Streptococcus pneumoniae
Based on the CSF findings—cloudy appearance, elevated opening pressure (27 cm H2O), markedly elevated WBC count (3,013 cells/µL) with polymorphonucleocyte predominance, low glucose (30 mg/dL with CSF/blood glucose ratio of 0.27), and elevated protein (62 mg/dL)—this presentation is classic for bacterial meningitis, with Streptococcus pneumoniae being the most likely causative organism in an immunocompetent adult. 1, 2
Why Streptococcus pneumoniae is Most Likely
S. pneumoniae is the most common cause of community-acquired bacterial meningitis in adults, accounting for the majority of cases in immunocompetent patients. 1 The CSF profile strongly supports bacterial rather than viral, fungal, or mycobacterial etiology:
- CSF/blood glucose ratio of 0.27 (30/110) is highly predictive of bacterial meningitis, with optimal cutoff of 0.36 having 92.9% sensitivity and specificity for bacterial meningitis 3
- Polymorphonucleocyte predominance is characteristic of acute bacterial meningitis 2, 4
- Elevated protein (62 mg/dL) and elevated opening pressure (27 cm H2O) further support bacterial etiology 2
- Cloudy appearance indicates high bacterial burden 4
Patients with pneumococcal meningitis are more likely to present with seizures, focal neurological symptoms, and reduced consciousness compared to other bacterial causes. 1 The clinical presentation of altered mental status, headache, and neck stiffness aligns with this pattern.
Why Not the Other Organisms
Cryptococcus neoformans
- Typically presents with lymphocytic predominance, not polymorphonucleocytes 1
- More common in immunocompromised patients (HIV/AIDS with CD4+ <150 cells/µL) 1
- CSF glucose may be low but usually not as dramatically decreased
- Opening pressure often markedly elevated (>25 cm H2O), but the PMN predominance excludes this diagnosis
Herpes Simplex Virus
- Causes lymphocytic pleocytosis, not neutrophilic 2
- CSF glucose is typically normal or only mildly decreased 2
- RBC elevation and xanthochromia are characteristic features not mentioned here
- Clinical presentation includes temporal lobe involvement/encephalitis features
Mycobacterium tuberculosis
- Presents with lymphocytic predominance in CSF 2
- More subacute/chronic presentation (weeks), not acute as described
- CSF glucose is low but protein is typically markedly elevated (>100 mg/dL) 2
- More common in immunocompromised patients or those with TB exposure history
Critical Management Points
The empiric antibiotic regimen of ceftriaxone plus vancomycin was appropriately initiated, as this covers S. pneumoniae including penicillin-resistant strains. 1, 5 The addition of corticosteroids (dexamethasone) is recommended for suspected pneumococcal meningitis to reduce mortality and neurological sequelae. 1
Common pitfalls to avoid:
- Do not delay antibiotics for imaging or LP results—treatment should begin within 1 hour of presentation 1
- Prior antibiotic administration may modify CSF glucose and protein but typically does not significantly affect WBC count until >12 hours of treatment 6
- Normal CSF parameters do not rule out bacterial meningitis in immunocompromised patients 2
The CSF should be sent for Gram stain (positive in 82.5% of bacterial meningitis cases) and culture (positive in 88.2% of cases) to confirm the diagnosis and guide definitive antibiotic therapy. 7