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Spinal Tap

Alternate Names

  • lumbar puncture
  • LP
  • Brain and spinal cord

Definition

A spinal tap is a procedure where a special needle is inserted between the vertebrae in the lower back and into the space that surrounds the spinal cord, usually to look for evidence of infection. The space surrounding the brain and spinal cord is filled with cerebrospinal fluid (CSF). The cerebrospinal fluid works like a shock absorber and filter.

Who is a candidate for the test?

When a person is thought to have a disease or injury that affects the brain, spinal cord, or their protective membranes, a spinal tap may help determine the cause. A spinal tap is a very useful procedure.
Healthcare professionals use a spinal tap to:
  • collect a small sample of a person's CSF
  • measure the pressure of someone's CSF
  • deliver anesthesia medication to a person before or after surgery
  • inject dye into a person's body. This allows the doctor to take special x-rays of the area surrounding the brain and spinal cord to detect an injury.

How is the test performed?

A spinal tap can be performed with a person bending over while sitting in a chair, or lying down on his or her side with the knees drawn up close to the chest. These positions help to separate the vertebrae of the lower back, giving the healthcare professional a larger space to insert the spinal needle. The area where the needle is to be inserted is cleaned and medication is injected into the skin to provide local anesthesia. A person may feel a burning sensation for a few seconds while the medication is administered.
After the area has been prepared, a special spinal needle is inserted. A person may feel the insertion of the spinal needle as a pushing or tugging sensation. Once the needle is in the proper place, the pressure can be measured and a sample of CSF can be collected or drugs injected. The procedure generally lasts 15 to 20 minutes. Once the procedure is completed, a dressing over the puncture site is applied and the person is instructed to lie flat.
Depending on the reason for the spinal tap, a person may be instructed to lie flat from 2 to 12 hours. This is done to avoid a post-procedure spinal headache. After the procedure is completed, the person is instructed to drink plenty of fluid. This will help the body replenish the cerebrospinal fluid which was removed for testing.
A person is also instructed to report any numbness and tingling to the legs, bleeding or oozing from the puncture site, or headaches that fail to improve. Normal activity can usually resume once a person has had enough rest. Sometimes the area where the spinal needle was inserted remains sore for a day or two. No other special home care is required.
The most common complication from a spinal tap is a post-procedure spinal headache. These headaches occur because of a drop in cerebrospinal fluid pressure as a result of removing some fluid for testing, and can usually be avoided if a person lies flat for several hours and drinks plenty of fluids. The spinal headache typically goes away in a few hours. Severe cases may require pain medication or another special procedure to repair the spinal tap site.

What is involved in preparation for the test?

A person should request specific instructions from his or her health care professional.

What do the test results mean?

Normally, spinal fluid will be clear and colorless and contain no blood and no organisms. Protein levels are usually 15 to 45 mg/dl of CSF, although in the elderly and children the level may be 70 mg/dl. Lactic dehydrogenase (LDH) levels are usually less than 2.0 to 7.2 U/ml.
Abnormal results may be caused by many different conditions:
  • Elevated protein, lactic dehydrogenase (LDH), and muddy or dense spinal fluid with malignant cells may indicate a brain tumor or cancer in the brain, spinal cord, or cancer that has spread to these areas.
  • CSF that looks muddy and may contain increased protein levels and increased LDH levels may indicate degenerative brain disease, autoimmune disorders, multiple sclerosis, neurosyphilis, or other diseases. In the case of neurosyphilis, the CSF should test positive for antibodies to syphilis.
  • CSF that has high protein levels, muddy color, and red blood cells may indicate subarachnoid bleeding, which is bleeding in the membranes surrounding the brain, cerebral bleeding, or a traumatic spinal tap (one in which the needle hit a blood vessel on the way into the spinal canal).
  • Elevated glutamine levels in the CSF may indicate encephalitis, which is swelling in brain, myelitis, which is inflammation of the spinal cord, or hepatic coma.
  • Elevated white blood cells and protein in the CSF may indicate meningitis, which is a very serious inflammation of the spinal cord or brain, encephalitis, or a brain abscess.

Sources

Isselbacher KJ, Braunwald E, Wilson, JD, et al: Harrison's principles of internal medicine. ed 13, New York, 1994, McGraw-Hill.

Miller DT, Lunde JR: Sheehy's emergency nursing: principles and practice, ed 4, St. Louis, 1998, Mosby.

Pagana KD, Pagana TJ: Mosby's diagnostic and laboratory test reference. ed 4, St. Louis, 1999, Mosby.

Thelan LA, Davie JK, Urden LD: Textbook of critical care nursing diagnosis and management. St. Louis, 1990, Mosby.

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