Spinal tap: Also known as a lumbar puncture or "LP", a spinal tap is a procedure whereby spinal fluid is removed from the spinal canal for the purpose of diagnostic testing. It is particularly helpful in the diagnosis of inflammatory diseases of the central nervous system, especially infections, such as meningitis. It can also provide clues to the diagnosis of stroke, spinal cord tumor and cancer in the central nervous system.
A lumbar puncture is so-called because the needle goes into the lumbar portion (the "small") of the back. Other names for a lumbar puncture (an LP) include spinal puncture, thecal puncture, and rachiocentesis.
An LP is most commonly done for diagnostic purposes, namely to obtain a sample of the fluid in the spinal canal (the cerebrospinal fluid) for examination.
An LP can also be done for therapeutic purposes, namely 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.
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.
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 but an LP can provide lifesaving information.