What is it?

Lumbar puncture (spinal tap) is the most common means of collecting a specimen of cerebral spinal fluid. You are positioned on your side with your knees curled up to your abdomen and your chin tucked in to your chest. (Occasionally this procedure is performed with the person sitting bent forward). The skin is scrubbed, and a local anesthetic is injected over the lower spine. The spinal needle is inserted, usually between the 3rd and 4th lumbar vertebrae. Once the needle is properly positioned in the subarachnoid space, pressures can be measured and fluid can be collected for testing. After the sample is collected, the needle is removed, the area is cleaned, and a bandage is applied. You will be asked to remain flat, or nearly flat, for 6 to 8 hours after the test. Lumbar puncture with fluid collection may also be part of other procedures, particularly a myelogram ( X-ray or CT scan after dye has been inserted into the cerebral spinal fluid).

Alternative methods of obtaining cerebral spinal fluid are rarely used, but may be indicated if there is a problem such as lumbar deformity or infection, which would make lumbar puncture impossible or unreliable. Cisternal puncture involves insertion of a needle below the occipital bone (back of the skull). It can be hazardous because the needle is inserted close to the brain stem. Ventricular puncture is even more rare, but may be indicated when sampling of cerebral spinal fluid is necessary in people with possible impending brain herniation. It is usually performed in the operating room. A hole is drilled in the skull and a needle is inserted directly into the lateral ventricle of the brain.