by Dr. C.H. Weaver M.D. updated 4/2019
A lumbar puncture, also known as a spinal tap, is a medical procedure in which a needle is inserted into the spinal canal, most commonly to collect cerebrospinal fluid for diagnostic testing. The main reason for a lumbar puncture is to help diagnose diseases of the central nervous system, including the brain and spine.
A lumbar puncture can help diagnose serious infections, such as meningitis or cancer involving the brain or spinal cord. Sometimes doctors use lumbar punctures to inject chemotherapy drugs into the cerebrospinal fluid to treat leukemia.
Why is a lumbar puncture done?
A lumbar puncture may be done to:
- Collect cerebrospinal fluid for laboratory analysis
- Measure the pressure of your cerebrospinal fluid
- Inject spinal anesthetics, chemotherapy drugs or other medications
- Inject dye (myelography) or radioactive substances (cisternography) into cerebrospinal fluid to make diagnostic images of the fluid's flow
Information gathered from a lumbar puncture can help diagnose:
- Serious bacterial, fungal and viral infections, including meningitis, encephalitis and syphilis
- Bleeding around the brain (subarachnoid hemorrhage)
- Certain cancers involving the brain or spinal cord
- Certain inflammatory conditions of the nervous system, such as multiple sclerosis and Guillain-Barre syndrome
What are the risks of a lumbar puncture?
Though lumbar punctures are generally recognized as safe, they do carry some risks. These include:
- Post-lumbar puncture headache. Up to 25 percent of people who have undergone a lumbar puncture develop a headache afterward due to a leak of fluid into nearby tissues.
The headache typically starts several hours up to two days after the procedure and may be accompanied by nausea, vomiting and dizziness. The headaches are usually present when sitting or standing and resolve after lying down. Post-lumbar puncture headaches can last from a few hours to a week or more.
- Back discomfort or pain. You may feel pain or tenderness in your lower back after the procedure. The pain might radiate down the back of your legs.
- Bleeding. Bleeding may occur near the puncture site or, rarely, into the epidural space.
- Brainstem herniation. Increased pressure within the skull (intracranial), due to a brain tumor or other space-occupying lesion, can lead to compression of the brainstem after a sample of cerebrospinal fluid is removed.
A computerized tomography (CT) scan or MRI prior to a lumbar puncture can be obtained to determine if there is evidence of a space-occupying lesion that results in increased intracranial pressure. This complication is rare.
What can you expect during a lumbar puncture?
A lumbar puncture is usually done in an outpatient facility or a hospital. Your doctor will talk to you about the potential risks, and any discomfort you might feel during the procedure.
Before the procedure
You're asked to change into a hospital gown. There are a few possible positions for this test. Usually, you lie on your side with your knees drawn up to your chest, or you sit and lean forward on a stable surface. These positions flex your back, widening the spaces between your vertebrae and making it easier for your doctor to insert the needle.
For an infant or young child, someone will hold the child in position during the procedure.
Your back is washed with antiseptic soap or iodine and covered with a sterile sheet.
During the procedure
- A local anesthetic is injected into your lower back to numb the puncture site before the needle is inserted. The local anesthetic will sting briefly as it's injected.
- A thin, hollow needle is inserted between the two lower vertebrae (lumbar region), through the spinal membrane (dura) and into the spinal canal. You may feel pressure in your back during this part of the procedure.
- Once the needle is in place, you may be asked to change your position slightly.
- The cerebrospinal fluid pressure is measured, a small amount of fluid is withdrawn and the pressure is measured again. If needed, a drug or substance is injected.
- The needle is removed, and the puncture site is covered with a bandage.
The procedure usually lasts about 45 minutes. Your doctor may suggest lying down after the procedure.
Sometimes, an ultrasound may be used as a guide during the procedure on infants and young children. The ultrasound can help prevent inserting the needle too far.
After the procedure
- Plan to rest. Don't participate in strenuous activities the day of your procedure. You may return to work if your job doesn't require you to be physically active. Discuss your activities with your doctor if you have questions.
- Take a pain medication. A nonprescription pain-relieving medication that contains acetaminophen can help reduce a headache or back pain.
Is a lumbar puncture safe with low platelet counts?
Results from a clinical study published in the Journal of the American Medical Association indicate that lumbar puncture, a procedure through which chemotherapy is administered directly into spinal fluid appears to be safe for children with acute lymphoblastic leukemia (ALL) who also have low platelet levels (thrombocytopenia).
Since low platelet levels may be associated with severe bleeding, there has been concern regarding the effects of thrombocytopenia on the safety of repeated lumbar punctures in patients with ALL. Patients with ALL may experience thrombocytopenia from the leukemia itself, or from treatments they receive for their disease.
Physicians from St. Jude’s Children’s Research Hospital recently evaluated the records of almost 1,000 children with ALL who also had thrombocytopenia. They attempted to determine if an association existed between thrombocytopenia and complications with a lumbar puncture. Over 5,000 lumbar punctures were performed on patients who had platelet levels as low as 10,000 (normal levels are over 100,000). There were no reports of any serious complications due to the procedure.
These physicians concluded that lumbar punctures were safe for children with ALL and thrombocytopenia. Children with platelet counts under 10,000 were not adequately evaluated in this study and may benefit from platelet transfusions prior to lumbar punctures. The importance of this study is to alleviate fears of parents concerning the risks of severe bleeding due to performing a lumbar puncture in children with thrombocytopenia. The risks of withholding appropriate treatment far outweigh the risks of the procedure.
Parents who have children with ALL may wish to speak with their doctor about the risks and benefits of a lumbar puncture or the participation in a clinical trial further evaluating this procedure or other promising treatments.
What tests results come from a lumbar puncture?
The spinal fluid samples are sent to a laboratory for analysis. Lab technicians check for a number of things when examining spinal fluid, including:
- General appearance. Spinal fluid is normally clear and colorless. If it's cloudy, yellow or pink in color, it might indicate abnormal bleeding. Spinal fluid that is green might indicate an infection or the presence of bilirubin.
- Protein (total protein and the presence of certain proteins). Elevated levels of total protein — greater than 45 milligrams per deciliter (mg/dL) — may indicate an infection or another inflammatory condition. Specific lab values may vary from medical facility to medical facility.
- White blood cells. Spinal fluid normally contains up to 5 mononuclear leukocytes (white blood cells) per microliter. Increased numbers may indicate an infection. Specific lab values may vary from medical facility to medical facility.
- Sugar (glucose). A low glucose level in spinal fluid may indicate infection or another condition.
- Microorganisms. The presence of bacteria, viruses, fungi or other microorganisms can indicate an infection.
- Cancer cells. The presence of abnormal cells in spinal fluid — such as tumor or immature blood cells — can indicate certain types of cancer.
Lab results are combined with information obtained during the test, such as spinal fluid pressure, to help establish a possible diagnosis.
Your health care provider generally gives you the results within a few days, but it could take longer. Ask your doctor when he or she expects to receive the results of your test.
Write down questions that you want to ask your doctor. Don't hesitate to ask questions during your visit. Questions you may want to ask include:
- Based on the results, what are my next steps?
- What kind of follow-up, if any, should I expect?
- Are there any factors that might have affected the results of this test and, therefore, may have altered the results?
- Will I need to repeat the test at some point?
Journal of the American Medical Association, Vol 284, No 17, pp 2222-2224, 2000)