May 16, 2012

Spinal tap gives clues to brain and spinal cord health

Anthony L. Komaroff, M.D.

---- — DEAR DOCTOR K: My doctor wants me to have a spinal tap to help confirm a diagnosis. What happens during a spinal tap?

DEAR READER: A spinal tap is also called a lumbar puncture, or "LP" for short. During this procedure, fluid known as cerebrospinal or spinal fluid is removed from the space surrounding your spinal cord. The term "spinal tap" sounds scary, but it is only briefly painful and mostly risk-free.

Why do doctors perform spinal taps? The spinal fluid bathes the entire brain and spinal cord. Therefore, the fluid can give important clues about diseases that affect the brain or spinal cord.

If a person has an infection of the brain or spinal cord, signs of infection (such as the presence of white blood cells) can be seen in the spinal fluid. The fluid can help identify the specific germ causing an infection. It can distinguish a life-threatening bacterial infection (bacterial meningitis) from milder viral infections. If a person has a brain hemorrhage, red and white blood cells will be in the spinal fluid.

Before the procedure, tell your doctor if you have any allergies, especially to injected anesthetics similar to what your dentist uses to numb your mouth.

You will wear a hospital gown for the spinal tap. Typically, you lie on your side with your knees curled up against your chest. Or your doctor may ask you to sit up and lean forward on a table.

An area on your lower back will be cleaned with an antiseptic solution. Then your doctor will use a tiny needle to inject numbing medicine just below the skin. This will numb the area where the spinal needle will be inserted. This medicine causes very brief stinging, but then the area is numb when the larger spinal needle is inserted.

The doctor will insert the thin spinal needle into your lower back, slowly pushing the needle forward until it reaches the spinal canal.

At this point, fluid from the spinal canal will begin to drip out of the spinal needle into a sterile test tube. Usually less than 3 tablespoons of spinal fluid are collected.

Because of the numbing medicine, you should experience only a little pressure when the fluid is removed. Let your doctor know if you feel any pain.

Your doctor may also want to measure the pressure of your spinal fluid, using a tube held against the spinal needle.

Once the fluid is collected, your doctor will remove the needle. You probably won't need more than a Band-Aid to cover the area where the needle was inserted.

The whole lumbar puncture takes 30 to 45 minutes. The spinal needle will be in place for about one minute. You may be told to lie flat for a while after the test. Let your doctor know if you get a headache or feel otherwise unwell after the test.

A spinal tap can be lifesaving. And in the hands of a skilled and experienced doctor, it should go smoothly.

Dr. Komaroff is a physician and professor at Harvard Medical School. To send questions, go to, or write: Ask Doctor K, 10 Shattuck St., Second Floor, Boston, MA 02115.

Distributed by Universal Uclick for UFS.