What are the characteristic cerebrospinal fluid (CSF) findings in viral meningitis?

Characteristic CSF Findings in Viral Meningitis

Viral meningitis typically presents with a CSF lymphocytic pleocytosis (5-1000 cells/μL), normal to mildly raised protein levels, and normal to slightly low CSF glucose with a normal CSF/plasma glucose ratio. 1

CSF Cell Count and Differential

• CSF white cell count in viral meningitis is typically elevated between 5-1000 cells/μL, which is generally lower than in bacterial meningitis 1
• Lymphocytes predominate in most cases of viral meningitis, though a predominance of neutrophils may be seen in early viral meningitis, especially with enteroviral infections 1, 2
• Even with neutrophil predominance early in the disease course, viral meningitis rarely presents with CSF white cell counts exceeding 2000 cells/μL 1
• A shift from polymorphonuclear leukocytes to mononuclear cells within 12 hours of symptom onset is characteristic of viral meningitis 2

CSF Biochemistry

• Opening pressure is typically normal or only mildly raised in viral meningitis, unlike bacterial meningitis where it is often significantly elevated 1
• CSF appearance is typically clear in viral meningitis, compared to turbid or cloudy in bacterial meningitis 1
• CSF protein is mildly raised in viral meningitis, but typically less than 0.6 g/L (values above this level are more suggestive of bacterial meningitis) 1
• CSF glucose is usually normal or only slightly decreased in viral meningitis 1
• CSF/plasma glucose ratio is typically normal or only slightly decreased, generally remaining above 0.36 (ratios below this level strongly suggest bacterial meningitis) 1
• CSF lactate is a valuable biomarker - levels below 3.5 mmol/L (or 35 mg/dL) strongly suggest viral rather than bacterial etiology 1, 3, 4

Diagnostic Considerations

• No single CSF parameter provides absolute differentiation between viral and bacterial meningitis; results must be interpreted in clinical context 1
• CSF lactate has high sensitivity (93%) and specificity (96%) for distinguishing between bacterial and viral meningitis when measured before antibiotic administration 1
• PCR testing of CSF for specific viral pathogens (HSV 1 & 2, VZV, and enteroviruses) can identify approximately 90% of viral causes 1
• In approximately 20% of cases, differentiation between viral and bacterial meningitis may be difficult based on initial CSF findings alone 3

Important Caveats

• Early viral meningitis, particularly enteroviral, may present with neutrophil predominance that can be mistaken for bacterial meningitis 1, 2
• If viral meningitis is suspected in a patient with initial neutrophil predominance, a repeat lumbar puncture within 12 hours showing a shift to lymphocyte predominance can help confirm the diagnosis 2
• Some viral causes (mumps, lymphocytic choriomeningitis virus, and herpes simplex) can occasionally present with low CSF glucose levels that mimic bacterial meningitis 2
• CSF findings may be atypical in up to 21.6% of viral meningitis cases, making biomarkers like CSF lactate particularly valuable 4
• If initial CSF findings are non-diagnostic, a second lumbar puncture performed 24-48 hours later is recommended 1