The Lumbar Puncture

A Lumbar Puncture, also known as a spinal tap or by the initials LP, is a procedure used to collect cerebrospinal fluid from the spine. 

For the procedure, a spinal needle is inserted into the lower part of the spinal column, usually between the 3rd and 4th or 4th and 5th lumbar vertebrae in the lower spine. 

In order to properly place the needle, it is extremely important to get the patient in the proper position.  The spine must be curved in order to allow as much space as possible between the lower vertebrae for the doctor to insert the needle.  Generally, the patient is laid on their side with their knees bent.  The head is bowed forward with the chin down so that the neck is bent. 

The area around the lower back is then disinfected and prepared using an antiseptic solution, in order to prevent infection.  The location of the LP is then determined and a local anesthetic is given under the skin and then injected along the intended patch of the spinal needle in order to minimize any pain.

The doctor then inserts the spinal needle, which is thin and hollow, into the space between the two vertebrae in the lower back and slowly advances it towards the spine.  Once the needle reaches the spinal canal, a steady flow of CSF will begin to fill the needle.

At this point, the doctor can measure the pressures in the ventricles in the brain and draw fluid for laboratory analysis if needed.

In Nathan's case, the doctor was going to withdraw enough fluid to "kick start" the ETV into working again.  By withdrawing the fluid from near the base of the spine, the intent of the LP was to "pull" the fluid down from the ventricles, thereby "forcing" the hole open and the CSF to go through it.