Lumbar puncture

Contraindications

• Increased intracranial pressure, with focal neurological deficits, new seizures, or papilledema (CT head first; see Investigations, below)
• Infection at the procedure site
• Coagulopathy
  • Platelets <30-40
  • INR ≥1.4-1.6
  • Hemophilia
  • Anticoagulation
    • Within 4 hours of prophylactic low molecular weight heparin (LMWH)
    • Within 24 hours of therapeutic LMWH
    • Within 7 days of clopidogrel (but no delay for aspirin)

Procedure

• Position appropriately: lateral decubitus or sitting and leaning over a table
• Landmark using posterior superior iliac crests (PSIS) (marks L4)
  • Can go above (L4/L5 space) or below (L3/L4 space)
• Inset spinal needle in the midline
• Measure opening pressure

Investigations

• Consider INR/PTT if concern about coagulopathy
• Consider platelet count (should be >100)
• Consider CT head if focal neurological signs, papilledema, GCS ≤12
  • IDSA: immunocompromised, history of CNS disease (including stroke and mass lesion), or new, uncontrolled, or continuous seizures

Complications

• Local pain
• Post-LP headache, better when supine
  • Treat with caffeine, fluids, and acetaminophen
• Brief radicular pain
• Rare:
  • Spinal epidural hematoma
  • Bacterial meningitis or spinal abscess
  • Cerebral herniation

Interpretation

Etiology	Gram stain	Cell count	CSF glucose	CSF protein
Bacterial meningitis	Positive in 60-80%	Elevated >1000 neutrophils	Reduced <0.4 CSF:serum	Elevated >1g/L
Viral meningitis	Negative	Moderate elevation <100 lymphocytes	Normal or mildly reduced	Normal or mildly elevated
Fungal or tuberculous meningitis	Negative, except for Cryptococcus on India ink or TB on acid-fast stain	Moderate elevation, usually lymphocytes	Often very low	Elevated

• Correction for a bloody tap is 1 excess WBC for every 700 RBCs
• Lymphocytosis may be seen in bacterial meningitis if Listeria infection or prior antibiotics
• Xanthochromia suggests subarachnoid hemorrhage
• Hypoglycorrhachia refers to low CSF glucose

Interpretation After Antibiotics

• Glucose increases first, then protein, and both are often normalized within 12 hours
• WBC normalizes last, and can still be useful even the following day

Opening Pressure

• Usual range is 5 to 20 cm H₂O
• See causes of increased intracranial pressure

Protein

• May be increased by disruption of the blood-brain barrier (most commonly), intracranial synthesis of IgG, or impaired CSF resorption of proteins by the arachnoid villi
• Highly suggestive of CNS disease

Correction for Traumatic Tap

• If increased RBCs are seen, may need to adjust
• Allow 1 WBC for every 700-1000 RBCs
• Allow 0.01 mg/mL protein for every 1000 RBCs

Further Reading

• Straus S, et al. How Do I Perform a Lumbar Puncture and Analyze the Results to Diagnose Bacterial Meningitis? JAMA. 2006;296(16):2012-2022.