Cerebrospinal fluid (CSF) analysis

Cerebrospinal fluid (CSF) analysis is a laboratory test to examine a sample of the fluid surrounding the brain and spinal cord. This fluid is a clear, watery liquid that protects the central nervous system from injury and cushionsit from the surrounding bone structure. It contains a variety of substances,particularly glucose (sugar), protein, and white blood cells from the immunesystem. The fluid is withdrawn through a needle in a procedure called a lumbar puncture.

The purpose of a CSF analysis is to diagnose medical disorders that affect the central nervous system. Some of these conditions include: meningitis, encephalitis, tumors or cancers of the nervous system, syphilis, bleeding (hemorrhaging) around the brain and spinal cord, multiple sclerosis, and Guillain-Barré syndrome.

In some circumstances, a lumbar puncture to withdraw a small amount of CSF for analysis may lead to serious complications. Lumbar puncture should be performed only with extreme caution, and only if the benefits are thought to outweigh the risks. For example, in people who have blood clotting (coagulation) or bleeding disorders, lumbar puncture can cause bleeding that can compress the spinal cord. If there is a large brain tumor or other mass, removal of CSFcan cause the brain to drop down within the skull cavity (herniate), compressing the brain stem and other vital structures, and leading to irreversible brain damage or death. These problems are easily avoided by checking blood coagulation through a blood test and by doing a computed tomography scan (CT) or magnetic resonance imaging (MRI) scan before attempting the lumbar puncture.

The procedure to remove cerebrospinal fluid is called a lumbar puncture, or spinal tap, because the area of the spinal column used to obtain the sample isin the lumbar spine, or lower section of the back. In rare instances, such as a spinal fluid blockage in the middle of the back, a doctor may perform a spinal tap in the neck.

In order to get an accurate sample of cerebrospinal fluid, it is critical that a patient is in the proper position. The spine must be curved to allow as much space as possible between the lower vertebrae, or bones of the back, forthe doctor to insert a lumbar puncture needle between the vertebrae and withdraw a small amount of fluid. The most common position is for the patient to lie on his or her side with the back at the edge of the exam table, head and chin bent down, knees drawn up to the chest, and arms clasped around the knees. It is important to be relaxed and to remain still during the entire procedure. If the patient is anxious or uncooperative, a short-acting sedative may be given.

During a lumbar puncture, the doctor drapes the back with a sterile coveringthat has an opening over the puncture site and cleans the skin surface with an antiseptic solution. Patients receive a local anesthetic (numbing medication) to minimize any pain in the lower back.

The doctor inserts a hollow, thin needle in the space between two vertebrae of the lower back and slowly advances it toward the spine. A steady flow of cerebrospinal fluid will begin to fill the needle as soon as it enters the spinal canal. The doctor measures the cerebrospinal fluid pressure with a specialinstrument called a manometer and withdraws several vials of fluid for laboratory analysis. The amount of fluid collected depends on the type and numberof tests needed to diagnose a particular medical disorder.

Normal CSF is clear and colorless. It may be cloudy in infections; straw- oryellow-colored if there is excess protein, as may occur with cancer or inflammation; blood-tinged if there was recent bleeding; or yellow to brown (xanthochromic) if caused by an older instance of bleeding.

After the procedure, the doctor covers the site of the puncture with a sterile bandage. Patients must avoid sitting or standing and remain lying down foras long as six hours after the lumbar puncture. They should also drink plentyof fluid to help prevent headache.

In most cases, this test to analyze CSF is a safe and effective procedure. Some patients experience pain, difficulty urinating, infection, or leakage of cerebrospinal fluid from the puncture site after the procedure. For most people, the most common side effect after the removal of CSF is a headache. This occurs in 10-30% of adult patients and in up to 40% of children. It is causedby a decreased CSF pressure related to a small leak of CSF through the puncture site. These headaches usually are a dull pain, although some people reporta throbbing sensation. A stiff neck and nausea may accompany the headache. Lumbar puncture headaches typically begin within two days after the procedureand persist from a few days to several weeks or months.

Since an upright position worsens the pain, patients with a lumbar puncture headache can control the pain by lying in a flat position and taking a prescription or non-prescription pain relief medication, preferably one containing caffeine. In rare cases, the puncture site leak is "patched" using the patient's own blood.

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