A lumbar puncture is used to diagnose meningitis, and is helpful in diagnosing some other brain and spinal cord disorders.
What is a lumbar puncture?
A lumbar puncture (sometimes called a spinal tap) is a procedure where a sample of cerebrospinal fluid (CSF) is taken for testing. CSF is the fluid that surrounds the brain (cerebrum) and spinal cord. This test is mainly used to diagnose meningitis (an infection of the meninges - the structure that surrounds the brain and spinal cord). It is also used to help diagnose some other conditions of the brain and spinal cord.
How is it done?
In most cases a lumbar puncture is done as an emergency procedure to diagnose meningitis quickly. In some cases it is done as an outpatient for other reasons.
Usually, you lie on a couch on your side with your knees pulled up against your chest. Sometimes it is done with you sitting up and leaning forward on some pillows. The doctor will clean an area of your lower back with antiseptic. They will then inject some local anaesthetic into a small area of skin which lies over a space between two lower vertebrae (spinal bones). This stings a little at first, but then makes the skin numb.
The doctor then pushes a needle through the skin and tissues between two vertebrae into the space around the spinal cord which is filled with CSF. Because the skin is numbed with local anaesthetic, most people do not feel pain, but you may feel pressure as the needle is pushed in. However, some people do get a sharp feeling in the back or leg when the needle is pushed through.
Some fluid leaks back through the needle and is collected in a sterile pot. If you have possible meningitis, this is sent to the laboratory to be examined under the microscope to look for bacteria. It is also 'cultured' to see if any bacteria grow and what type they are. The fluid can also be tested for protein, sugar and other chemicals if necessary. Sometimes the doctor will also measure the pressure of the fluid. This is done by attaching a special tube to the needle, which can measure the pressure of the fluid coming out.
The needle is usually in for about 1-2 minutes. As soon as the required amount of fluid is collected, the needle is taken out and a sticking plaster put over the site of needle entry.
Are there any side-effects or risks from a lumbar puncture?
Some people develop a headache after the test. This usually goes after a few hours. It is best to lie down for a few hours after the test, as this makes a headache less likely to develop. Other problems are rare - for example, infection or bleeding of the site of the needle entry. Any damage to the spinal cord or brain as a result of lumbar puncture is rare.
Dr Tim Kenny
Dr Michelle Williams
Prof Cathy Jackson