To obtain samples of cerebrospinal fluid (CSF), the substance that surrounds the brain and spine, doctors perform a procedure called a lumbar puncture, better known as a spinal tap. The test involves inserting a needle into the space between two vertebrae of the lower back and into the CSF-filled space surrounding the spinal cord and withdrawing fluid for testing. To avoid injuring the spinal cord, the patient must lie still, in a fetal position, during the half-hour procedure. Although often uncomfortable, spinal taps are rarely as painful as they sound, and they provide important information that helps doctors detect a variety of serious diseases.

Lumbar punctures are performed to diagnose meningitis, an infection of the membrane that surrounds the brain. They can also be used to test for neurological disorders such as multiple sclerosis, neurosyphilis, or a type of neuropathy called Guillain-Barré syndrome. A local anesthetic is used, although mild discomfort or pain may still occur.

CSF is usually clear, so a cloudy or colored sample can indicate infection, bleeding, or a buildup of protein or cells. Higher-than-normal CSF pressure may result from increased pressure within the skull; low CSF pressure may be a sign of shock, fainting, or a diabetic coma.

Basic results from a lumbar puncture are available within an hour of the test, but bacteria cultures generally take about 2 days for analysis.

In many cases, doctors will recommend that patients lie down after a spinal tap. Headaches occur in 5 to 10 percent of patients, while bleeding or infections after the procedure occur rarely. However, the test is more risky for people who take blood thinners or are particularly susceptible to infection.

ADDITIONAL FACTS

  1. The lumbar puncture procedure was brought to the United States by the Harvard Medical School professor Arthur Wentworth. His career was nearly destroyed when he was prosecuted (although he was later acquitted) for having obtained spinal fluid from children.
  2. During a procedure called a myelogram, dye is injected into the CSF and then x-rays or CT scans are performed.
  3. Syphilis involving the brain is usually diagnosed by performing a lumbar puncture and testing the CSF.