Lab Tests Online - Explaining Pathology

Glossary

Lumbar puncture


Also known as: Spinal tap

The lumbar puncture is a special but relatively routine procedure. It is usually performed while you are lying on your side in a curled up fetal position, but may sometimes be performed in a sitting position. It is crucial that you remain still during the procedure. Once you are in the correct position, your back is cleaned with an antiseptic and a local anaesthetic is injected under the skin. When the area has become numb, a special needle is inserted through the skin, between two vertebrae, and into your spinal canal. It is gently advanced until it enters the subarachnoid space (located between the arachnoid and pia mater layers of the meninges) and cerebrospinal fluid (CSF) begins to flow. You may be asked to straighten out your legs at this point and relax your muscles. It is important not to move unless you are instructed to do so. An “opening” or initial pressure reading of the CSF is obtained. The doctor then collects a small amount of CSF in multiple sterile vials. A “closing” pressure is obtained, the needle is withdrawn, and a sterile dressing and pressure are applied to the puncture site. You will then be asked to lie quietly in a flat position, without lifting your head, for one or more hours to avoid a potential post-test spinal headache.

The lumbar puncture procedure usually takes less than half an hour. For most patients it is a moderately uncomfortable to somewhat painful procedure. The most common sensation is a feeling of pressure when the needle is introduced. Let your doctor know if you experience a headache or any abnormal sensations, such as pain, numbness, or tingling in your legs, or pain at the puncture site.

The lumbar puncture is performed low in the back, well below the end of the spinal cord - usually between lumbar (L) vertebrae L4 and L5. There are spinal nerves in the location sampled, but they have room to move away from the needle. There is the potential for the needle to contact a small vein on the way in. This can cause a “traumatic tap,” which just means that a small amount of blood may leak into one or more of the samples collected. While this is not ideal, it may occur a certain percentage of the time. The evaluation of your results will take this into account.

Blood from the collection procedure (spinal tap) may contaminate the first portion of CSF sample that is collected. However, there are usually three or more separate tubes used to collect CSF samples during one spinal tap procedure. The last tube that is collected during a spinal tap is least likely to have blood cells present due to the procedure and is usually the sample used to test for the presence of blood cells in the CSF. Likewise, the last sample collected is used for infectious disease testing since it will not be contaminated with microorganisms from inserting the needle through the skin.