Patients will be asked to either sit up or lie down on their side with back parallel to the bed and 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. 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, a local anesthetic cream to numb the skin, 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 surrounding the spinal cord and encasing the fluid). At this point, fluid will flow out of the needle and 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 This may be due to irritation of a nerve root and subsides when the needle is withdrawn. Following the procedure, the patient is advised to remain lying down for an hour or two to prevent low-pressure headache. The entire procedure takes approximately 20 minutes.
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