Viral Encephalitis (Most Likely HSV) - Initiate Acyclovir Immediately
This CSF profile (glucose 48 mg/dL, protein 28 mg/dL, mononuclear pleocytosis) is most consistent with viral encephalitis, particularly HSV encephalitis, and you should start empiric acyclovir immediately while awaiting confirmatory HSV PCR testing. 1
CSF Profile Analysis
The presented CSF findings are classic for viral CNS infection:
- Normal glucose (48 mg/dL is normal, assuming plasma glucose ~60-100 mg/dL - though you must verify with simultaneous plasma glucose) 1
- Mildly elevated or normal protein (28 mg/dL is actually normal to minimally elevated)
- Mononuclear (lymphocytic) pleocytosis - the hallmark of viral encephalitis 2, 1
This pattern strongly suggests viral encephalitis rather than bacterial meningitis, which typically shows:
- Low CSF:plasma glucose ratio
- Markedly elevated protein (usually >100 mg/dL)
- Often neutrophilic predominance (though partially treated bacterial meningitis can mimic viral patterns) 1
Differential Diagnosis Priority
Most Likely: HSV Encephalitis
HSV encephalitis is the most critical diagnosis to consider because:
- It's the most common cause of sporadic viral encephalitis in adults 1
- Untreated HSV encephalitis has 70% mortality - this is why immediate treatment is essential
- CSF findings in HSV typically show moderate pleocytosis (tens to hundreds of cells), mildly elevated protein, and normal glucose ratio 1
Other Viral Causes to Consider:
- VZV, enteroviruses - together with HSV, these account for 90% of identified viral encephalitis cases 1
- West Nile virus - can show persistent neutrophilic pleocytosis (atypical) 2, 3
Important Bacterial Mimics:
The mononuclear pleocytosis pattern can also occur with:
- Tuberculous meningitis - usually shows lower glucose and higher protein than your case 1, 4
- Listeria, brucellosis - less common 1
- Partially treated bacterial meningitis - critical not to miss 1
CSF lactate <2 mmol/L essentially rules out bacterial disease if available 1
Immediate Management Algorithm
1. Start Acyclovir NOW
- Do not wait for PCR results to initiate treatment
- HSV PCR sensitivity is 96-98% but can be falsely negative early in disease 2
- 5-10% of HSV encephalitis cases have normal initial CSF with negative PCR 1
- Critical pitfall: Absence of pleocytosis does not rule out HSV encephalitis - 23.7% of HSV encephalitis patients lack pleocytosis, yet delayed acyclovir was associated with worse outcomes 5
2. Send Comprehensive CSF Testing
- HSV-1 and HSV-2 PCR (mandatory) 1
- VZV PCR 1
- Enterovirus PCR 1
- Bacterial culture and gram stain
- Mycobacterium tuberculosis testing (6 mL) if clinical suspicion 1
- Consider CSF lactate to help exclude bacterial meningitis 1
3. Verify Glucose Interpretation
You must obtain simultaneous plasma glucose to properly interpret the CSF glucose of 48 mg/dL 1. Without this, you cannot determine if the CSF:plasma glucose ratio is normal or reduced. This is a commonly missed step that makes CSF interpretation difficult 1.
4. If Initial LP Non-Diagnostic
Repeat LP in 24-48 hours - HSV PCR often converts from negative to positive, and pleocytosis may develop 1
Clinical Context Matters
Direct further testing based on:
- Travel history (arbovirus, TB exposure)
- Immunocompromised status (CMV, EBV, VZV more likely; acellular CSF more common) 2, 1
- Seasonal/geographic factors (West Nile virus, tick-borne diseases)
- Animal/insect exposure 1
- Age (younger patients more likely to have neutrophilic pleocytosis with enterovirus) 6
Key Pitfalls to Avoid
- Don't delay acyclovir waiting for "more typical" CSF findings - 25% of all-cause encephalitis lacks pleocytosis 5
- Don't assume normal CSF rules out HSV - up to 10% of HSV encephalitis has normal initial CSF 1
- Don't forget simultaneous plasma glucose - CSF glucose interpretation is meaningless without it 1
- Don't dismiss TB meningitis - it can present with similar findings but usually has lower glucose and higher protein 1, 4