What is a lumbar puncture (LP)?
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.
Why is a lumbar puncture done?
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).
How is the LP performed?
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).
What is done next?
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 (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.
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)
Lumbar Puncture
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.
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 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.
What are the benefits of an
LP?
The benefits of the LP depend upon the exact situation but an LP
can provide lifesaving information.
