A lumbar puncture (an LP) is the insertion of a needle into the fluid within the spinal canal. It is termed a "lumbar puncture" because the needle goes into the lumbar portion (the "small") of the back.
Other names for a lumbar puncture (an LP) include spinal tap, spinal puncture, thecal puncture, and rachiocentesis.
An LP is most commonly peformed to diagnose a disease, namely to obtain a sample of the fluid in the spinal canal (the cerebrospinal fluid) for examination.
An LP can also be done to treat diseases. For example, 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, for examples, normal-pressure hydrocephalus and benign intracranial hypertension).
The patient is typically lying down sideways for the procedure. Less often, the procedure is performed while the patient is sitting up. LPs in infants are often done upright.
After local anesthesia is injected into the small of the back (the lumbar area), a needle is inserted in between the nearby bony building blocks (vertebrae) into the spinal canal. (The needle is usually placed between the 3rd and 4th lumbar vertebrae).
Spinal fluid pressure can then be measured and cerebrospinal fluid (CSF) removed for testing.
The CSF circulates around the brain and spinal cord (the 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 contains various chemicals, such as protein and sugar (glucose), and few if any cells. The spinal fluid also has a normal pressure when first removed.
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.
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 brain or spinal cord, etc. Sometimes spinal fluid can indicate diseases of the immune system, such as multiple sclerosis.
What are the risks of an LP? 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 LP. Headaches occur less frequently when the patient remains lying flat 1-3 hours after the procedure.
The benefits of the LP depend upon the exact situation for which it is performed, but an LP can provide lifesaving information.
Harrison's Principles of Internal Medicine, McGraw-Hill, edited by Eugene Braunwald, et. al., 2001.