Lumbar Puncture
(or Spinal Tap)
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What is the
test?
A lumbar puncture, also known as a spinal tap,
uses a needle to remove a sample of fluid from
the space surrounding the spinal cord. This fluid
is known as cerebrospinal fluid (CSF). The test
is used to diagnose meningitis infections and
some neurological conditions.
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How
do I prepare for the test?
You will need to sign a consent form, which
is generally required when the procedure is done
outside of an emergency situation. Tell your
doctor ahead of time if you have ever had an
allergic reaction to lidocaine or the numbing
medicine used at the dentist's office.
Doctors routinely do a physical examination
and in some cases order a brain scan before recommending
a lumbar puncture, to make sure you do not have
a medical condition that could put you at risk
for movement of the brain during the procedure,
a very rare but serious complication.
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What
happens when the test is performed?
Most patients wear a hospital gown. Typically,
you lie on your side with your knees curled up
against your chest. In some cases, the doctor
asks you to sit on the bed or a table instead,
leaning forward against some pillows.
The doctor feels your back to locate your lower
vertebrae and feels the bones in the back of
your pelvis. An area on your lower back is cleaned
with soap. Medicine is injected through a small
needle to numb the skin and the tissue underneath
the skin in the area from which the sample is
to be removed. This causes some very brief stinging.
A different needle is then placed in the same
area and moved forward until fluid can be obtained
through it from the spinal canal. Because the
needle must be placed through a small opening
between two bones, the doctor must sometimes
move the needle in and out several times to locate
the opening. Because of the numbing medicine
used in this area, most patients experience only
a sense of pressure from this movement. Occasionally
some patients do get a sharp feeling in the back
or (rarely) in the leg. Let your doctor know
if you feel any pain.
Sometimes the doctor measures the pressure of
the fluid before taking a sample. The pressure
is measured with a tube that looks like a large
thermometer held against the needle. The fluid
sample collected is usually less than three tablespoons.
You will not feel any discomfort when it is removed.
After this, the needle is taken out. Usually
a Band-Aid is the only dressing necessary.
The whole lumbar puncture, including set-up
time, takes 30–45 minutes. The needle is
in place for close to one minute.
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What
risks are there from the test?
The most common risk of a lumbar puncture is
that it can cause a temporary headache. Lying
down for a few hours after the test can make
a headache less likely to occur. Other problems
are rare and include infection or bleeding. Because
the volume of fluid is small, a lumbar puncture
almost never causes movement of the brain or
spinal cord, a serious complication.
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Must
I do anything special after the test is over?
You may be told to lie flat for a while after
the test, sometimes for a few hours.
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How
long is it before the result of the test is
known?
Depending on the tests being done on the fluid
sample, results take anywhere from a few hours
to a few days.
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