Spinal fluid is a liquid that surrounds your brain and spinal cord. Spinal fluid helps cushion your brain if you hit your head or fall. Spinal fluid circulates freely around your brain and spinal cord. Eventually, the fluid enters your blood vessels.
Many brain problems cause changes in your spinal fluid. For example, a burst blood vessel in the brain causes red blood cells to appear in your spinal fluid.
A spinal tap is a test to remove a sample of your spinal fluid. It's sometimes called a lumbar puncture. Doctors do tests on the fluid to look for problems such as:
During a spinal tap, doctors numb your skin and then put a long, thin needle in your lower back to get the sample of spinal fluid. They take about a tablespoon (15 milliliters) of fluid. Your brain quickly makes more fluid.
Doctors test spinal fluid for many substances, including:
About 1 in 10 people get a headache after a spinal tap. This headache may last a few days to a few weeks.
For a spinal tap:
You lie curled up on your side, so your knees are toward your chest
Sometimes doctors have you sit up leaning forward
Your doctor numbs your back with a shot of numbing medicine
You lie still as your doctor puts a thin, hollow needle into your lower back, between your backbones
The needle goes into the area around the spinal cord to draw out a small amount of spinal fluid
A spinal tap takes about 15 minutes. Your doctor may have you lie down for a while afterward.
Some people (about 1 in 10) get a headache after having a spinal tap. This is called a low-pressure headache. It usually goes away in a few days or a few weeks. Doctors can treat it by injecting a small amount of blood into the area of the spinal tap. This usually eases the headache.
You don't have to worry that the needle will poke into your spinal cord. Doctors do the spinal tap just below the point where your spinal cord ends.