Patients will be asked to either sit up or lie down on their sides with the back parallel to the bed and the chin tucked under and the legs curled in the fetal position. The healthcare provider will use the bony protrusions of the hip to determine the most appropriate location to remove fluid. This is usually at what is known as the L3-L4 vertebral level.

Subsequently iodine will be used to clean off the area and a sterile sheet will be draped over the surrounding area. In infants and small children, EMLA cream, a local anesthetic cream will be used to numb the skin and will be applied about 1 hour prior to the procedure. In adults, lidocaine will be injected under the skin initially, and then deeper such that tissue beneath the skin will also be numbed.

A thin 20-gauge needle will then be inserted into the interspace between the vertebra. Depending on how much subcutaneous tissue there is, this may require shallow or deeper penetration of the needle. There may be an audible "pop" when the needle penetrates the dura (membrane that surrounds the spinal cord and encases the fluid). At that point there will be fluid flowing out of the needle and this will be collected in plastic containers. Under normal circumstances, the opening pressure will be measured with a manometer and then about 20-30 cc of spinal fluid will be collected and sent for evaluation under the microscope.

Under experienced hands there should not be any pain. There may be a feeling of pressure when the needle is inserted but with appropriate anesthetic, this should not hurt. Occasionally, some people may feel numbness shooting down the leg. This may be due to irritation of a nerve root.

The needle is withdrawn, the sensation subsides and is not permanent. Following the procedure, the patient is advised to remain lying down for an hour or two to prevent low-pressure headaches. The entire procedure takes approximately 20 minutes.

• Risks of lumbar puncture include:
  • hypersensitivity (allergic) reaction to the anesthetic
  • discomfort during the test
  • headache after the test
  • bleeding into the spinal canal
• Brain herniation (if performed on a person with increased intracranial pressure), and resulting in brain damage and/or death. This test is not performed in patients with CT/MRI or clinical findings suggestive of increased intracranial pressure.
• Damage to the spinal cord does not occur because the needle is inserted below the level that the spinal cord ends. The needle may irritate a nerve root which results in transient numbness or tingling in the leg. This goes away when the needle is re-adjusted.