The patient is asked to either sit up or lie down on his side with back parallel to the bed, chin tucked under, and 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 the L3-L4 vertebral level. Next, iodine is used to clean off the area and a sterile sheet is draped over the surrounding area. In infants and small children, a local anesthetic cream is used to numb the skin and is applied about 1 hour prior to the procedure. In adults, lidocaine is first injected right below the skin, and then deeper such that tissue beneath the skin will also be numbed. A thin, 20-gauge needle is then inserted into the interspace between the vertebrae (spine bones). 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). Fluid will then flow out of the needle to be collected in plastic containers. Under normal circumstances, the opening pressure will be measured with a manometer. About 20-30 cc of spinal fluid is collected and sent for evaluation under the microscope. In 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 This may be due to irritation of a nerve root and will go away when the needle is withdrawn. Following the procedure, the patient will be asked to remain lying down for an hour or two to prevent low-pressure headaches. The entire procedure takes approximately 20 minutes.
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