What is a lumbar puncture (LP)?
A lumbar puncture (LP) is the insertion of a needle into the fluid within the spinal canal to collect and examine the fluid that surrounds the brain and spinal cord (cerebral spinal fluid). It is termed a "lumbar puncture" because the needle is placed in the lumbar portion of the back and used to puncture through tissues to enter the spinal canal.
Other names for a lumbar puncture (an LP) include spinal tap, spinal puncture, thecal puncture, and rachiocentesis.
Why is a lumbar puncture done?
An LP is most commonly performed to obtain a sample of the fluid that surrounds the brain and spinal cord. Examination of the fluid can be crucial in diagnosing diseases such as meningitis, neurologic diseases, or effects of systemic disease on the brain and spinal fluid. An LP can also be done to treat diseases, as a way of administering antibiotics, cancer drugs, or anesthetic agents into the spinal canal. Spinal fluid is sometimes removed by LP for the purpose of decreasing spinal fluid pressure in patients with uncommon conditions (such as, normal-pressure hydrocephalus and benign intracranial hypertension, for example).
Sometimes a lumbar puncture is performed in patients with migraines to assure that no other underlying pathology exists.
How is the LP procedure performed?
The patient is typically lying down on their side for the procedure. Less often, the procedure is performed while the patient is sitting up and leaning slightly forward.
After local anesthesia is injected into the lumbar area of the back , a long needle is inserted in between the bones of the spine (vertebrae) into the spinal canal. (The needle is most commonly placed between the third and fourth lumbar vertebrae.)
Spinal fluid pressure can then be measured and cerebrospinal fluid (CSF) removed for testing.
What is the cerebrospinal fluid (CSF)?
The CSF circulates around the brain and spinal cord (central nervous system). This "water bath" acts as a support of buoyancy for the brain and spinal cord. The support of the CSF helps to protect the brain from injury.
The normal CSF normally appears clear and contains various substances, such as protein and sugar (glucose), and few if any cells. The spinal fluid also has a normal pressure when first removed.
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Lumbar Puncture: Testing for Meningitis
A lumbar puncture is the most common method of obtaining a sample of the fluid in the spinal canal (the cerebrospinal fluid or CSF) for examination. The evaluation of the spinal fluid is usually necessary for the definitive diagnosis and to help make optimal treatment decisions (such as the appropriate choice of antibiotics).
The diagnosis is confirmed by abnormal spinal fluid results and, in the case of an infection, by identifying the organism causing the infection. In patients with meningitis, the CSF fluid often has a low glucose (sugar) level and increased white blood cell count.
What is normal CSF?
Normal values for spinal fluid examination are as follows:
- Protein (15-45 mg/dl)
- Glucose (50-75 mg/dl)
- Cell count (0-5 mononuclear cells)
- Initial pressure (70-180 mm)
These normal values can be altered by injury or disease of the brain, spinal cord, or adjacent tissues. The values are routinely evaluated during examination of the spinal fluid obtained from the lumbar puncture. Additionally, spinal fluid is tested for infection in the microbiology laboratory.
A health-care provider will usually review and interpret the test results with the patient. These results often will need to be evaluated in relation to other blood and radiologic test results.
What diseases are diagnosed by examination of the CSF?
Spinal fluid obtained from the lumbar puncture can be used to diagnose many important diseases such as bleeding around the brain; increased pressure from hydrocephalus; inflammation of the brain, spinal cord, or adjacent tissues (encephalitis, meningitis); tumors of the brain or spinal cord, etc. Sometimes spinal fluid can indicate diseases of the immune system, such as multiple sclerosis.
What are possible risks, complications, or side effects of a lumbar puncture?
When spinal fluid is removed during an LP, the risks include headache, brain herniation, bleeding, and infection. Each of these complications are uncommon with the exception of headache, which can appear from hours to up to a day after a lumbar puncture. Up to 25 % of patients will get headaches after the procedure. Headaches occur less frequently when the patient remains lying flat several hours after the procedure.
What are the benefits of a lumbar puncture?
The benefits of the LP depend upon the exact situation for which it is performed, but a lumbar puncture can provide lifesaving information.
Kasper, D.L., et al., eds. Harrison's Principles of Internal Medicine, 19th Ed. United States: McGraw-Hill Education, 2015.