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Lumbar Puncture (Spinal Tap)

By Michael C. Levin, MD

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Patient Education

Lumbar puncture is used to do the following:

Cerebrospinal Fluid Abnormalities in Various Disorders

Condition

Pressure*

WBCs/µL*

Predominant Cell Type

Glucose

Protein*

Normal

100–200 mm H2O

0–3

L

50–100 mg/dL (2.78–5.55 mmol/L)

20–45 mg/dL

Acute bacterial meningitis

100–10,000

PMN

> 100 mg/dL

Subacute meningitis (eg, due to TB, Cryptococcus infection, sarcoidosis, leukemia, or carcinoma)

N or

100–700

L

Acute syphilitic meningitis

N or

25–2000

L

N

Paretic neurosyphilis

N or

15–2000

L

N

Lyme disease of CNS

N or

0–500

L

N

N or

Brain abscess or tumor

N or

0–1000

L

N

Viral infections

N or

100–2000

L

N

N or

Idiopathic intracranial hypertension

N

L

N

N or

Cerebral hemorrhage

Bloody

RBC

N

Cerebral thrombosis

N or

0–100

L

N

N or

Spinal cord tumor

N

0–50

L

N

N or

Multiple sclerosis

N

0–50

L

N

N or

Guillain-Barré syndrome

N

0–100

L

N

> 100 mg/dL

Lead encephalopathy

0–500

L

N

*Figures given for pressure, cell count, and protein are approximations; exceptions are common. Similarly, PMNs may predominate in disorders usually characterized by lymphocyte response, especially early in the course of viral infections or tuberculous meningitis. Alterations in glucose are less variable and more reliable.

Up to 14% of patients may have a CSF protein level < 100 mg/dL in the initial lumbar puncture sample.

L = lymphocyte; N = normal; PMN =polymorphonuclear leukocyte; = increased; = decreased.

Relative contraindications include

  • Infection at the puncture site

  • Bleeding diathesis

  • Increased intracranial pressure due to an intracranial mass lesion, obstructed CSF outflow (eg, due to aqueductal stenosis or Chiari I malformation), or spinal cord CSF blockage (eg, due to tumor cord compression)

If papilledema or focal neurologic deficits are present, CT or MRI should be done before lumbar puncture to rule out presence of a mass that could precipitate transtentorial or cerebellar herniation (see Figure: Brain herniation.).

Lumbar puncture procedure

For the procedure, the patient is typically in the left lateral decubitus position. A cooperative patient is asked to hug the knees and curl up as tightly as possible. Assistants may have to hold patients who cannot maintain this position, or the spine may be flexed better by having patients, particularly obese patients, sit on the side of the bed and lean over a bedside tray table.

An area 20 cm in diameter is washed with iodine, then wiped with alcohol to remove the iodine and prevent its introduction into the subarachnoid space. A lumbar puncture needle with stylet is inserted into the L3-to-L4 or L4-to-L5 interspace (the L4 spinous process is typically on a line between the posterior-superior iliac crests); the needle is aimed rostrally toward the patient’s umbilicus and always kept parallel to the floor. Entrance into the subarachnoid space is often accompanied by a discernible pop; the stylet is withdrawn to allow CSF to flow out.

Opening pressure is measured with a manometer; 4 tubes are each filled with about 2 to 10 mL of CSF for testing. The puncture site is then covered with a sterile adhesive strip.

A post–lumbar puncture headache occurs in about 10% of patients.

Lumbar puncture.

For the procedure, the patient is typically in the left lateral decubitus position. A lumbar puncture needle with stylet is inserted into the L3-to L4 or L4-to-L5 interspace (the L4 spinous process is typically on a line between the posterior-superior iliac crests); the needle is aimed rostrally toward the patient’s umbilicus and always kept parallel to the floor. Entrance into the subarachnoid space is often accompanied by a discernible pop; the stylet is withdrawn to allow CSF to flow out.

CSF color

Normal CSF is clear and colorless; 300 cells/μL produces cloudiness or turbidity.

Bloody fluid may indicate a traumatic puncture (pushing the needle in too far, into the venous plexus along the anterior spinal canal) or subarachnoid hemorrhage. A traumatic puncture is distinguished by

  • Gradual clearing of the CSF between the 1st and 4th tubes (confirmed by decreasing RBC count)

  • Absence of xanthochromia (yellowish CSF due to lysed RBCs) in a centrifuged sample

  • Fresh, uncrenated RBCs

With intrinsic subarachnoid hemorrhage, the CSF remains uniformly bloody throughout collection; xanthochromia is often present if several hours have passed after ictus; and RBCs are usually older and crenated. Faintly yellow fluid may also be due to senile chromogens, severe jaundice, or increased protein (> 100 mg/dL).

CSF cell count and glucose and protein levels

Cell count and differential and glucose and protein levels aid in the diagnosis of many neurologic disorders (see Table: Cerebrospinal Fluid Abnormalities in Various Disorders).

Normally, CSF:blood glucose ratio is about 0.6, and except in severe hypoglycemia, CSF glucose is typically > 50 mg/dL (> 2.78 mmol/L).

Increased CSF protein (> 50 mg/dL) is a sensitive but nonspecific index of disease; protein increases to > 500 mg/dL in purulent meningitis, advanced TB meningitis, complete block by spinal cord tumor, or a bloody puncture. Special examinations for globulin (normally < 15%), oligoclonal banding, and myelin basic protein aid in diagnosis of a demyelinating disorder.

CSF staining, testing, and culture

If infection is suspected, the centrifuged CSF sediment is stained for bacteria (Gram stain), for TB (acid-fast stain or immunofluorescence), and for Cryptococcus sp (India ink). Larger amounts of fluid (10 mL) improve the chances of detecting the pathogen, particularly acid-fast bacilli and certain fungi, in stains and cultures. In early meningococcal meningitis or severe leukopenia, CSF protein may be too low for bacterial adherence to the glass slide during Gram staining, producing a false-negative result. Mixing a drop of aseptic serum with CSF sediment prevents this problem. When hemorrhagic meningoencephalitis is suspected, a wet mount is used to search for amebas. Latex particle agglutination and coagglutination tests may allow rapid bacterial identification, especially when stains and cultures are negative (eg, in partially treated meningitis). CSF should be cultured aerobically and anaerobically and for acid-fast bacilli and fungi.

Except for enteroviruses, viruses are seldom isolated from the CSF. Viral antibody panels are available.

Venereal Disease Research Laboratories (VDRL) testing and cryptococcal antigen testing are often routinely done. PCR tests for herpes simplex virus and other CNS pathogens are increasingly available.

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* This is the Professional Version. *