Modes of Acquisition for Fungal Meningitis
Fungal meningitis is primarily acquired through inhalation of fungal spores from the environment, which then spread hematogenously to the central nervous system, though it can also occur through direct inoculation from trauma, neurosurgery, or contiguous spread from adjacent infections.
Primary Routes of Acquisition
1. Inhalation and Hematogenous Dissemination
- Most common route of acquisition for fungal meningitis
- Fungal spores are aerosolized and inhaled into the lungs
- Initial pulmonary infection occurs, which is often self-limited
- Subsequent hematogenous dissemination to the CNS occurs, particularly in:
- Immunocompromised patients
- Patients with underlying medical conditions
- Sometimes in immunocompetent individuals (depending on fungal species)
2. Direct Inoculation
- Less common route of acquisition
- Occurs through:
- Traumatic injury
- Neurosurgical procedures
- Lumbar puncture complications
- Contaminated medical devices or medications
3. Contiguous Spread
- Rare route of acquisition
- Local extension from adjacent infections:
- Paranasal sinusitis
- Mastoiditis
- Orbital infections
Risk Factors for Fungal Meningitis
Immunocompromised States
- HIV/AIDS
- Hematologic malignancies
- Organ transplant recipients
- Long-term corticosteroid therapy
- Cytotoxic chemotherapy
- Hyperalimentation
Other Risk Factors
- Injection drug use
- Recent neurosurgical procedures
- Presence of neurosurgical devices
- Travel to endemic areas for specific fungi
- Advanced age (particularly for Listeria, though bacterial)
Clinical Presentation Differences from Bacterial Meningitis
Fungal meningitis typically presents as a subacute or chronic process, unlike the acute presentation of bacterial meningitis 1. Key CSF findings that distinguish fungal from bacterial meningitis include:
| Parameter | Bacterial Meningitis | Fungal Meningitis |
|---|---|---|
| WBC count | 1000-5000/mm³ | 40-400/mm³ |
| Cell type | Predominantly neutrophils | Predominantly lymphocytes |
| Protein | High (>100 mg/dL) | Elevated |
| Glucose | Low (<40 mg/dL) | Low |
| CSF:blood glucose ratio | <0.4 | <0.5 |
Common Causative Fungi
Cryptococcus neoformans/gattii
- Most common cause of fungal meningitis worldwide
- Acquired through inhalation of spores from soil contaminated with bird droppings
Coccidioides immitis/posadasii
- Endemic to southwestern United States and parts of Central/South America
- Inhaled from soil in endemic regions
Histoplasma capsulatum
- Endemic to Ohio and Mississippi River valleys
- Inhaled from soil contaminated with bird or bat droppings
Blastomyces dermatitidis
- Endemic to southeastern and south-central states, Great Lakes, and St. Lawrence River
- Inhaled from moist soil rich in organic matter
Candida species
- Usually associated with neurosurgical procedures, indwelling catheters, or hematogenous spread
- Often hospital-acquired
Aspergillus species
- Typically causes focal brain abscesses rather than meningitis
- Inhaled from environment, particularly in construction areas
Diagnostic Approach
Diagnosis of fungal meningitis requires:
- CSF analysis with cell count, glucose, protein
- Fungal culture (may take weeks and have low sensitivity)
- Fungal antigen testing (particularly for Cryptococcus)
- PCR for fungal pathogens (increasingly available)
- Consideration of brain imaging to identify focal lesions
Treatment Considerations
The treatment of fungal meningitis depends on the causative organism but generally involves:
- Amphotericin B formulations as primary therapy for most fungal meningitis cases 3
- Azole antifungals (fluconazole, voriconazole) as step-down therapy or for specific fungi
- Extended duration of therapy (weeks to months)
- Monitoring for complications such as hydrocephalus
Prevention
- Avoid activities that disturb soil in endemic areas
- Use of HEPA filters in hospital rooms for highly immunocompromised patients
- Antifungal prophylaxis in selected high-risk patients
- Strict aseptic technique during neurosurgical procedures
Understanding the mode of acquisition is crucial for prevention strategies and for identifying patients at risk of fungal meningitis, which can be difficult to diagnose but devastating if not treated promptly.