Medical Q&As

Lumbar puncture - how's it done?

Can you explain what a lumbar puncture is and why is it performed?

A lumbar puncture or spinal tap is performed for the purpose of obtaining a sample of CSF or cerebrospinal fluid. The CSF is the fluid that bathes the brain and spinal cord and it can exhibit changes when diseases of the nervous system are present. For example in the case of bacterial meningitis it is possible to cultivate the bacteria causing the meningitis by culturing a sample of CSF in the microbiology laboratory. A further example would be the detection of a particular type of protein in the CSF in the case of multiple sclerosis. The lumbar region of the spine is the safest and least hazardous area from which to take a sample of CSF. Attempting to take the sample from a higher point in the spine could be extremely dangerous. At the commencement of the procedure you are positioned on your side with your knees drawn up and the chin drawn down onto the chest. This foetal-type position renders the spine convex and opens up the spaces between the vertebrae in the lower spine. This slightly uncomfortable position enables the doctor to gain easier access to the spinal canal. A local anaesthetic is then injected into the skin overlying the interspace between the 3rd and 4th lumbar vertebrae. Once the area is adequately anaesthetised a needle is passed through the skin into the spinal canal. Once the spinal canal has been entered the CSF pressure can be measured and some of the fluid can be extracted. The needle and syringe used for this procedure are similar to the type of needle and syringe used for taking a blood sample from the arm. The fluid sample is drawn into the syringe and the needle is then withdrawn and a dressing is applied to the skin. The person is usually required to lie flat for a number of hours after the test. If the person were to rise or sit up this could precipitate a severe headache due to the reduction in fluid volume in the spinal canal from the procedure. For the very same reason the procedure can result in some relief of the headache that is associated with meningitis.