A 32-year-old woman from Bangalore with a 3-year history of SLE (ANA positive, anti-dsDNA positive) presents with sudden onset of severe headache, confusion, and a generalized tonic-clonic seizure. She is afebrile. CT head is normal. Lumbar puncture reveals: CSF protein 120 mg/dL, glucose 55 mg/dL (serum glucose 95 mg/dL), WBC 45/μL (70% lymphocytes), RBC 8/μL, negative bacterial and viral cultures. What is the most likely diagnosis?

Explanation

## Diagnosis: Lupus Cerebritis with Aseptic Meningitis ### Clinical Context: CNS Involvement in SLE **Key Point:** Central nervous system (CNS) lupus occurs in 20–75% of SLE patients and includes lupus cerebritis, aseptic meningitis, transverse myelitis, and cognitive dysfunction. ### CSF Analysis Interpretation | Parameter | Finding | Significance | |-----------|---------|-------------| | **Protein** | 120 mg/dL (elevated) | Consistent with inflammation; not as high as bacterial meningitis (>200) | | **Glucose** | 55 mg/dL with serum 95 (ratio 0.58) | Low CSF:serum ratio suggests CNS inflammation; typical of viral/autoimmune, not bacterial (which is <0.4) | | **Cell count** | 45/μL, 70% lymphocytes | Lymphocytic pleocytosis typical of viral/autoimmune meningitis | | **RBC** | 8/μL | Mild hemorrhage or contamination; not significant | | **Cultures** | Negative (bacterial & viral) | Rules out infectious meningitis | **High-Yield:** The combination of **lymphocytic pleocytosis, elevated protein, low CSF:serum glucose ratio, and negative cultures** in a known SLE patient is diagnostic of lupus cerebritis. ### Pathophysiology of Lupus Cerebritis 1. **Immune complex deposition** in cerebral vasculature 2. **Complement activation** (C3, C4 consumption) 3. **Vasculitis** of small cerebral vessels 4. **Blood-brain barrier disruption** → increased protein and cells in CSF 5. **Neuronal dysfunction** → seizures, confusion, headache **Clinical Pearl:** Lupus cerebritis is a **diagnosis of exclusion** — infections (TB, bacterial, viral) and other CNS pathology must be ruled out first. ### CSF Findings in Lupus Cerebritis - **Protein:** 50–200 mg/dL (usually <150) - **Glucose:** Normal or mildly low (CSF:serum ratio 0.5–0.9) - **Cells:** 10–500/μL, predominantly lymphocytes - **LE cells:** May be present in CSF (rare, not routinely tested) - **Antinuclear antibodies:** May be detected in CSF - **Cultures:** Always negative ### Differential Diagnosis Flowchart ```mermaid flowchart TD A[Meningitis in SLE patient]:::outcome --> B{Cultures negative?}:::decision B -->|Yes| C{CSF glucose low?}:::decision B -->|No| D[Bacterial meningitis]:::urgent C -->|Mildly low 0.5-0.9| E[Lupus cerebritis likely]:::outcome C -->|Very low <0.4| F[Consider TB meningitis]:::urgent E --> G[MRI brain + CSF ANA]:::action G --> H[Treat with IV methylprednisolone + Cyclophosphamide]:::action ``` ### Why This Is Lupus Cerebritis (Not TB or HSV) **Lupus Cerebritis:** - CSF:serum glucose ratio 0.58 (mildly low, typical of autoimmune) - Lymphocytic pleocytosis - Negative cultures - Known SLE with positive anti-dsDNA - Acute onset with seizure and confusion **TB Meningitis (ruled out):** - CSF:serum glucose ratio would be <0.4 (very low) - CSF protein typically >200 mg/dL - Subacute presentation over weeks - AFB smear/culture would be positive (though may take days) **HSV Encephalitis (ruled out):** - Would typically show **hemorrhagic necrosis** on MRI (temporal lobe) - CSF would have higher RBC count (>500/μL) - PCR for HSV would be positive - Seizures are more focal ### Management of Lupus Cerebritis 1. **Immediate:** IV methylprednisolone 1 g daily × 3–5 days 2. **Induction:** Cyclophosphamide 500–1000 mg/m² IV monthly × 6 months 3. **Maintenance:** Azathioprine or mycophenolate mofetil 4. **Supportive:** Antiepileptics (levetiracetam preferred), management of seizures 5. **Monitoring:** Repeat CSF analysis, MRI brain, cognitive assessment **Key Point:** Early recognition and immunosuppression are critical to prevent permanent neurological damage.