What is the most likely cause and recommended initial management for cerebrospinal fluid showing a mononuclear (predominantly monocytes/macrophages) pleocytosis with normal‑to‑high glucose and elevated protein?

CSF with Mononuclear Pleocytosis, Normal-to-High Glucose, and Elevated Protein

The most likely diagnosis is viral encephalitis (particularly HSV, VZV, or enterovirus), though tuberculosis, fungal meningitis, partially treated bacterial meningitis, and certain non-infectious conditions (Lyme disease, sarcoidosis, malignancy) must be excluded based on clinical context.

Differential Diagnosis Framework

The CSF profile you describe—mononuclear (monocyte/macrophage) predominance with normal-to-high glucose and elevated protein—creates a specific diagnostic pattern:

Primary Considerations:

Viral Encephalitis is the leading diagnosis. HSV encephalitis typically shows moderate mononuclear pleocytosis (tens to hundreds of cells), mildly elevated protein, and normal CSF:plasma glucose ratio 1. While lymphocytes usually predominate, monocytes/macrophages can be prominent, especially in certain viral infections (VZV, EBV, CMV) 1. Recent evidence confirms that viral infections promote lymphocytic/mononuclear profiles through interferon and CXCR3 ligand pathways 2.

Critical caveat: Approximately 5-10% of HSV encephalitis cases have normal initial CSF findings, requiring repeat lumbar puncture at 24-48 hours 1.

Important Bacterial Mimics:

Tuberculosis, listeriosis, brucellosis, and partially treated acute bacterial meningitis can all produce lymphocytic/mononuclear CSF patterns 1. The key distinguishing features:

• TB meningitis: Usually shows LOW glucose (not normal-to-high), though early cases may have preserved glucose
• CSF lactate <2 mmol/L effectively rules out bacterial disease 1
• Clinical setting and higher protein levels typically suggest these bacterial etiologies

Fungal and Chronic Infections:

Fungal pathogens (Cryptococcus, Candida, Aspergillus) and mycobacteria produce mixed or mononuclear pleocytosis due to chronic antigen exposure 2. However, these typically present with LOW glucose, making them less likely with your described profile.

Non-Infectious Causes:

• Lyme meningoradiculitis: Pronounced mononuclear pleocytosis with T lymphocytes, plasma cells, and IgM-positive B cells 3
• Histiocytosis: Rare but important—monocytic meningitis with normal glucose can occur in Erdheim-Chester disease or CNS primary malignant histiocytosis 4
• Sarcoidosis, malignancy, autoimmune encephalitis: Must be considered when infectious workup is negative

Initial Management Algorithm

Immediate Actions (Within 1 Hour):

1. Start empiric IV acyclovir immediately for presumed HSV encephalitis—do not wait for PCR results 1

   • Dose: 10 mg/kg IV every 8 hours
   • HSV encephalitis has >95% sensitivity/specificity for CSF PCR between days 2-10 of illness 1

2. Obtain comprehensive CSF studies 1:

   • Opening pressure
   • Cell count with differential (correct for bloody tap if needed: subtract 1 WBC per 7000 RBCs)
   • Protein and glucose (with simultaneous plasma glucose)
   • Gram stain, bacterial culture
   • PCR for HSV-1, HSV-2, VZV, and enteroviruses (identifies 90% of viral cases) 1
   • AFB smear and TB culture (6 mL sample) if clinically indicated
   • Fungal studies if immunocompromised
   • Store additional sample for future testing

3. Neuroimaging before LP if contraindications exist:

   • Focal neurologic deficits, papilledema, immunocompromised state, seizures, altered consciousness
   • CT head to exclude mass effect; MRI brain preferred for encephalitis diagnosis

Diagnostic Refinement (24-48 Hours):

If initial CSF is non-diagnostic, repeat LP at 24-48 hours 1. This is particularly important because:

• Early HSV encephalitis may have normal initial CSF
• PCR sensitivity improves after day 2 of illness
• Cell differential may evolve from early neutrophil to later mononuclear predominance

Pathogen-Specific Testing Based on Clinical Context:

• Travel history/endemic areas: Arbovirus IgM (West Nile, Japanese encephalitis), rickettsial diseases
• Immunocompromised: CMV, EBV, JC virus, Toxoplasma
• Tick exposure: Lyme serology (acute and convalescent), ehrlichiosis
• Animal contact: Rabies (if appropriate exposure)
• Chronic symptoms: TB, fungal, sarcoidosis, malignancy workup

Adjunctive Studies:

• MRI brain with contrast: Look for temporal lobe involvement (HSV), brainstem lesions, leptomeningeal enhancement
• EEG: Temporal lobe periodic lateralizing epileptiform discharges suggest HSV 5
• Serum and CSF paired samples: For antibody index calculations if viral PCR negative

Treatment Modifications Based on Results:

• HSV/VZV positive: Continue acyclovir for 14-21 days
• Enterovirus positive: Supportive care (no specific antiviral)
• TB suspected: Add empiric anti-TB therapy (rifampin, isoniazid, pyrazinamide, ethambutol)
• Fungal confirmed: Amphotericin B ± flucytosine, then fluconazole
• All PCR negative with persistent symptoms: Consider autoimmune encephalitis panel, repeat LP, brain biopsy if deteriorating

Common Pitfalls to Avoid:

1. Delaying acyclovir while awaiting PCR results—mortality increases significantly with treatment delay
2. Assuming normal glucose excludes all bacterial causes—partially treated meningitis and some atypical bacteria can have preserved glucose
3. Single negative HSV PCR ruling out HSV encephalitis—repeat LP if high clinical suspicion
4. Not obtaining simultaneous plasma glucose—CSF glucose interpretation is impossible without it 1
5. Overlooking non-infectious causes when infectious workup is negative—particularly autoimmune encephalitis in younger patients

The key principle: Start empiric acyclovir immediately, obtain comprehensive CSF studies including viral PCR, and be prepared to repeat LP at 24-48 hours if initial results are non-diagnostic.