Lumbar Puncture

Cerebrospinal fluid (CSF) Examination

Why is a Lumbar-Puncture (LP) performed?

LP

LP (also known as a spinal tap) is important in the diagnosis of infections and certain inflammatory diseases (e.g. multiple sclerosis , Guillain-Barre syndrome , vasculitis).

LP with CSF analysis is also an important diagnostic tool in sudden onset headache primarily to test for a type of bleeding around the brain called subarachnoid hemorrhage and and certain malignant conditions.

Lumbar-Puncture itself can be therapeutic, particularly in benign intracranial hypertension (ie. pseudotumor cerebri) An LP is performed in the interspaces between the lumbar vertebrae, usually at the L4-L5 level.

The spinal cord typically ends at the L1 level in adults (slightly lower in children).

Preparation

The spinal needle used is disposable.

A 20-gauge needle for adults, or a 22-gauge needle for children, is used typically. Overall, using a blunt needle (vs a sharper "cutting" type) and a smaller needle size is recommended, as these decrease the chance of post– lumbar-puncture headache.

Sedative medication may be required in children or in the less co-operative patient.

The bed should be flat, and the patient should be lying on his/her side. The patient's body needs to be perfectly perpendicular to the bed. The patient should assume the fetal position with knees flexed as much as possible.

Procedure

Local anesthetic should be infiltrated and then the area should be prepared carefully and draped. The spinal needle then is positioned between the 2 vertebral spines at the L4-L5 level and introduced into the skin with the bevel of the needle facing up.

Accurate placement of the needle is rewarded by a flow of fluid, which normally is clear and colorless.

As with any procedure, experience improves the success rate. One of the primary goals is to prevent the introduction of blood into the CSF sample.

A measurement of opening pressure should be made; the normal range is 80-180 mm H2O.

Following determination of CSF pressure, CSF samples should be obtained.

Collecting the fluid

If the fluid appears to be bloody, several specimens should be collected. If the blood clears in successive tubes then the blood, at least in part, was traumatic in origin.

When sufficient fluid is obtained, the needle is withdrawn and a dry sterile dressing applied to the puncture site. Prolonged compression of the site, or keeping the patient supine for an extended period, has not been proved to reduce the incidence of spinal headache.

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Separate specimens should be sent for microscopic study and for biochemical analysis as well as other specific tests (such as oligoclonal band testing) if required.

Normal CSF may contain as many as 5 white blood cells (WBCs)s per cubic millimeter.

A larger-than-usual number of WBCs suggests infection or inflammation or, neoplastic infiltration.

A traumatic tap will, of course, introduce both WBCs and RBCs into the CSF.

An approximation of1 WBC per 1000 RBCs can be made, although a repeat tap may be preferable.

Number of Red Blood cells in CSF Roughly how many white blood cells should this should"add" to the CSF ? Roughly how much CSF protein this should add to the CSF?
1000 1 1 mg / dl

Xanthochromia

The best way to distinguish RBCs related to intracranial bleeding is examination of the CSF for xanthochromia (yellow color). Although xanthochromia can be confirmed visually, it is identified and quantified more accurately in the laboratory after spinning the sample in a centrifuge.

While xanthochromia can be produced by spillover from a very high serum bilirubin level (ie, >15 mg/dL), patients with severe hyperbilirubinemia usually have been identified prior to the LP (eg, jaundice, known liver disease). With this exception, the presence of xanthochromia in a freshly spun specimen is evidence of preexistent blood in the subarachnoid space. However, note that an extremely high CSF protein level, as seen in a lumbar-puncture below a complete spinal block, also renders the fluid xanthochromic, though without RBCs.

Xanthochromia can persist up to several weeks following a subarachnoid hemorrhage (SAH). Thus it has greater diagnostic sensitivity than a CT scan of the head without contrast, especially if the SAH has occurred more than 3-4 days prior to presentation. Patients with aneurysmal leaks (ie, sentinel hemorrhages) may present days after headache onset, increasing the likelihood of a false-negative head CT scan.

Other tests

Assuming the CSF has been collected under sterile conditions, microbiologic studies can be performed. Stains, cultures, and immunoglobulin titers can be obtained. The latter are of special importance in diseases in which peripheral manifestations fade while CNS symptoms persist (eg, syphilis, Lyme disease).

Assessment of CSF protein level, while nonspecific, can be a clue to otherwise unsuspected neurologic disease. The high protein levels in demyelinating polyneuropathies , or postinfectious states, can be informative. A traumatic tap can introduce protein into the CSF.

An approximation of 1 mg of protein per 750 RBCs may be used, although a repeat tap is preferable.

CSF glucose level normally approximates 60% of the peripheral blood glucose level at the time of the tap. A simultaneous measurement of blood glucose (especially if the CSF glucose level is likely to be low) is recommended. Low CSF glucose level usually is associated with bacterial infection (probably due to enzymatic inhibition rather that actual bacterial consumption of the glucose). It also is seen in tumor infiltration, and may be one of the hallmarks of cancerous invasion of the lining of the brain (meningeal carcinomatosis), even with negative cytologic findings.

Leptomeningeal malignancies: Multiple lumbar punture examinations may be required in this situation. At least 3 negative cytologic evaluations (ie, 3 separate samplings) are required to rule out leptomeningeal malignancy (eg, leptomeningeal carcinomatosis).

Risks associated with a Lumbar Puncture

This headache is characterized by pulsatile head pain , with or without nausea, relieved by lying down and aggravated by standing and "Valsalva" maneuvers such as coughing and straining at stool. It is self-limited but may last up to a week (or rarely longer).

The placement of an epidural blood patch using the patient's own venous blood often corrects this problem. However, the need for a blood patch is uncommon. The use of intravenous caffeine benzoate (500 mg infusion over 1 h) also has been found to treat post Lumbar-Puncture headaches effectively in double-blind, controlled trials.

  • This may cause temporary pain or tingling in the skin from which the nerve root is transmitting information (i.e. the "dermatome" of that particular nerve root).

Lumbar puncture risks

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