Characteristic CSF Picture of Viral Meningitis
CSF Parameters in Viral Meningitis
- The characteristic cerebrospinal fluid (CSF) picture in viral meningitis typically shows a mild to moderate lymphocytic pleocytosis (5-1000 cells/μL), mildly elevated protein, normal or slightly low glucose, and normal to mildly raised opening pressure 1
- CSF white cell count is typically elevated between 5-1000 cells/μL 1
- Lymphocytes predominate in the cell differential, though neutrophils may predominate in early viral meningitis, especially in enteroviral disease 2, 1
- Even in cases where neutrophils predominate early, the total CSF white cell count is unlikely to exceed 2000 cells/μL in viral meningitis 3
- CSF appearance is typically clear, unlike the turbid or purulent appearance seen in bacterial meningitis 1
- Opening pressure is usually normal or mildly raised 1
- CSF protein is mildly elevated but typically less than 0.6 g/L 2
- CSF glucose is normal or slightly low 1
- CSF/plasma glucose ratio is typically normal or slightly low, but remains above 0.36 (which is an important differentiator from bacterial meningitis) 2, 1
- CSF lactate has high negative predictive value for bacterial meningitis; levels below 35 mg/dL suggest viral etiology 2
Comparison with Other Types of Meningitis
| Parameter | Viral | Bacterial | Tuberculous | Fungal |
|---|---|---|---|---|
| Opening Pressure | Normal/mildly raised | Raised | Raised | Raised |
| Appearance | Clear | Turbid, cloudy | Clear or cloudy | Clear or cloudy |
| WCC (cells/μL) | 5-1000 | >100 (typically) | 5-500 | 5-500 |
| Cell type | Lymphocytes* | Neutrophils | Lymphocytes | Lymphocytes |
| Protein (g/L) | Mildly raised | Raised | Markedly raised | Raised |
| Glucose | Normal/slightly low | Very low | Very low | Low |
| CSF/plasma glucose ratio | Normal/slightly low | Very low | Very low | Low |
*May be neutrophilic in early disease, especially with enteroviral infections 1
Diagnostic Pearls and Pitfalls
- In approximately 5-10% of viral encephalitis cases (particularly HSV), initial CSF findings may be normal with no pleocytosis 4
- If the first CSF is normal in patients with suspected viral CNS infection, a second CSF examination 24-48 hours later is likely to show the characteristic abnormalities 4
- PCR testing of CSF for HSV (1 and 2), VZV, and enteroviruses should be performed in suspected viral meningitis, as these tests will identify 90% of cases due to known viral pathogens 4
- CSF red cell count is elevated in approximately 50% of HSV encephalitis cases due to its hemorrhagic nature 4
- A CSF lactate <2 mmol/L effectively rules out bacterial disease and supports viral etiology 4