Lumbar Puncture (Spinal Tap)

A lumbar puncture (LP), often called a spinal tap, is a procedure in which a small amount of the fluid that surrounds the brain and spinal cord, called the cerebrospinal fluid (CSF), is removed and examined.

In infants and children, a lumbar puncture is typically done to look for meningitis, an infection of the meninges (the membrane covering the brain and spinal cord). Lumbar punctures also might be done to remove fluid and relieve pressure with certain types of headaches, to look for other diseases in the central nervous system, or to place chemotherapy medications into the spinal fluid.

What Happens During a Lumbar Puncture

Steps are taken to make sure that a child doesn't feel pain during the lumbar puncture. For a nonemergency lumbar puncture, a nurse will rub a topical anesthesia cream on the lower back about 30 minutes to 1 hour before the actual procedure. When the needle goes into the skin, the child won't feel sharp pain, only perhaps some pressure. After the skin is numbed, some doctors also inject liquid anesthesia such as lidocaine into the tissues right under the skin to prevent any further pain.

After the preparation, the child is positioned on an exam table so that the spaces between the vertebrae (bones of the spine) are as wide as possible. Infants and small kids lie on their sides curled up with their knees under their chin, like the letter C. (Sometimes nurses or aides have to hold kids in the position to get the best test results.) Teens may lie down or sit with their heads resting on a pillow placed on a table at waist level.

Next, the doctor places a small needle through the skin and then forward through the space between the vertebrae in the lower back until it enters the space that contains the spinal fluid. The spinal fluid drips out through the needle into tubes, is collected, and sent to a lab for analysis. The pressure in the fluid surrounding the brain and spinal cord also can be measured, which can provide important information in some cases. After the sample is collected (which usually takes several minutes), the needle is withdrawn and a bandage is placed on the site.

It is important to note that the needle does not enter the spinal cord or nerves because the test is done in the lower back, below the level to which the spinal cord extends.

Understanding the Results

Fluid collected from a lumbar puncture is immediately sent to the lab and analyzed for evidence of an infection. Some results are available in 30 to 60 minutes. However, bacterial culture is necessary to watch for germ growth. Those results are usually available in 48 hours. In the meantime, if an infection is suspected the doctor will start antibiotic treatment and then make any necessary adjustments once the culture results are in.

The lab technicians look for a number of things when examining the spinal fluid sample, including:

General appearance: CSF is usually clear and colorless and looks like water. Cloudy spinal fluid may indicate infection because of increased cells and proteins suspended in the fluid.
Cell count: This includes the number and type of white blood cells and the number of red blood cells present. CSF normally does not contain more than a few of these types of cells. The presence of too many white cells indicates an infection.
Protein: Large amounts of protein in the spinal fluid also suggest an infection or other disease.
Glucose: In bacterial infections of the spinal fluid, the glucose level of the fluid is often low.
Gram stain and culture: In a gram stain, dye is added to the CSF sample, which is then examined under the microscope for bacteria. To confirm an infection, the CSF is also cultured for organism growth.

CSF may also be processed for other tests for which results are not available immediately.

Complications From Spinal Taps

Most spinal taps are done without complications. In rare instances, complications can include:

Headache: This can occur because of continued leaking at the site where the fluid was taken. If this occurs, you may want to give your child acetaminophen or another non-aspirin type of pain reliever. If the headache persists for more than 2 days, call the doctor.
Infection: In rare cases, infection can occur if bacteria enter the skin during the puncture. This is very rare, because doctors always use sterile techniques to perform the test.
Bleeding: If a small blood vessel under the skin is nicked, there may be some bleeding.

Again, these complications are uncommon. If you have any questions or concerns regarding lumbar punctures, talk to your doctor.

Reviewed by: Steven Dowshen, MD
Date reviewed: November 2007