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Lumbar puncture (spinal tap)

What is a lumbar puncture?

A lumbar puncture, also called a spinal tap, is a procedure in which the fluid surrounding the spinal cord (called the cerebrospinal fluid or CSF) is withdrawn through a needle and examined in a laboratory.

Why is the lumbar puncture procedure performed?

This procedure may be performed to:

  • Obtain CSF for testing.
  • Measure the pressure around the brain and spinal cord.
  • Relieve pressure inside the head.
  • Give spinal anaesthesia.
  • Inject dye for an X-ray diagnostic test.
  • Inject medication.

Testing the CSF can help in the diagnosis of disorders of the central nervous system that may involve the brain, spinal cord or their coverings ( meninges). The CSF contains glucose ( sugar), proteins and other substances found in the blood. When the CSF is examined in the laboratory, the following are evaluated:

  • The number and types of white blood cells.
  • The level of glucose.
  • The types and levels of proteins.
  • The presence of bacteria, fungi or abnormal cells.

Testing of the CSF may not be necessary if the lumbar puncture procedure is done only to inject a medication or produce spinal anaesthesia.

What to expect: Preparation for the procedure

  • Many lumbar punctures are done as emergency procedures, such as to help diagnose meningitis. In such cases there is no opportunity for specific preparatory instructions for the patient.
  • If the test is routine and has been planned in advance, you may be able to maintain your regular eating schedule without dietary or fluid restrictions beforehand. However, there are situations where dietary and fluid restrictions are appropriate in the few hours before the test is carried out. Your hospital or outpatient department will issue written instructions if this is the case.
  • Ask your doctor for specific guidelines about discontinuing alcohol use, aspirin products, and anticoagulant medications before the procedure.
  • Tell your doctor if you are allergic to latex or any medications.
  • Make arrangements for transportation, as you should not drive immediately after the test.

Description of the procedure

You will either lie on your side with your knees drawn as close to your chest as possible and your chin toward your chest or sit with your arms and head resting on a table. After cleaning your back with an antiseptic solution, sterile cloths (called drapes) will be placed around the area. A local anaesthetic ( pain-relieving medication) will be injected into the area of your back. You may feel a slight burning sensation.

When the area is numb, a hollow needle is inserted in the lower back between two lumbar vertebrae. This sometimes causes a feeling of pressure. The spinal canal is penetrated and fluid is collected or medication is injected. The needle does not touch the spinal cord during the test. You may feel some discomfort or have a minor headache.

The needle is removed after the medication has been injected or fluid is removed. The area will be covered with a small dressing. A blood sample may be taken from a vein in your arm and tested, along with the spinal fluid, in the laboratory. This is not usually done if the procedure was to inject medication or perform spinal anaesthesia.

WebMD Medical Reference

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