device called a manometer, which measures the patient's CSF pressure. In some cases, depending on the patient's medical history and symptoms, the physician might choose to insert a needle at the base of the skull or to drill a hole in the patient's skull and place a catheter directly into one of the brain's ventricles rather than perform a lumbar puncture. CSF pressure also can be tested by measuring either the cisternal pressure or the ventricular pressure in the skull. All of these methods use a catheter hooked to a manometer to measure CSF pressure, and they help the doctor determine the best course of action for treating the patient.
A doctor will perform a lumbar puncture to measure cerebrospinal fluid pressure if he or she suspects that the patient has hydrocephalus, or "water on the brain." Hydrocephalus in babies is caused by a congenital defect, and adults can develop hydrocephalus as a result of conditions such as infection, stroke, tumor or head injury. For a lumbar puncture, the patient lies on one side with his or her knees pulled toward his or her chest, and the physician sterilizes the skin over the lumbar spine and injects a local anesthetic. Then the physician inserts a needle between two of the lumbar vertebrae and into the subarachnoid space of the spinal cord, which contains cerebrospinal fluid. The manometer gives a reading of the CSF pressure, which should be between 70 and 180 millimeters of water (mm H2O), meaning that the cerebrospinal fluid pressure causes the water in the manometer tube to rise 70-180 millimeters.
Before performing a lumbar puncture to test cerebrospinal fluid pressure, the doctor will look into the patient's eyes using an ophthalmoscope. If the doctor sees that the patient's optic nerve is bulging, he or she will not perform a lumbar puncture, because the procedure would not be safe. This is because a bulging optic nerve indicates high intracranial pressure — pressure inside the head — and inserting a needle into the spine can cause the CSF pressure in the spine to drop suddenly. A sudden drop in spinal CSF pressure can cause brain herniation, which is when part of the brain is pushed into the opening at the base of the skull, causing brain damage or death. If a lumbar puncture would not be safe, the doctor will test CSF pressure at a different site.
One alternate way of testing cerebrospinal fluid pressure is to measure cisternal pressure, which a doctor will do by inserting a needle into the cistern magna just below the occipital bone at the back of the skull. The doctor uses fluoroscopy to see where to place the needle, because the cistern magna is very close to the brain stem. Another way of testing cerebrospinal fluid pressure is to measure ventricular pressure. The doctor performs this test in an operating room, where he or she drills a hole in the skull and inserts a catheter directly into one of the ventricles, which are spaces within the brain that contain CSF. In some cases, such as with a serious head injury, the doctor might leave the catheter in to continuously monitor CSF pressure while the patient is in the hospital.