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What Is a Spinal Tap?

Lumbar Puncture Provides Cerebrospinal Fluid for Diagnosis

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Updated July 14, 2012

Written or reviewed by a board-certified physician. See About.com's Medical Review Board.

When trying to reach a diagnosis, it is sometimes necessary to perform more invasive testing to come up with answers. One such test that might yield important diagnostic information is a spinal tap. This can be especially useful in certain neurological conditions and even, surprisingly, some sleep disorders. What is a spinal tap or lumbar puncture? Are there risks or complications related to the procedure that you should know about? Learn how a spinal tap may be used to diagnose your condition.

What Is a Spinal Tap?

A spinal tap, or lumbar puncture (LP), refers to a specialized procedure in which fluid is removed with a needle from the sac surrounding the spinal cord. This cerebrospinal fluid (CSF) bathes the brain and spinal cord, protecting these vital structures from harm. Analyzing the fluid can yield important clues about underlying neurological conditions.

The procedure may be performed by a physician in the emergency department, hospital, or clinical setting. A spinal tap typically takes less than an hour, but it may last longer if the fluid is collected slowly. It is most often performed with the patient lying on her side, often with her knees drawn towards her chest. It may also be done in a seated position. This positioning separates the bones in the lower back (called the lumbar vertebrae), allowing easier access. After the skin is sterilized, numbing medication is injected. It is at this point that a larger needle is placed to remove the CSF.

Why Is a Spinal Tap Necessary?

A spinal tap may be needed to aid in the diagnosis of various conditions, especially problems related to the nervous system. Most commonly, it is used to assess whether there is an infection in the brain or surrounding tissues. These infections are called encephalitis or meningitis.

Spinal taps are also used if a severe headache is thought to be due to a small amount of bleeding in the head. This is called a subarachnoid hemorrhage. It can be overlooked by scans and further insight can be gained by analyzing the CSF for the presence of blood.

There are also chronic medical conditions that can be diagnosed by doing a spinal tap. The most common is multiple sclerosis. People with multiple sclerosis may have changes in special markers in their CSF called oligoclonal bands or myelin basic protein. Measures of protein, glucose, and cell counts are routinely made in all lumbar punctures.

Even the pressure at which the CSF comes out can be informative; an elevation may suggest pseudotumor cerebri. A lowered pressure may occur in dehydration.

In the realm of sleep medicine, there are relatively few indications to perform a spinal tap. The diagnosis of narcolepsy may be aided by measuring orexin and hypocretin levels in the CSF. In the infectious disease called African sleeping sickness, the procedure can also be useful in determining the degree of the illness. It is otherwise typically not a necessary diagnostic test to assess sleep.

Risks and Complications of Spinal Taps

Spinal taps are intimidating. Most people have no interest in undergoing them, and many have heard horror stories. Moreover, when they are required, it is typically in a serious setting. It is possible to allay some of these fears.

The most common concern is: Does a spinal tap hurt? The tolerance for the procedure varies, as we all have different thresholds for pain or discomfort. The application of the numbing medicine may feel like a bee sting. After that point, it may feel more like pressure than a sharp pain. Many will comment at the conclusion, "That wasn’t so bad." In skilled hands, with a welcome measure of good fortune, a spinal tap can in fact be conducted rather easily. This may not always be possible, especially when obesity or degenerative changes in the spine create a challenge.

There are relatively few risks associated with the procedure itself. It is important that a doctor identify the risks that may be present prior to doing a spinal tap. The most dangerous complication occurs when the brain shifts due to increased pressure within the skull. This may result in death. For this reason, a thorough neurological examination and sometimes a computerized tomography (CT) scan or magnetic resonance image (MRI) may be done. Though serious, this complication is rare.

More commonly, a small amount of surface bleeding may occur at the site of the puncture. Your doctor may check blood counts and have you avoid taking blood thinners just prior to the procedure. There is also a small risk of infection. The most frequent complication associated with a spinal tap is the complaint of a headache that develops afterward. This is often worsened by sitting up or standing. It responds well to rest, hydration, caffeine, and pain medicine.

Another worry might be the risk of becoming paralyzed with the procedure. Although a twinge of pain or numbness shooting into the leg may occur as the spinal tap is performed, more persistent discomfort does not typically occur. These twinges of discomfort occur when the nerves leaving the spinal cord are contacted. This will not lead to weakness. In the vast majority of people, the spinal cord itself ends several vertebral levels above the point at which the fluid is drawn, so the risk of damage or paralysis is exceptionally rare.

If your doctor recommends a spinal tap, it is likely for an important reason. Discuss any concerns that you might have about the procedure and ask questions so that you understand how it may be helpful in obtaining a diagnosis. Consider the risks carefully and take comfort in knowing that this common procedure may provide important clues about the underlying condition your doctor hopes to identify.

Sources:

"Neurological Diagnostic Tests and Procedures." National Institute of Neurological Disorders and Stroke, National Institutes of Health. Accessed: July 14, 2012.

Dugdale, C et al. "Cerebral spinal fluid (CSF) collection." MedLinePlus, National Institutes of Health. Accessed: July 14, 2012.

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