During a lumbar puncture (also known as a spinal tap), a small amount of cerebrospinal fluid is extracted from the spinal canal using a special needle. This fluid surrounds the brain and the spinal cord, acting as a shock absorber. After being extracted, the cerebrospinal fluid is analyzed in a laboratory. Doing so can help detect diseases of the brain and spinal cord.

Lumbar punctures can be helpful when trying to confirm or rule out conditions like the following:

• Meningitis (inflammation of the membranes covering the brain and spinal cord)
• Encephalitis (inflammation of the brain tissue itself)
• Bleeding in the brain
• Dementia
• Myelitis (inflammation of the spinal cord)
• Leukemia (cancer of the blood)
• Autoimmune diseases such as multiple sclerosis

Lumbar punctures can also be used for treatments: For instance, drugs such as local anesthetics, antibiotics, or cancer medications can be injected through the lumbar puncture needle. This allows the drug to enter the nervous system directly, instead of through the bloodstream. When local anesthetics are given in this way it is called spinal anesthesia.

Before doing a lumbar puncture, your doctor will check whether your blood clots normally. This is important in order to prevent bleeding that could damage the nerves where the needle is inserted. If you take medication that affects blood clotting, you should tell your doctor.

Sometimes the doctor will check your intracranial (brain) pressure to make sure it isn't too high. This is usually done by examining the ocular fundus (the back of the eye), or – less commonly – by x-raying the brain (cranial computed tomography, CCT).

Unless it's an emergency, you should be informed about the purpose of the lumbar puncture and any associated risks at least one day beforehand.

Lumbar punctures are usually done in hospitals, but nowadays some neurologists perform them as outpatient procedures in their practices. The doctor inserts a thin, hollow needle into the lower part of the lumbar spine, usually between the 3rd and 4th or the 4th and 5th lumbar vertebrae. The spinous processes of these vertebrae in the lower spine can easily be felt through the skin. To make enough room for the needle, the vertebrae have to be spread apart as much as possible, so the person is asked to bend forward. The easiest way to do this is by arching your back forwards while sitting down or lying sideways.

The skin around the site of insertion is disinfected and numbed using a local anesthetic. After inserting the needle between two vertebrae, the doctor pushes it about three to four centimeters towards the spinal cord. The cerebrospinal fluid then automatically trickles through the hollow needle into a sterile container. About 10 to 15 milliliters of the fluid are typically collected. The needle is then carefully removed and the site of insertion is covered with a bandage, using a bit of pressure to help the wound heal faster. A lumbar puncture takes about 15 minutes in total.

Afterwards it's important to lie down for at least an hour, take it easy for about 24 hours and drink lots of fluids. After several hours your doctor will examine the site of insertion and check whether you can move your legs. This is done in case any bleeding in the spinal canal is pressing against the nerves. After a lumbar puncture, you will usually stay in the hospital or practice for at least one hour, but more typically for up to four hours, depending on how much cerebrospinal fluid was extracted and whether the puncture was used for a treatment.

Lumbar puncture in the lumbar (lower back) region of the spine

The color of the cerebrospinal fluid (CSF) and its individual components are examined. CSF is normally as clear as water. Blood-colored or cloudy CSF could be a sign of bleeding or an inflammation in the brain. Any changes in the number of cells in the CSF or in the mix of different substances (e.g. proteins, glucose and lactate) are checked for in a laboratory. The CSF is also tested for bacteria that can cause meningitis, or for proteins that may be a sign of certain types of dementia.

Serious complications of this procedure are rare. The spinal canal in the lower part of the lumbar spine only contains fluid because the spinal cord ends further up. This means the spinal cord can't be damaged in the lumbar spine area.

You may feel some pain for a short while when the needle is being inserted, or if the needle touches a nerve root deeper inside the tissue. The pain will then shoot down into a leg, but immediately go away again.

A few hours or even days after the lumbar puncture, you may experience headaches, nausea, a fast heart rate, or low blood pressure. This is known as a "post-lumbar puncture headache." But these delayed effects usually go away on their own after about five days. The headaches typically get a lot better when you lie down.

In very rare cases, inflammations, bleeding or other complications that need to be treated in a hospital may arise after a lumbar puncture.

Lumbar punctures should not be performed if the skin at the site of insertion is inflamed, if the intracranial pressure is too high, or if there is an increased risk of bleeding.