Definition of Spinal Tap
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.
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. A
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.
Last Editorial Review: 10/13/1998 8:45:00 PM
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