A lumbar puncture is used to diagnose meningitis, and is helpful in diagnosing some other brain and spinal cord disorders where inflammation or infection can occur.
Often, a lumbar puncture is feared as a terrible procedure. In most cases, this is not necessary, as the procedure is done using anesthesia to block feelings of pain, and the needle is fairly thin. Many patients will not have the bad experience that they anticipate.
What is a lumbar puncture?
A lumbar puncture (sometimes called a spinal tap) is a procedure to allow a sample of cerebrospinal fluid (CSF) to be taken for testing. CSF is the fluid that surrounds the brain and spinal cord. If a condition occurs somewhere in the nervous system, the CSF can be helpful even though the fluid is taken only from the lumbar spinal region.
How is a lumbar puncture performed?
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 in which a slower inflammatory or infectious cause is suspected.
Usually, you lie on a couch on your side with your knees pulled up against your chest, called a fetal position (see below picture). Sometimes it is done while you are sitting up and leaning forward on some pillows.
A lumbar puncture can be performed while lying on your side in the fetal position (left) or while sitting and leaning forward (right). If CSF pressure is needed, then the lying position is always used. The spinal needle passes through the spaces between the vertebral processes to enter the dural sac and measure the pressure before removing some CSF for testing.
Before the needle is even touched, you will have an area of your lower back cleaned with antiseptic. Then, injection of some local anaesthetic into a small area of skin lying over a space between two lower vertebrae (spinal bones) at L3 and L4 levels. The anesthetic can sting a little at first, but then makes the skin numb so little else can be felt afterwards.
There will then be a needle pushed through the skin and tissues between two vertebrae into the space around the spinal cord which is filled with CSF. At this level of the spine, the spinal cord has already ended, so there is no chance of touching the spinal cord. Sometimes, the nerve roots can be touched, which will be felt as a tingling or electrical sensation into the hip or leg briefly.
Because the skin is numb after local anaesthetic, most people do not feel pain, but it is still possible to 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 regardless of the amount of anesthetic given.
Once the dural sac is entered, some CSF leaks back through the needle and is collected in a sterile vial. 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. The pressure can be measured with a manometer, with a normal pressure between 5-20 cm of water.
The needle is usually in for about 2-10 minutes. As soon as the required amount of fluid is collected, the needle is taken out and a bandage is placed over the site of needle entry.
Are there any side-effects or risks from a lumbar puncture?
It is possible to develop a headache after the test. When present, this usually goes away after a few hours. However, it is best to lie down for a few hours after the test, as this makes a headache less likely to develop. This problem is more commonly in young females. If persistant, this can indicate a leak in the CSF which may require an easily performed procedure called a blood patch.
Other problems are rare - infection or bleeding of the site of the needle entry can rarely occur.
Any damage to the spinal cord or brain as a result of lumbar puncture is extremely rare.
Sometimes, despite anesthetic, pain is felt
Sometimes, a nerve root is touched, giving a quick tingling or electrical sensation into the hip or leg
Rarely, the lumbar puncture cannot obtain fluid as the level of the spinal sac ending is higher up.