Acute Meningitis

Indications for Testing

Fever, headache, altered sensorium, and stiff neck

Laboratory Testing

• CDC meningitis overview
• Initial testing is nonspecific – CBC, electrolytes
  • Normal WBC does not rule out meningitis
• Cerebrospinal fluid (CSF) exam – necessary to determine presence of meningitis
  • CSF opening pressure – limited value if normal; usually >300 mm in bacterial but ≤300 in all others
  • Microscopic exam – white count >1,000 cells/µL in >90% of patients with bacterial meningitis
    • Neutrophils (typically >80%) usually predominate in bacterial meningitis
      • Lymphocytes/monocytes predominate in viral and fungal meningitis
        • Early viral disease may have ≥50% neutrophils, but shift toward lymphocytes/monocytes
      • Immunocompromised patients may not demonstrate elevated WBC results in bacterial meningitis
    • Immunocompromised patients and those with Listeria monocytogenes may have normal CSF WBC results
      • Listeria may also present with normal WBC results and high protein CSF
    • Normal WBC result does not exclude bacterial meningitis
  • Protein – usually elevated (>200 mg/dL) in bacterial and fungal meningitis; usually <200 in viral
  • Glucose – usually low (<10mg/dL) in bacterial and tuberculous meningitis; normal to minimally low in viral and fungal meningitis
  • Gram stain – useful if positive
  • Culture
    • Bacterial culture – gold standard for diagnosis of bacterial meningitis
      • Anaerobic culture may be important for post-neurosurgical meningitis or shunt meningitis
    • Fungal and AFB cultures require HIGH VOLUME taps (at least 10cc fluid)
    • Viral culture from CSF not indicated
  • CSF antigen antibody testing, when appropriate (eg, pneumococcal antigen, dimorphic fungi serology, cryptococcal antigen)
• PCR testing for enterovirus, Epstein-Barr virus, herpes simplex virus, varicella-zoster virus, cytomegalovirus, arboviruses
• Blood cultures – may be positive in up to 2/3 of patients in Western countries in bacterial meningitis
  • Less sensitive if antibiotics have been administered
• Other tests to consider
  • Rapid serum HIV antibody and plasma viral load testing – rule out acute HIV infection
    • Use for patients with risk factors and aseptic meningitis
  • RPR – rule out syphilis
  • Urinalysis – may reveal urinary tract infection as etiology of bacteremia
  • Malaria blood film – in areas where malaria is endemic
    • Has a negative predictive value of 98%

Imaging Studies

• If focal findings are present or patient is significantly immunocompromised, consider CT/MRI prior to CSF tap
• Chest x-ray – may be useful in diagnosing pneumonia as etiology (usually S. pneumoniae)

Differential Diagnosis

• Encephalitis
• Connective tissue CNS disease
  • Sjögren syndrome
  • Systemic lupus erythematosus
• Vasculitic CNS disease
  • Behçet disease
  • Granulomatosis with polyangiitis
• Granulomatous CNS disease
  • Sarcoidosis
• Migraine headache
• Malignant meningitis
  • Carcinoma
  • Lymphoma
    • B-cell
    • T-cell
  • Leukemia
    • Acute lymphoblastic leukemia
    • Acute myeloid leukemia
• Subarachnoid hemorrhage
• Stroke
• Seizure disorder
• Febrile seizures in children
• Fabry disease
• Medications – numerous drugs can cause aseptic meningitis