Fungal Meningitis (Cryptococcal)
The most likely diagnosis is fungal meningitis, specifically cryptococcal meningitis, given the IV drug use history, severe hypoglycemia (blood glucose <0.4), lymphocytic predominance, and CSF glucose at the lower end of normal. 1
Critical Diagnostic Reasoning
Why Fungal (Cryptococcal) Meningitis is Most Likely
IV drug use is a major risk factor for HIV infection and subsequent opportunistic infections, with cryptococcal meningitis being the most common fungal CNS infection in immunocompromised patients with CD4 counts <200. 1
The severe systemic hypoglycemia (blood glucose <0.4) is a critical clue that distinguishes this case from typical viral meningitis, suggesting a more severe systemic illness consistent with disseminated fungal infection or underlying immunocompromise. 2
Lymphocytic predominance with hypoglycorrhachia (CSF glucose 250 mg/L = 25 mg/dL, which is markedly low) creates a pattern consistent with fungal meningitis, which typically shows lymphocytic pleocytosis with very low CSF glucose and markedly elevated protein. 3, 4
The CSF/blood glucose ratio is critically low (<0.1 in this case), far below the 0.36 cutoff that distinguishes bacterial from viral causes, and even lower than typical bacterial meningitis, suggesting fungal etiology. 5
Why Other Diagnoses Are Excluded
Viral meningitis (Option A) is unlikely because:
- Viral meningitis characteristically maintains normal CSF glucose with CSF:plasma ratio of 0.6-0.7, not the severe hypoglycorrhachia seen here. 3, 6
- While viral meningitis can occasionally show low glucose (mumps, LCM, HSV), the CSF glucose is typically >50% of blood glucose, not the <10% seen in this case. 6, 7
- The severe systemic hypoglycemia is not explained by viral meningitis. 2
Tuberculous meningitis (Option B) is possible but less likely because:
- TB meningitis does cause lymphocytic predominance and hypoglycorrhachia, but the acute presentation and IV drug use history make cryptococcal infection more probable in this demographic. 1, 8
- TB meningitis typically has a more subacute course over weeks, not the acute presentation implied here. 1
Bacterial meningitis (Option C) is excluded because:
- Bacterial meningitis typically shows neutrophil predominance (not lymphocytic) in 64% of cases. 3
- While bacterial meningitis causes hypoglycorrhachia, the lymphocytic predominance argues strongly against typical bacterial pathogens. 5, 4
- The CSF/blood glucose ratio of <0.36 does suggest bacterial disease, but combined with lymphocytic predominance, this points toward atypical organisms like fungi. 5
Critical Clinical Caveats
Empiric antibiotics including ampicillin must be started immediately while awaiting definitive diagnosis, as bacterial meningitis (including Listeria, which can present with lymphocytic predominance) cannot be definitively excluded and carries 20-30% mortality if untreated. 3, 9
HIV testing should be performed urgently given the IV drug use history, as this fundamentally changes the differential diagnosis and management. 1
Cryptococcal antigen testing and India ink staining of CSF should be performed immediately, as these have high sensitivity and specificity for cryptococcal meningitis. 1
Immediate Management Algorithm
Start empiric therapy immediately with ceftriaxone 2g IV q12h + vancomycin 15-20 mg/kg IV q8-12h + ampicillin 2g IV q4h (for Listeria coverage in IV drug users). 9
Add empiric antifungal therapy with amphotericin B 0.7-1.0 mg/kg/day + flucytosine 25 mg/kg q6h if cryptococcal meningitis is strongly suspected based on clinical presentation. 1
Obtain CSF cryptococcal antigen, India ink stain, fungal culture, and bacterial cultures to guide definitive therapy. 1
Perform HIV testing and CD4 count measurement urgently. 1
Measure opening pressure on lumbar puncture, as elevated opening pressure (>25 cm H2O) is common in cryptococcal meningitis and requires therapeutic drainage. 1