Performing the Lumbar Puncture (Part 2)

By palpating onto the iliac crest and spinal processes of the back, the site of needle puncture for lumbar puncture (LP) is identified. As the spinal cord ends at the level of L2 in adults, the spinal needle can be inserted safely between the L3/4 or L4/5 levels. The highest point of the iliac crest is a very useful landmark, as an imaginery line joining the two iliac crests corresponds to the level of the forth lumbar vertebral body.

In the video, after identifying the appropriate level, local anesthetic is injected into the subcutaneous tissues. Note that before injection of local anesthetic, the physician aspirates from the needle to ensure that he is not injecting local anesthetic directly into a blood vessel.

The spinal needle (either 20 or 22 gauge) is then advanced slowly, angling slightly towards the head as if aiming towards the umbilicus. The flat surface of the bevel of the needle should be positioned to face the patient’s sides, to allow the needle to spread rather than cut the dural sac. The needle is then advanced slowly, and the physician removes the stylet every now and then to check for cerebrospinal fluid (CSF) flow. Once the subarachnoid space is entered, one would be able to see CSF flowing from the spinal needle.

Once CSF begins to flow through the spinal needle, a manometer is then connected in order to measure the opening pressure. CSF is normally colorless and the normal opening pressure of adults ranges from 6-20cm H₂0. One can see in the video here that the patient’s opening pressure is 13cmH₂0.

After noting the CSF opening pressure, the physician disconnects the manometer from the spinal needle. The CSF is allowed to flow freely into a sterile CSF bottle. This can take some time and at least 5mls of CSF is collected and sent for further analysis (such as total cell count, CSF protein and glucose, microbiology studies, oligoclonal bands etc.).

Once the CSF has been collected, the stylet is replaced followed by removal of the spinal needle. Dressing is applied and the patient is advised for bed rest afterwards.