What is the most likely diagnosis for a patient with lymphocytic predominance in CSF, worsening headache, confusion, and cough?

Viral Meningitis

Based on the CSF findings of lymphocytic predominance (64%), mildly elevated protein (56 mg/dL), and normal glucose (68 mg/dL with CSF:plasma ratio of 0.63), this patient most likely has viral meningitis. 1

Key CSF Findings Supporting Viral Meningitis

The CSF profile demonstrates classic features of viral meningitis:

• Lymphocytic predominance (64%): While bacterial meningitis typically shows neutrophil predominance, viral meningitis characteristically presents with lymphocytic pleocytosis 1
• Moderate WBC elevation (410 cells/mm³): Falls within the typical range of 5-1000 cells/µL for viral meningitis, and patients with viral meningitis are unlikely to exceed 2000 cells/mm³ 1, 2
• Normal CSF glucose (68 mg/dL) with preserved CSF:plasma ratio (0.63): The normal CSF glucose is highly characteristic of viral infection, as bacterial meningitis typically shows very low glucose with CSF:plasma ratio <0.36 1, 3
• Mildly elevated protein (56 mg/dL): Consistent with viral meningitis, which shows only mild protein elevation compared to the marked elevation seen in bacterial meningitis 1, 2

Why Other Diagnoses Are Less Likely

Bacterial meningitis is unlikely because:

• The CSF:plasma glucose ratio of 0.63 is well above the 0.36 cutoff that indicates bacterial disease 1
• While lymphocytic predominance can occur in bacterial meningitis (particularly Listeria or partially treated cases), this represents only a minority of cases 1, 3
• The protein elevation is mild rather than markedly elevated 1

Fungal meningitis is less likely because:

• Fungal infections typically present with very low CSF glucose and markedly elevated protein 1
• The clinical presentation lacks mention of immunocompromised state or subacute/chronic course typical of fungal meningitis 3

Subarachnoid hemorrhage is excluded by:

• Zero RBCs in the CSF (0 cells/mm³) 1

Migraine with aura is excluded by:

• The presence of significant CSF pleocytosis and confusion, which are not features of migraine 1

Critical Clinical Caveat

Despite the viral pattern, empiric antibiotics including ampicillin should be started immediately if bacterial meningitis cannot be definitively excluded, particularly given the patient's confusion. 3 The four-day history of cough raises concern for atypical presentations, and Listeria monocytogenes can present with lymphocytic predominance and accounts for 5% of bacterial meningitis overall (20-40% in immunocompromised, elderly, or diabetic patients) 1, 3. Listeria is resistant to cephalosporins, making ampicillin coverage essential if the patient has risk factors 1, 3.

Recommended Next Steps

• CSF PCR testing for HSV-1, HSV-2, VZV, and enteroviruses should be performed immediately, as this identifies 90% of viral meningitis cases 1
• CSF lactate measurement can help differentiate: a level <2 mmol/L effectively rules out bacterial disease 1, 3
• Repeat lumbar puncture at 24-48 hours if initial CSF PCR is negative and clinical suspicion remains high 1
• Brain MRI with contrast to evaluate for parenchymal involvement or alternative diagnoses 3