How should I diagnose the cause of meningitis in a patient presenting with acute meningeal signs such as headache, fever, neck stiffness, altered mental status, photophobia, or focal neurologic deficits?

Diagnosing the Cause of Meningitis

Obtain cerebrospinal fluid (CSF) examination immediately in all patients with suspected meningitis, as clinical features alone cannot reliably distinguish bacterial from viral causes or rule out bacterial meningitis. 1

Initial Clinical Assessment

Document Key Clinical Features

Document the presence or absence of the following symptoms, recognizing that the classic triad of fever, neck stiffness, and altered mental status occurs in less than 50% of bacterial meningitis cases: 1, 2

• Headache (present in 58-87% of adults, 78% of children) 1
• Fever (present in 77-97% of cases, but can be absent especially in elderly) 1, 3
• Neck stiffness (present in 65-83% of adults, 60-82% of children) 1
• Altered mental status (present in 54-69% of adults, 13-56% of children) 1
• Rash (petechial/purpuric rash indicates meningococcus in >90% when present with meningitis) 1, 2, 3
• Seizures (occur in 10-56% of children, 10-25% of adults) 1, 2, 3
• Focal neurological deficits (occur in 11-34% of bacterial cases, uncommon in viral meningitis) 1, 4, 3

Critical Pitfall to Avoid

Do not rely on Kernig's or Brudzinski's signs—they have extremely poor sensitivity (5-11%) and cannot rule out meningitis. 1, 3 Neck stiffness itself has only 31% sensitivity, missing 69% of actual cases. 2, 3

Diagnostic Algorithm Based on Clinical Presentation

High-Risk Features Suggesting Bacterial Meningitis or Encephalitis

If the patient presents with any of the following, prioritize bacterial meningitis and obtain neuroimaging before lumbar puncture: 4, 5

• Altered consciousness or confusion (more common in bacterial meningitis; suggests encephalitis if severe) 4
• Focal neurological deficits (present in 11-34% of bacterial cases, not typical of viral meningitis) 4, 3
• Papilledema 5
• History of immunocompromising conditions or CNS disease 5
• Seizures 5

Lower-Risk Features Suggesting Viral Meningitis

If the patient presents with preserved mental status, gradual-onset headache, fever, and myalgias without focal deficits, viral meningitis is more likely, but CSF examination is still mandatory. 4

Essential Diagnostic Testing

Immediate Actions Before Lumbar Puncture

1. Obtain blood cultures immediately 6
2. Check serum leukocytes, procalcitonin, and C-reactive protein (elevated values suggest bacterial etiology) 6, 5
3. Perform neuroimaging (CT/MRI) if any high-risk features are present before lumbar puncture 5

CSF Analysis: The Gold Standard

CSF examination remains the principal diagnostic test with the highest accuracy (AUC 0.95) for bacterial meningitis. 2 Key CSF findings that suggest bacterial meningitis include: 5

• CSF leukocytes >2000/μL
• CSF granulocytes >1180/μL
• CSF protein >2.2 g/L
• CSF glucose <34.23 mg/dL
• Positive Gram stain (diagnostic but sensitivity only 50-90%)

Additional Diagnostic Considerations

• CSF culture and sensitivity (definitive pathogen identification)
• CSF PCR (especially for viral pathogens and when antibiotics given before LP)
• Serum glucose (to compare with CSF glucose ratio)

Age-Specific Considerations

Neonates

Neonates often present with nonspecific symptoms only (poor feeding, irritability, lethargy), making clinical diagnosis unreliable—maintain a very low threshold for lumbar puncture. 1

Children Beyond Neonatal Age

Younger children have more subtle and atypical symptoms—headache occurs in only 2-9% of children <1 year versus 75% in children >5 years. 1

Elderly Patients (>65 years)

Elderly patients are less likely to present with fever or neck stiffness but more likely to have altered consciousness. 1, 2 Consider Listeria monocytogenes as a more common pathogen in this age group. 1, 2

Critical Management Principle

Never delay empiric antibiotic therapy while awaiting diagnostic confirmation—initiate antibiotics immediately after obtaining blood cultures if bacterial meningitis is suspected, even before lumbar puncture is performed. 2, 3, 6, 5 The combination of vancomycin and ceftriaxone (plus ampicillin if age >50, immunocompromised, or other risk factors for Listeria) should be started within one hour of presentation. 2, 5

Key Diagnostic Pitfall

Clinical features alone cannot distinguish between viral and bacterial meningitis—95% of patients with bacterial meningitis have at least two of four symptoms (headache, fever, neck stiffness, altered mental status), but the absence of these features does not exclude bacterial disease. 2, 7 Therefore, proceed with CSF examination unless contraindications exist. 1