What is a lumbar puncture?

Indications and Procedures

A lumbar puncture, often referred to as a spinal tap, is indicated for the diagnosis of meningitis
because it is the best method for detecting meningeal irritation. A laboratory examination of the cerebrospinal fluid
(CSF) harvested through a lumbar puncture can detect problems relating to the brain and spinal cord. Meningitis cannot be diagnosed through imaging methods such as computed tomography (CT) scanning.

Similarly, some subarachnoid (brain) hemorrhages

involve the loss of so little blood that they are not detectable radiologically. When this is the case, the CSF may contain small amounts of blood that continues to be present in successive testing. If diagnosis is delayed, then the blood from a subarachnoid hemorrhage may have dissipated, making the CSF yellowish. This signifies that the blood in it is being metabolized. When this is the case, an immediate angiogram is indicated.

CSF withdrawn through a lumbar puncture should be examined immediately by a pathologist or hematologist to determine if blood is present or, if it is xanthochromic or yellowish, to ascertain if the protein level is abnormally high or a subarachnoid hemorrhage exists and blood is spreading over the surface of the brain. Such leakage may indicate an aneurysm and generally requires prompt surgery.

Although lumbar puncture is essentially a diagnostic tool, it is sometimes indicated when CSF has built up to dangerous levels in the spinal canal and the patient is hydrocephalic. In such instances, excess CSF can be withdrawn from the spinal canal, but it is imperative that the cause of the buildup be immediately determined and treated.

Patients undergoing lumbar punctures lie on their side, with their chin down and their knees drawn up to separate the vertebrae. Local anesthetic is used to numb the area surrounding the lower vertebrae. A hollow needle is inserted between two lower vertebrae and pushed into the spinal canal. The entire procedure takes about twenty minutes and involves minimal discomfort. The puncture wound left by the needle is covered with a sterile bandage. Patients may experience a headache following the procedure, but it usually disappears quickly. If it does not disappear within a few days, a small amount of the patient’s blood can be injected into the site, creating a patch that should eliminate the headache.

Uses and Complications

Lumbar puncture is used to inject dyes into the spinal canal to serve as a contrast medium in diagnostic procedures involving x-rays, particularly myelography. It is also used to introduce medications into the CSF for the treatment of certain types of cancer. This use, often appropriate in cases of leukemia and carcinomas of the nervous system, is frequently employed in pediatric care.

Some surgical procedures require that patients be awake during surgery. In such cases, local anesthetics are introduced into the spinal canal through lumbar puncture, allowing patients to remain conscious while rendering them insensate.

As with any invasive procedure, there is risk of infection, but it is minimal. The procedure is performed under sterile conditions. One danger, in the case of a subarachnoid hemorrhage, is blood loss. Bleeding, in rare cases, may become uncontrollable and result in death. One cautionary note is that a lumbar puncture should never be performed in cases where a brain abscess is suspected unless reliable CT scans or other tests have failed to reveal a mass or reveal only a small mass.

Lumbar puncture in and of itself is not a high-risk procedure, although the conditions that require its use often involve high risks for the patient. It is advisable in most cases that the procedure be performed in a location where immediate surgery can be carried out if the patient’s condition deteriorates suddenly.

Perspective and Prospects

The earliest use of CSF in diagnosis was in the nineteenth century, when such primitive tools as sharpened bird quills were used to penetrate the lumbar region. The technique came into its own in the mid-twentieth century, when most problems of the central nervous system were diagnosed through an examination of the CSF.

Because of the importance of CSF in diagnosing meningitis, cerebral hemorrhages, and other dangerous conditions, the lumbar puncture procedure has seen considerable progress and become increasingly sophisticated. However, the imaging tools currently available to surgeons, neurologists, hematologists, and pathologists are so advanced and accurate that lumbar puncture is used as a diagnostic tool less often than in the past.

Bibliography

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Dugdale, David C., Kevin Sheth, and David Zieve. “Cerebral Spinal Fluid (CSF) Collection.” MedlinePlus, June 18, 2011.

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