What are the diagnostic criteria for viral meningitis?

Diagnostic Criteria for Viral Meningitis

Viral meningitis is diagnosed by the presence of meningeal symptoms (headache, neck stiffness, photophobia) with CSF pleocytosis and bacteriologically sterile cultures, confirmed definitively by CSF PCR testing for viral pathogens. 1

Clinical Diagnostic Criteria

The clinical presentation requires:

• Meningeal symptoms: neck stiffness, headache, and photophobia 1
• Fever (though not always present) 1
• Absence of altered consciousness - any reduction in conscious level suggests bacterial meningitis, encephalitis, or other intracranial pathology rather than viral meningitis 1

Additional non-specific symptoms may include:

• Diarrhea, vomiting, muscle pain, and sore throat 1
• For HSV-2 meningitis: rarely concurrent genital ulcers 1
• For VZV meningitis: may occur with or without chickenpox or shingles rash 1

Laboratory Diagnostic Criteria

CSF Examination Requirements

The CDC defines aseptic meningitis as acute onset of meningeal symptoms, fever, and CSF pleocytosis with bacteriologically sterile cultures. 1, 2

Key CSF findings include:

• CSF pleocytosis (elevated white blood cell count) 1, 2
• Negative bacterial cultures - no evidence of bacterial or fungal meningitis 1, 2
• Negative CSF Gram stain 1

Confirmatory Testing

CSF PCR is the gold standard for confirmation of viral meningitis. 1, 3

Standard PCR panel should test for:

• Enteroviruses (most common cause) 1
• HSV-1 and HSV-2 1
• VZV 1

Additional testing based on clinical context:

• Stool and/or throat swabs for enterovirus PCR 1
• PCR or serological assays for other viruses (CMV, EBV, mumps) guided by immune status and travel history 1

Diagnostic Algorithm

Step 1: Clinical Assessment

Evaluate for meningeal symptoms without altered consciousness 1

Step 2: Lumbar Puncture

Perform LP if no contraindications exist 1

Step 3: Initial CSF Analysis

• Cell count and differential (looking for pleocytosis) 1
• Gram stain (should be negative) 1
• Bacterial culture (should be sterile) 1
• Glucose and protein levels 1

Step 4: Viral Testing

• CSF PCR for enterovirus, HSV-1, HSV-2, and VZV 1
• Additional viral testing based on epidemiological factors 1

Step 5: Confirmation

A confirmed case requires clinically compatible illness diagnosed by a physician as aseptic meningitis with no laboratory evidence of bacterial or fungal meningitis. 1

Important Caveats

No cause is identified in 30-50% of patients with presumed viral meningitis despite appropriate testing. 1 This does not exclude the diagnosis if clinical and CSF findings are consistent.

Diagnostic algorithms for differentiating bacterial from viral meningitis are not 100% sensitive, meaning bacterial cases can be missed if relying solely on algorithms. 1 Clinical judgment remains paramount when deciding whether to initiate empiric antibiotics.

Kernig's and Brudzinski's signs should not be relied upon - they have high specificity (up to 95%) but very low sensitivity (as low as 5%). 1

Elderly patients may present atypically with altered consciousness, less neck stiffness, and absence of fever compared to younger patients. 1

Specialist Consultation

All patients with viral meningitis should be assessed by an infection or neurological specialist. 1