Lumbar Puncture: Indications and Procedure

Primary Indications

Lumbar puncture is essential when CNS infection (bacterial meningitis, viral encephalitis) or subarachnoid hemorrhage is suspected, as it provides critical diagnostic information that directly guides life-saving treatment. 1, 2

Diagnostic Indications:

Suspected bacterial meningitis - most critical indication requiring immediate CSF analysis 1
Suspected viral encephalitis - particularly HSV encephalitis where early diagnosis changes management 1
Subarachnoid hemorrhage when CT is normal or unavailable 2
Suspected CNS malignancy (meningeal carcinomatosis, lymphoma) 3
Demyelinating diseases (multiple sclerosis evaluation) 4
Alzheimer's disease biomarker assessment in specialized settings 1
Autoimmune/paraneoplastic encephalitis workup 1, 5

Contraindications to Immediate Lumbar Puncture

Absolute Contraindications Requiring CT First:

Perform cranial CT before LP if any of the following are present: 1

Focal neurologic deficits (excluding isolated cranial nerve palsies) 1
New-onset seizures 1
Severely altered mental status (Glasgow Coma Scale <10) 1
Severely immunocompromised state (organ transplant recipients, advanced HIV/AIDS) 1
Papilledema on fundoscopic examination 1

Note: There is controversy regarding the exact GCS threshold, with some guidelines suggesting GCS <13, others <9, but the ESCMID guideline (2016) recommends GCS <10 as the most evidence-based cutoff. 1

Other Absolute Contraindications:

Coagulopathy or platelet count <100 × 10⁹/L (some accept ≥50 × 10⁹/L with hematology consultation) 1
Active anticoagulation (warfarin, heparin) without adequate reversal 1
Local skin infection at puncture site 1
Hemodynamic instability or respiratory insufficiency requiring stabilization first 1
Suspected spinal cord compression 1

Critical Timing Consideration

If LP must be delayed due to need for CT imaging, start empiric antibiotics immediately after obtaining blood cultures - do not wait for imaging or LP results. 1 Delayed antibiotic administration is strongly associated with death and poor outcome, and treatment should begin within 1 hour of hospital arrival. 1

Procedure Technique

Patient Positioning:

Lateral recumbent position is strongly preferred over sitting position to minimize complications 1, 4
Measure opening pressure only in lateral recumbent position (normal: 10-20 cm H₂O) 5

Needle Selection (Critical for Safety):

Use an atraumatic (pencil-point, blunt-tip) needle of 22-24 gauge for optimal safety profile. 1, 4, 6

Atraumatic needles (Sprotte, Whitacre design) significantly reduce post-LP headache compared to cutting-bevel needles 1, 4
22-24 gauge diameter balances low complication rates with reasonable CSF flow 1
Avoid needles <27 gauge (too slow) or >22 gauge (higher complication rates) 1
Standard length 70-90 mm for most adults; longer needles may be needed for obese patients 1

Procedural Steps:

Identify L3-L4 or L4-L5 interspace (below conus medullaris termination) 4
Sterile preparation with chlorhexidine or povidone-iodine 4
Local anesthesia with lidocaine 4
Insert needle with bevel parallel to spine (separates rather than cuts dural fibers) 1, 4
Limit attempts to ≤4 punctures to minimize complications 1, 4
Passive CSF collection preferred when feasible over active aspiration 1, 4
Collect <30 mL total CSF (typically 10-20 mL sufficient for all diagnostic tests) 1, 4

Essential CSF Studies to Obtain

Always send CSF for: 1, 5

Cell count with differential (WBC, RBC) 1, 5
Protein and glucose (with simultaneous serum glucose) 1, 5
Gram stain and bacterial culture 1, 5
Opening pressure measurement 5

Additional studies based on clinical suspicion: 1, 5

HSV PCR if encephalitis suspected (sensitivity >95%) 5
PCR for other pathogens if cultures negative 1
Fungal/TB studies if immunocompromised or chronic presentation 5
Cytology if malignancy suspected 1
Oligoclonal bands if demyelinating disease suspected 1
Alzheimer's biomarkers (Aβ1-42, tau, p-tau) in appropriate clinical context 1, 5

Key Interpretation Principles

Neutrophilic predominance (>50%) = bacterial meningitis until proven otherwise; start antibiotics immediately 5
Lymphocytic predominance = viral, TB, fungal, or partially treated bacterial meningitis 5
CSF glucose <40 mg/dL or <50% of serum = bacterial, TB, or fungal meningitis 5
CSF lactate <2 mmol/L effectively rules out bacterial disease 1, 5
Elevated RBC count in ~50% of HSV encephalitis cases 1, 5

Common Pitfalls to Avoid

Never delay antibiotics waiting for LP if bacterial meningitis suspected and LP must be delayed 1
Do not rely on CT to rule out raised intracranial pressure - use clinical criteria instead 1
Avoid LP in patients on anticoagulation without reversal - risk of spinal hematoma 1
Do not use cutting-bevel needles - significantly higher post-LP headache rates 1, 4, 6
Recognize that 5-10% of HSV encephalitis cases have normal initial CSF - repeat LP in 24-48 hours if suspicion remains high 1

Post-Procedure Management

Patient communication is critical - explain procedure thoroughly to reduce anxiety and perceived complications 1, 4
Post-dural puncture headache is most common complication; treat conservatively first, then epidural blood patch if severe 6
Bed rest after LP does not prevent headache and is not recommended 4

What are the indications for and procedure of a lumbar tap (lumbar puncture)?

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