What is the appropriate procedure for an adult patient presenting with neurological symptoms, considering an Lp (lumbar puncture) analysis to diagnose or rule out conditions such as meningitis, encephalitis, or subarachnoid hemorrhage?

Lumbar Puncture Analysis in Adults with Neurological Symptoms

When to Perform LP Immediately

All patients with suspected encephalitis or meningitis should undergo lumbar puncture as soon as possible after hospital admission, unless specific clinical contraindications are present. 1, 2

Absolute Contraindications to Immediate LP

• Glasgow Coma Scale ≤12 or decline in GCS >2 points – indicates potential raised intracranial pressure 1, 2
• Focal neurological signs including unequal, dilated, or poorly responsive pupils 1, 2
• Papilledema – direct indicator of raised ICP 1, 2
• Signs of severe sepsis or rapidly evolving rash 1
• Respiratory or cardiac compromise 1
• Continuous or uncontrolled seizures 1
• Coagulopathy: platelet count <100 × 10⁹/L or anticoagulant therapy 1, 2
• Local infection at LP site 1, 2

Critical point: Clinical assessment, not CT scan, should be the primary determinant of LP safety. 2 Inability to visualize the fundus is NOT a contraindication to LP. 1

Management Algorithm When Contraindications Present

Step 1: Imaging Before LP

If any contraindications exist, obtain non-contrast CT head immediately to exclude mass lesions or obstructive hydrocephalus. 1 However, relying solely on CT to rule out raised ICP is inadequate, as it may miss cases of increased pressure. 2

Step 2: Antibiotic Administration

If bacterial meningitis is suspected and LP is delayed for ANY reason (including awaiting imaging), start empirical antibiotics immediately after obtaining blood cultures. 1 This is critical for rapidly fatal etiologies like S. pneumoniae. 1

Step 3: Post-CT Decision Making

• If CT shows significant brain shift, tight basal cisterns, or raised ICP: defer LP 1, 2
• If CT is normal but clinical contraindications persist: consider LP on case-by-case basis with neurology/neurosurgery consultation 1
• Review situation every 24 hours and perform LP when safe 1

Step 4: Anticoagulation Reversal

For anticoagulated patients, adequate reversal is mandatory before LP: 1, 2

• Heparin: protamine
• Warfarin: vitamin K, prothrombin complex concentrate, or fresh frozen plasma
• Bleeding disorders: replacement therapy with hematology consultation

CSF Collection and Analysis

Volume and Technique

Collect at least 10-22 mL of CSF – CSF is produced at ~15 mL/hour, making this volume safe and avoiding repeat procedures. 1, 2 Use atraumatic needles meeting National Patient Safety Agency standards. 1, 2

Essential CSF Studies

Basic tests for all patients: 1

• Cell count with differential
• Glucose and protein concentrations
• Gram stain
• Bacterial culture
• Opening pressure (measured in lateral decubitus position; normal <200 mm H₂O) 1

Additional tests based on clinical suspicion: 1

• Cryptococcal antigen
• Fungal stains and cultures
• Acid-fast bacillus smears
• PCR for HSV, CMV, enterovirus (especially in immunocompromised)
• CSF lactate (if antibiotics not yet given)

CSF Interpretation

Bacterial meningitis typically shows: 1

• Polymorphonuclear pleocytosis (though 10% have <100 cells/mm³) 1
• CSF glucose <2.6 mmol/L 1
• CSF:plasma glucose ratio <0.36 (93% sensitivity/specificity) 1
• CSF protein >0.6 g/L 1
• Opening pressure >20 cm H₂O 1

CSF lactate >35 mg/dL has 93% sensitivity and 96% specificity for bacterial vs. viral meningitis if obtained before antibiotics. 1

Special Considerations

Subarachnoid Hemorrhage vs. Meningitis

Critical pitfall: Meningitis can mimic subarachnoid hemorrhage on imaging. 3, 4, 5 If CT suggests SAH but clinical picture is atypical (fever, ear pain, mastoiditis), LP is essential to differentiate – CSF analysis will reveal infection rather than blood. 3, 5

HSV Encephalitis

5-10% of HSV encephalitis cases have normal initial CSF findings. 2 If clinical suspicion persists despite normal first LP, repeat LP may be valuable. 2

Patients with Intracranial Devices

In febrile patients with ventriculostomies showing stupor or meningitis signs, remove catheter and culture the tip. 1 Obtain CSF from the reservoir, and consider lumbar CSF if flow is obstructed. 1

Contingency/Crisis Standards

When resources are constrained, prioritize LP for patients with ≥2 of the following: severe headache, altered mental status, meningeal signs, or other neurological symptoms. 1 In crisis situations, diagnose meningitis clinically based on any single feature. 1