I couldn't resist title's corny riff on name of rock band and their movie, but 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 brain) by draining away excess fluid, but from outset he was also interested in lumbar puncture's use as a diagnostic tool.
To understand 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 meninges. Within meninges, a watery fluid called cerebrospinal fluid (CSF) bathes inside and outside of brain and outside of spinal cord. Within brain's fluid chambers (ventricles), body perpetually manufactures new CSF from constituents of bloodstream. Once CSF has percolated through openings to get outside brain, it is reabsorbed and recycled into 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 CSF's makeup could be useful in diagnosing infections and other diseases affecting central nervous system (brain plus spinal cord). Measuring 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 meninges. Most, but not all, instances of meningitis are due to infections, but kinds of infections seen have evolved over years. In Quincke's lifetime tuberculosis and syphilis germs were common causes of meningitis, but presently, in developed countries these are uncommon. Nowadays, 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 fluid that can track down specific, infecting organisms.
Another important use of lumbar puncture is to diagnose subarachnoid hemorrhage, an abrupt, devastating, and potentially lethal bleed into 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 doctor is still suspicious that a bleed occurred, 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 diagnosis of multiple sclerosis, a disease in which patient's own immune system attacks central nervous system. In this condition immune reaction produces abnormal proteins that can be detected and measured in CSF.
How is test performed? Well, first step, of course, is informed consent process in which your doctor explains risks and benefits of test and you sign a permission form. In this author's opinion, lumbar puncture is 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.