A spinal tap is usually performed in a hospital, although it can be performed in an outpatient surgical center in some cases. Prior to the procedure, parents of the child will be asked to sign a consent form. Parents are also urged to explain to their child that it may be necessary to lie flat for six to eight hours following the procedure. This helps to prevent headaches that sometimes occur following a spinal tap. In some cases, the child will be sedated to help them relax and lie still for this long time period.

Parents should follow the physician’s advice about any other preparatory steps that the child should take before a spinal tap. These may include dietary restrictions and modifications to a child’s regimen for taking over-the-counter and prescription medications. In rare cases, a spinal tap may be performed on a child using general anesthesia. This means that the child will not be conscious during the procedure. If this type of procedure is planned, the child will not be allowed to eat or drink anything from the night before the test until after the procedure is over.

Just prior to the procedure, children are asked to remove all clothing, jewelry and glasses and to wear a hospital gown. Children are then positioned with their back to the physician and rounded out, so that the spaces between the vertebrae are as wide as possible. The optimal position will vary depending on the patient’s age. For example:

• Infants and small children. Lie on their sides curled up in a position that resembles the letter “C,” with their knees under their chin.
  
  
• Teens and adults. Lie on their sides with their knees drawn up to their chest and chin resting on their chest. 

Pillows will be positioned under the patient to support the spine. To help maintain this position, children will be instructed to place one arm around their knees and the other around their neck. Although this position may be uncomfortable, it is vital that children remain curled (to open up the space between the vertebrae) and still (to avoid injuring the spinal cord) during the procedure. Children should also take care to breathe normally, because hyperventilation can skew pressure readings.

The area is first cleaned with an antibacterial solution and alcohol, and draped with sterile cloths to prevent any infection from entering the spine. A local anesthetic is used to numb the area, which may cause an initial burning or stinging feeling. A small, hollow needle is inserted near the waistline (below the spinal cord) between the third and fourth lumbar vertebrae and into the spinal canal (arachnoid space). 

The needle is pushed forward gently until the cerebrospinal fluid (CSF) is found. The needle may require some repositioning to locate the spinal canal. The child will feel pressure as the needle is inserted and some brief pain as the needle penetrates the meninges – the three membranes that cover the spinal cord and brain. This pain should dissipate after a few seconds and the overall discomfort of this procedure is mild to moderate.

The physician inserts a device called a manometer into the lumbar-puncture needle to measure the pressure of the CSF within the spinal canal. Two pressure readings are taken, one just after the initial puncture and another after the CSF has been withdrawn. One to two teaspoonfuls of fluid are then drawn into several sterile collection tubes and the needle is withdrawn. A bandage is placed on the injection site. The procedure usually takes at least 30 minutes.

Variations on the standard spinal tap procedure include the following:

• A sitting position may be used as an alternative, although it is rarely employed because the position is hard for patients to maintain, and movement could lead to complication or injury from the spinal tap. In the sitting position, children sit straight and then bend their head and chest toward their knees.
  
  
• Infants may be held upright for a spinal tap.
  
  
• For children with lower back problems who cannot use either the curled or sitting position, the needle may be inserted below the occipital bone at the back of the skull (cisternal puncture).

In extremely rare cases where the physician suspects an impending brain herniation (a condition that causes the brain to push through the opening at the back of the skull where the spinal cord is attached), the physician will drill a hole into the skull and insert a needle directly into the brain. This is called a ventricular puncture.