What are the possible causes of meningoencephalitis with cerebrospinal fluid showing a lymphocytic pleocytosis of >1000 white blood cells, markedly elevated protein (~1244 mg/dL), and normal glucose?

Differential Diagnosis: Severe Lymphocytic Meningoencephalitis

This CSF profile—with >1000 lymphocytes, markedly elevated protein (1244 mg/dL), and normal glucose—is atypical and demands urgent consideration of tuberculous meningitis, autoimmune encephalitis (particularly anti-GFAP), partially treated bacterial meningitis, or less commonly, severe viral encephalitis or neurobrucellosis.

Key Clinical Context

The CSF findings you describe are highly unusual because most viral encephalitis cases have <1000 WBC/mm³ 1. This extreme lymphocytic pleocytosis with massively elevated protein but preserved glucose creates a specific pattern that narrows the differential significantly.

Primary Diagnostic Considerations

1. Tuberculous Meningitis (Most Critical)

• This is the most dangerous diagnosis to miss given the mortality implications
• Classic CSF pattern: lymphocytic pleocytosis, markedly elevated protein (often >500 mg/dL), and LOW glucose
• However, early TB meningitis or partially treated cases can present with normal glucose initially
• The protein level of 1244 mg/dL is particularly concerning for TB, as this degree of elevation is uncommon in viral infections
• Look for: subacute onset over days-weeks, basilar meningeal enhancement on MRI, cranial nerve palsies, hydrocephalus, history of TB exposure or immunosuppression

2. Autoimmune Encephalitis (Anti-GFAP)

• Anti-GFAP encephalitis specifically presents with marked lymphocytic pleocytosis and can have WBC counts >1000 2
• Often follows viral prodrome, which may explain initial infectious presentation
• Look for: linear perivascular radial gadolinium enhancement on MRI, rapid progression to coma, preceding viral-like illness
• CSF anti-GFAP antibodies are diagnostic
• Critical: This requires immunotherapy (IVIG, steroids, plasma exchange, rituximab) rather than antimicrobials 2

3. Partially Treated Bacterial Meningitis

• Prior antibiotic exposure can convert neutrophilic to lymphocytic predominance
• Protein elevation of this magnitude can occur with bacterial meningitis
• Crucially, bacterial meningitis with normal CSF glucose is rare but documented, particularly in immunocompromised patients 3
• CSF Gram stain and culture remain essential—bacteria may still be visible even with normal WBC counts in some cases 3

4. Severe Viral Encephalitis (Less Likely)

• While most viral encephalitis has <1000 WBC 1, exceptions exist
• HSV encephalitis typically has lower cell counts but can occasionally present atypically
• Important: 23.7% of HSV-1 encephalitis cases have NO pleocytosis 4, so the presence of >1000 cells doesn't exclude HSV but makes it less typical
• Normal glucose favors viral over bacterial, but protein of 1244 is unusually high for typical viral causes

5. Neurobrucellosis

• Endemic regions: consider in stockmen, shepherds, or those with animal exposure 5
• Typically presents with mild CSF pleocytosis (<50 cells common), but severe cases can occur
• Protein >500 mg/dL is rare in neurobrucellosis 5, making this less likely with protein of 1244
• Normal to mildly decreased glucose

Critical Diagnostic Algorithm

Immediate actions while awaiting specific test results:

1. Start empiric therapy immediately covering:

   • HSV: Acyclovir 10 mg/kg IV q8h (do NOT delay for lack of pleocytosis 4)
   • TB: If any clinical suspicion (subacute course, basilar enhancement, immunosuppression), start RIPE therapy
   • Bacterial: If any concern for partially treated bacterial meningitis, continue/start appropriate antibiotics

2. Send CSF for:

   • HSV PCR (sensitivity 96-98% 6)
   • TB: AFB smear, culture, GeneXpert/PCR, adenosine deaminase
   • Autoimmune panel: anti-GFAP, anti-NMDAR, other neuronal antibodies
   • Bacterial culture (even if prior antibiotics given)
   • Brucella serology if epidemiologically relevant

3. MRI brain with contrast:

   • Temporal lobe involvement → HSV
   • Basilar meningeal enhancement, hydrocephalus → TB
   • Linear perivascular enhancement → anti-GFAP 2

4. Clinical history priorities:

   • Timeline: hyperacute (hours) vs acute (days) vs subacute (weeks)
   • Prior antibiotics (any amount can alter CSF profile)
   • Immunosuppression status
   • TB exposure or risk factors
   • Recent viral illness (suggests autoimmune)
   • Animal/occupational exposure (brucellosis)

Common Pitfalls

• Do not withhold acyclovir based on atypical CSF findings—HSV can present without pleocytosis 4 and delayed treatment increases mortality
• Do not assume viral based on lymphocytic predominance and normal glucose alone—TB and autoimmune causes are critical alternatives
• Do not wait for culture results to start TB therapy if clinical suspicion exists—TB meningitis has high mortality if treatment is delayed
• The protein level of 1244 mg/dL is exceptionally high and should raise suspicion beyond typical viral causes 1

Immunocompromised Patients

If the patient is immunocompromised, expand differential to include:

• CMV encephalitis (though typically has minimal or no pleocytosis 7)
• Cryptococcal meningitis (check CSF cryptococcal antigen, India ink)
• Toxoplasmosis
• Lymphomatous meningitis

The combination of >1000 lymphocytes with protein >1200 mg/dL and normal glucose is sufficiently atypical that empiric coverage for TB and autoimmune causes should be strongly considered while pursuing definitive diagnosis 1, 6, 2.