What are the indications for a lumbar puncture (LP) in the emergency room (ER)?

Indications for Lumbar Puncture in the Emergency Room

Lumbar puncture (LP) in the emergency room is primarily indicated for suspected central nervous system infections (meningitis/encephalitis) and subarachnoid hemorrhage when CT is negative, but should only be performed after careful assessment for contraindications. 1

Primary Indications for LP in the ER

1. Suspected CNS Infections

   • Suspected bacterial meningitis
   • Suspected viral encephalitis
   • Suspected fungal meningitis

2. Suspected Subarachnoid Hemorrhage (SAH)

   • When CT scan is negative but clinical suspicion remains high
   • For xanthochromia assessment when SAH is suspected 2

3. Other Indications

   • Suspected inflammatory conditions (e.g., multiple sclerosis)
   • Suspected carcinomatous meningitis
   • Idiopathic intracranial hypertension (for both diagnostic and therapeutic purposes)

Contraindications to Immediate LP

Absolute Contraindications

• Local skin infection at puncture site
• Signs of increased intracranial pressure due to mass lesion
• Clinical suspicion of spinal cord compression
• Clinically unstable patient (circulatory shock or respiratory insufficiency)

Relative Contraindications Requiring CT Before LP

• New onset seizures
• Focal neurological signs (excluding isolated cranial neuropathies)
• Moderate to severe impairment of consciousness (GCS ≤10)
• Papilledema
• Immunocompromised state (e.g., advanced HIV) 1

Coagulation-Related Contraindications

• Coagulopathy
• Platelet count <100 × 10⁹/L (though some guidelines accept >50 × 10⁹/L)
• Anticoagulation with heparin or warfarin (requires reversal)
• Rapidly falling platelet count 1

Decision Algorithm for LP in the ER

1. Initial Assessment

   • Evaluate for clinical contraindications to immediate LP
   • If no contraindications → proceed with LP
   • If contraindications present → proceed to step 2

2. CT Scan Evaluation

   • Perform CT scan if clinical contraindications to immediate LP exist
   • If CT shows mass effect, significant brain shift, or tight basal cisterns → defer LP
   • If CT normal → proceed with LP on case-by-case basis within 6 hours 1

3. LP Procedure Considerations

   • Use atraumatic needles to reduce post-LP headache risk 3
   • Reinsert stylet before needle removal to reduce headache risk 3
   • Collect appropriate samples for analysis (cell count, glucose, protein, Gram stain, culture)
   • Consider "three tube test" if traumatic tap is suspected 2

CSF Analysis Interpretation

• Bacterial Meningitis: Elevated WBC (often >500/μL), elevated protein, decreased glucose ratio (<0.4), elevated lactate (>31.53 mg/dL) 3
• Viral Encephalitis: Moderate pleocytosis (tens to hundreds of cells), mildly elevated protein, normal glucose ratio 1
• Subarachnoid Hemorrhage: Xanthochromia, elevated red blood cells that don't clear in sequential tubes 2

Important Caveats and Pitfalls

• CT scan is not reliable for diagnosing increased intracranial pressure; clinical assessment should be the primary determinant of LP safety 1
• 5-10% of adults with proven HSV encephalitis may have normal initial CSF findings; consider repeat LP after 24-48 hours if clinical suspicion remains high 1
• Post-LP headache is the most common complication but can be reduced with proper technique 4, 3
• Traumatic taps can be distinguished from true SAH using the "three tube test" and checking for xanthochromia 2
• Bacterial meningitis has been reported following LP in children with bacteremia, emphasizing the need for sterile technique 4

Remember that while LP is a valuable diagnostic tool, the information gained must be weighed against potential risks, particularly in patients with possible increased intracranial pressure.