Lumbar Puncture: This (Really) Is Spinal Tap

ouldn't resist the title's corny riff on the name of the rock band and their movie, but the kind of spinal tap featured in this article was a spinal tap before Spinal Tap was Spinal Tap. (Does that make any sense?)

Known more formally as a lumbar puncture, this kind of spinal tap is a valuable medical test with an interesting history. In 1891 Heinrich Quincke, of Kiel, Germany, introduced this procedure as we know it today. His original intent was to help babies suffering from hydrocephalus (water on the brain) by draining away excess fluid, but from the outset he was also interested in lumbar puncture's use as a diagnostic tool.

To understand the usefulness of this test and why you might someday need to have one, a little background is helpful. The brain and spinal cord are wrapped in a membrane called the meninges. Within the meninges, a watery fluid called the cerebrospinal fluid (CSF) bathes the inside and outside of the brain and the outside of the spinal cord. Within the brain's fluid chambers (ventricles), the body perpetually manufactures new CSF from constituents of the bloodstream. Once the CSF has percolated through openings to get outside the brain, it is reabsorbed and recycled into the bloodstream. The entire volume of CSF-about 150 milliliters or five ounces-is made and reabsorbed several times per day.

Dr. Quincke understood that analyzing the CSF's makeup could be useful in diagnosing infections and other diseases affecting the central nervous system (brain plus spinal cord). Measuring the CSF's protein and glucose (sugar) content along with inspecting a sample of CSF under a microscope to count red and white blood-corpuscles soon became standard practices.

The premier use of lumbar puncture in both Quincke's time and ours has been to diagnose meningitis. The suffix "-itis" signifies inflammation, so meningitis means inflammation of the meninges. Most, but not all, instances of meningitis are due to infections, but the kinds of infections seen have evolved over the years. In Quincke's lifetime tuberculosis and syphilis germs were common causes of meningitis, but presently, in developed countries these are uncommon. Nowadays, the usual causes of meningitis are other bacteria, viruses or even funguses. In cases of suspected infection, CSF protein, glucose and blood-corpuscle measurements are supplemented by other tests on the fluid that can track down the specific, infecting organisms.

Another important use of lumbar puncture is to diagnose subarachnoid hemorrhage, an abrupt, devastating, and potentially lethal bleed into the CSF space caused by rupture of an aneurysm or other abnormal blood vessel. In suspected cases-classically presenting with "the worst headache of my life"-a computed tomographic (CT) scan is usually performed first. While very sensitive in detecting subarachnoid hemorrhages, CT scans can still miss cases. So if the doctor is still suspicious that a bleed occurred, the next step is to do a lumbar puncture which is 100% sensitive in detecting this condition. That is, it never misses.

Lumbar puncture with CSF analysis can also help in the diagnosis of multiple sclerosis, a disease in which the patient's own immune system attacks the central nervous system. In this condition the immune reaction produces abnormal proteins that can be detected and measured in the CSF.

How is the test performed? Well, the first step, of course, is the informed consent process in which your doctor explains the risks and benefits of the test and you sign a permission form. In this author's opinion, lumbar puncture is the most benign test for which written permission is traditionally required and is less risky than some other procedures-like drawing blood from a high-pressure artery-for which written permission is traditionally omitted.

The next step is to lie on your side on a bed or procedure table with your knees tucked up to your chest. The skin of your lower back is painted with an iodine-based solution to produce a sterile field. If you have an allergy to iodine, an alcohol-based solution is substituted. The surrounding area is then covered with sterile paper or cloth. The skin and the tissue beneath the skin are then numbed with local anesthetic, and then everything is ready to insert the spinal needle.

The reason the lower back (lumbar spine) is chosen is because here the sac of meninges can be entered without risk of poking a hole in the spinal cord. This is because the spinal cord ends several inches higher within the spinal canal. The composition of the CSF is nearly the same throughout its system. Thus, CSF from the lumbar region is as good for diagnosis as from anywhere else, yet safer to obtain.

Once the spinal needle enters the lumbar sac of fluid, cor