Test Overview

A lumbar puncture (also called a spinal tap) is a procedure to collect and look at the fluid (cerebrospinal fluid, or CSF) surrounding the brain and spinal cord.

During a lumbar puncture, a needle is carefully inserted into the spinal canal low in the back (lumbar area). Samples of CSF are collected. The samples are studied for color, blood cell counts, protein, glucose, and other substances. Some of the sample may be put into a special culture cup to see if any infection, such as bacteria or fungi, grows. The pressure of the CSF also is measured during the procedure.

Why It Is Done

A lumbar puncture is done to:

• Find a cause for symptoms possibly caused by an infection (such as meningitis), inflammation, cancer, or bleeding in the area around the brain or spinal cord (such as subarachnoid hemorrhage).
• Diagnose certain diseases of the brain and spinal cord, such as multiple sclerosis or Guillain-Barré syndrome.
• Measure the pressure of cerebrospinal fluid (CSF) in the space surrounding the spinal cord. If the pressure is high, it may be causing certain symptoms.

A lumbar puncture may also be done to:

• Put anesthetics or medicines into the CSF. Medicines may be injected to treat leukemia and other types of cancer of the central nervous system.
• Put a dye in the CSF that makes the spinal cord and fluid clearer on X-ray pictures (myelogram). This may be done to see whether a disc or a cancer is bulging into the spinal canal.

In rare cases, a lumbar puncture may be used to lower the pressure in the brain caused by too much CSF.

How To Prepare

Before you have a lumbar puncture, tell your doctor if you:

• Are taking any medicines. If you take medicines every day, ask your doctor whether you should take these medicines on the day of the lumbar puncture.
• Are allergic to any medicines, such as those used to numb the skin (anesthetics).
• Have had bleeding problems or take blood-thinners, such as aspirin, warfarin (Coumadin), or clopidogrel (Plavix).
• Are or might be pregnant.
• Take any herbal remedies. Some of these remedies may thin the blood.

You will empty your bladder before the procedure.

For a lumbar puncture, you will be asked to sign a consent form. Talk to your doctor about any concerns you have regarding the need for the procedure, its risks, how it will be done, or what the results will mean. This procedure is often done in an emergency situation. If you are scheduled to have this procedure, you can understand the importance of it by filling out the medical test information form(What is a PDF document?).

How It Is Done

A lumbar puncture may be done in your doctor's office, in an emergency room, or at your bedside in the hospital. It may also be done in the radiology department if fluoroscopy is used.

You will lie on a bed on your side with your knees drawn up toward your chest. Or you may sit on the edge of a chair or bed and lean forward over a table with your head and chest bent toward your knees. These positions help widen the spaces between the bones of the lower spine so that the needle can be inserted more easily. If fluoroscopy is used, you will lie on your stomach so the fluoroscopy machine can take pictures of your spine during the procedure. See a picture of a lumbar puncture.

Your doctor marks your lower back (lumbar area) with a pen where the puncture will occur. The area is cleaned with a special soap and draped with sterile towels. A numbing medicine (local anesthetic) is put in the skin.

Then a long, thin needle is put in the spinal canal. When the needle is in place, the solid central core of the needle (stylet) is removed. If the needle is in the right spot in the spinal canal, a small amount of cerebrospinal fluid (CSF) will drip from the end of the needle. If not, the stylet will be put back in and the needle will be moved in a little farther or at a different angle to get to the fluid. Your doctor may need to move to another area of your spine if it is hard to get to the spinal fluid.

When the needle is in the spinal canal, a device called a manometer is hooked to the needle to measure the pressure of the CSF. You may be asked to straighten your legs while you are lying down. Your doctor takes the pressure reading, called the opening pressure, and checks whether the fluid is clear, cloudy, or bloody. Several small samples of fluid are collected and sent to the lab for study.

A final pressure reading, called the closing pressure, may be taken after the fluid samples are done. The needle is taken out and the puncture site is cleaned and bandaged.

The entire procedure takes about 30 minutes.

To lower your chance of getting a headache following a lumbar puncture, you may be told to lie flat in bed or with your head slightly raised for 1 to 4 hours. Since your brain makes new CSF all the time and replaces it 2 to 3 times a day, the small amount of fluid that is removed will be quickly replaced. You may be told to drink extra fluids after the procedure to help prevent or to reduce the severity of a headache.

How It Feels

Some people find it uncomfortable to lie curled up on their side. The soap may feel cold on your back. You will probably feel a brief pinch or sting when the numbing medicine is given. You may feel a brief pain when the spinal needle is inserted or repositioned.

During the procedure, the needle may touch one of your spinal nerves and cause a tingling feeling, like a light electrical shock, running down one of your legs. The needle will not touch or damage the spinal cord.

Some people (10% to 25%) develop a headache after having a lumbar puncture. Of those who do get headaches, only about half report that they are severe. These headaches last 24 to 48 hours and go away on their own. Pain medicine does not help control the headache, but lying flat in bed for several hours after the procedure may help the headache.

You may feel tired and have a mild backache the day after the procedure. Some people have trouble sleeping for 1 to 2 days.

Risks

A lumbar puncture is generally a safe procedure. In some cases, a leak of cerebrospinal fluid (CSF) may develop after a lumbar puncture. Symptoms of this problem are a headache that does not go away after 1 to 2 days. A CSF leak can be treated with a blood "patch," in which the person's own blood is injected into the area where the leak is occurring in order to seal the leak.

About 1 in 1,000 people who have a lumbar puncture have a minor nerve injury. This heals on its own with time. There is also a small chance of infection of the CSF (meningitis), bleeding inside the spinal canal, or damage to the cartilage between the vertebrae. Your doctor will talk with you about these risks.

People who have bleeding problems and those who are taking blood-thinning medicine (such as warfarin or heparin) have a higher chance of bleeding after the procedure. A lumbar puncture may not be done unless it is needed for a life-threatening illness.

A lumbar puncture may cause serious problems for people who have high pressure in the brain caused by a tumor, a pocket of infection in the brain (abscess), or major bleeding inside the brain. Your doctor will check your nervous system, spinal cord and brain before doing a lumbar puncture. In some cases, a computed tomography (CT) scan or magnetic resonance imaging (MRI) scan may be done before the lumbar puncture to know that it is safe to do the puncture.

After the procedure

Call your doctor immediately if you have:

• Chills or a fever.
• A stiff neck. This may be a sign of a developing infection.
• Any drainage or bleeding from the puncture site.
• A severe headache.
• Any numbness or loss of strength below the puncture site.

Results

A lumbar puncture (also called a spinal tap) is a procedure to collect and look at the fluid (cerebrospinal fluid, or CSF) surrounding the brain and spinal cord. Many different tests can be done on the CSF. Some results will be ready right away, some will take a few hours after the procedure, and others will take several weeks.

The normal values listed here—called a reference range—are just a guide. These ranges vary from lab to lab, and your lab may have a different range for what's normal. Your lab report should contain the range your lab uses. Also, your doctor will evaluate your results based on your health and other factors. This means that a value that falls outside the normal values listed here may still be normal for you or your lab.

Your doctor may order other special tests on the CSF fluid depending on your symptoms and past health.

What Affects the Test

Reasons you may not be able to have the biopsy or why the results may not be helpful include:

• Not being able to lie still during the procedure.
• Having conditions such as obesity, dehydration, spinal disease, severe arthritis, or recent spinal surgery. These conditions may make it hard to do a lumbar puncture with the person lying down. The procedure may need to be done with the person sitting up and bent forward. In some cases, the procedure may need to be done with a special X-ray method called fluoroscopy. This is used to guide the placement of the spinal needle.
• Bleeding into the cerebrospinal fluid (CSF). The needle puncture can cause bleeding into the CSF. Several samples of the fluid can show the difference between bleeding from the puncture and bleeding caused by a brain hemorrhage.
• Not being able to collect a sample of the fluid. This is called a "dry tap."

What To Think About

• A lumbar puncture is not done if the person has:
  • A suspected tumor, swelling, or increased pressure in the brain. A computed tomography (CT) scan or magnetic resonance imaging (MRI) scan may be done first to see if it is safe to do the lumbar puncture.
  • A skin infection on the lower back. Doing a lumbar puncture in this case may allow the infection to spread into the spinal canal.
  • Bleeding disorders, which may increase the chance of bleeding during the procedure.
• Other tests that may be done include:
  • A cisternal or ventricular puncture. Cerebrospinal fluid (CSF) can be collected from the upper spinal canal at the base of the skull (cisternal puncture) or from within the skull (ventricular puncture) when it is not possible to get CSF from the lumbar area. These procedures are done by a trained radiologist or neurosurgeon. Special X-ray methods are used to guide the placement of the needle.
  • Bacterial marker studies. In cases of suspected bacterial meningitis, bacterial marker (bacterial antigen) studies can quickly find the common types of bacteria that cause meningitis. Antibiotic treatment for that bacteria can be started immediately.

References

Credits