http://www.health.harvard.edu/diagnostic-tests/lumbar-puncture.htm
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.
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.
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.
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.
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.
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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