A spinal tap, also called a lumbar puncture, removes a tiny amount of cerebrospinal fluid (CSF) for laboratory analysis. CSF is the clear, watery fluid that surrounds the brain and spinal cord. Although a spinal tap is not generally painful, it can cause discomfort.

CSF forms in the areas of the brain called ventricles and flows down into the area around the spinal cord. It is usually clear and contains small amounts of proteins and glucose (blood sugar). Its main function is to cushion and protect the brain and spinal cord by maintaining uniform pressure within the skull and spine.

Spinal taps in children are most often performed to diagnose meningitis, an infection and inflammation of the membranes (meninges) covering the brain and spinal cord. Children who have seizures caused by a high fever (febrile seizures) usually receive a spinal tap to look for signs of meningitis.

However, a spinal tap should be performed only after a neurological examination and should never delay potentially lifesaving treatment such as the administration of antibiotics and steroids to patients with suspected bacterial meningitis. 

In some cases, a spinal tap may also be used to determine whether a child’s ongoing headaches are caused by spinal fluid pressure that is too high or too low.

Spinal taps also may be used to diagnose various cancers of the brain and spinal cord. For example, this test can help detect tumors of the choroid plexus, which is the tissue located in the spaces of the brain called ventricles. Such tumors are rare, but make up a significant percentage of brain tumors that affect children during the first year of life. The choroid plexus produces CSF, and a spinal tap can help determine whether cancer cells have started to spread to other parts of the body via this fluid.

A spinal tap can also be used to detect other cancers that often affect children, such as leukemia (cancer of the blood and bone marrow) and retinoblastoma (tumor of the retina). By analyzing CSF, it can be determined whether cancer has spread to the brain and spine.

In some cases, spinal taps are actually used to treat a condition rather than to just diagnose it. For example, a spinal tap can be used to remove excess CSF within the skull. This is a condition that affects some babies and is known as hydrocephalus, or “water on the brain.”

Other conditions that may be diagnosed or treated with a spinal tap include:

• Viral or bacterial infections of the brain and spinal cord. In addition to meningitis, such conditions include Lyme disease, polio and neurosyphilis.
  
  
• Bleeding around the brain (subarachnoid hemorrhage)and in the central nervous system (CNS).
  
  
• CNS disorders characterized by tissue destruction, such as multiple sclerosis (autoimmune disease that often results in disability and impaired function) or nerve diseases, such as Guillain-Barre syndrome (a rare inflammatory disorder of the peripheral nerves).
• 
  
• An obstruction in CSF circulation.

Spinal taps are used to administer spinal anesthetic for some kinds of surgery. They are also sometimes used to administer chemotherapy medications designed to treat or prevent brain and central nervous system cancers. In this procedure, known as intrathecal chemotherapy, the drug is injected directly into the CSF and flows freely to the brain and spinal cord.