Most Likely Diagnosis: Viral Meningitis
Based on the CSF findings of lymphocytic predominance with normal glucose, viral meningitis is the most likely diagnosis in this IV drug user presenting with meningitis symptoms. 1
Diagnostic Reasoning
CSF Pattern Analysis
The combination of lymphocytic predominance and normal CSF glucose essentially excludes bacterial meningitis and points strongly toward viral etiology:
Bacterial meningitis characteristically shows neutrophil predominance (80-95%), not lymphocytic predominance, along with very low CSF glucose and markedly elevated protein 1, 2
Viral meningitis typically presents with lymphocytic pleocytosis (tens to hundreds of cells), normal or only slightly low glucose, and mildly elevated protein 1, 3
The CSF/plasma glucose ratio in viral meningitis remains normal or slightly low but stays above 0.36, whereas bacterial meningitis shows a ratio <0.36 and tuberculous meningitis shows a ratio <0.5 2, 4
Excluding Other Diagnoses
Tuberculous meningitis is unlikely despite the lymphocytic predominance because:
TB meningitis characteristically presents with very low CSF glucose (typically <2.2 mmol/L) and a CSF/plasma glucose ratio <0.5, which contradicts the normal glucose finding in this case 1, 4
TB meningitis also shows markedly raised protein (typically >1 g/L), which is not mentioned in this presentation 1, 4
Fungal meningitis is less likely because:
While fungal meningitis can present with lymphocytic predominance, it typically shows low CSF glucose and raised protein, not normal glucose 1
Fungal meningitis is more common in severely immunocompromised patients (advanced HIV, transplant recipients), and IV drug use alone does not confer the same level of risk 4
Critical Clinical Caveat
Early viral meningitis can show neutrophil predominance initially, particularly with enteroviral infections, but typically shifts to lymphocytic predominance within 12-24 hours 3, 5:
If there is any diagnostic uncertainty, repeat lumbar puncture in 12-24 hours will demonstrate a shift from polymorphonuclear to mononuclear cells, confirming viral etiology 2, 5
The total CSF white cell count in viral meningitis rarely exceeds 2000 cells/μL, unlike bacterial meningitis 2
Immediate Management Approach
Initiate IV acyclovir empirically until viral PCR results confirm the diagnosis and exclude HSV-2, which is the most important treatable cause of viral meningitis 2:
HSV-2 is more likely to cause viral meningitis syndrome, while HSV-1 typically causes meningoencephalitis 5
Early acyclovir treatment significantly improves outcomes, particularly in immunocompromised patients 2
Send CSF PCR immediately for HSV, VZV, and enteroviruses, which identifies approximately 90% of viral causes 2:
Nucleic acid amplification techniques are the diagnostic methods of choice, as they are more rapid and sensitive than traditional culture methods 6
PCR remains positive even when CSF microscopy is completely normal, which occurs in 5-10% of viral CNS infections 3, 7
Antibiotics are not indicated given the lymphocytic predominance and normal glucose, which essentially exclude bacterial meningitis 2:
- The only exception would be if the absolute CSF glucose is <2.6 mmol/L or CSF/plasma ratio is <0.36, in which case empiric therapy should be reconsidered 2