A 32-year-old woman from Mumbai with a 3-year history of SLE (ANA+, anti-dsDNA+, anti-Sm+) on hydroxychloroquine and prednisolone presents with acute-onset severe headache, confusion, and seizures. MRI brain shows no acute infarct or hemorrhage. CSF analysis reveals: protein 120 mg/dL (normal <45), glucose 52 mg/dL (serum glucose 95 mg/dL), WBC 85/μL (80% lymphocytes), Gram stain and culture negative. Which of the following is the most likely diagnosis?

Explanation

## Diagnosis: Lupus Cerebritis (CNS Lupus) ### Clinical Context **Key Point:** CNS lupus (neuropsychiatric lupus) occurs in 14–75% of SLE patients and is a major cause of morbidity. Cerebritis presents with acute neurological symptoms (headache, seizures, confusion, psychosis, stroke) in the setting of active SLE. ### CSF Profile Analysis | Parameter | Finding | Interpretation | |-----------|---------|----------------| | Protein | 120 mg/dL (elevated) | Indicates BBB disruption; consistent with inflammation | | Glucose | 52 mg/dL (low); CSF:serum ratio ~0.55 | Hypoglycorrhachia suggests CNS inflammation or infection | | WBC | 85/μL, 80% lymphocytes | Lymphocytic pleocytosis; rules out acute bacterial meningitis | | Gram stain, culture | Negative | Excludes bacterial and fungal infection | | Imaging | No infarct/hemorrhage | Excludes stroke or hemorrhage | **High-Yield:** The combination of **lymphocytic pleocytosis + elevated protein + hypoglycorrhachia + negative cultures** in an SLE patient = lupus cerebritis until proven otherwise. ### Pathophysiology of CNS Lupus ```mermaid flowchart TD A[SLE with active disease<br/>High anti-dsDNA, low C3/C4]:::outcome --> B[Immune complex deposition<br/>in cerebral vasculature]:::outcome B --> C[Complement activation<br/>C1q, C3, C4 consumption]:::outcome C --> D[Vasculitis + BBB disruption]:::outcome D --> E{Mechanism of CNS injury}:::decision E -->|Vasculitis| F[Cerebral infarction<br/>TIA, stroke]:::urgent E -->|Inflammation| G[Cerebritis, meningitis<br/>Seizures, confusion]:::urgent E -->|Thrombosis| H[Antiphospholipid syndrome<br/>Stroke, thromboembolism]:::urgent G --> I[CSF: elevated protein<br/>lymphocytic pleocytosis<br/>low glucose]:::outcome ``` ### Diagnostic Criteria for CNS Lupus **Mnemonic:** **BRAIN** — Behavioral changes, Reversible encephalopathy, Acute confusional state, Inflammation (meningitis), Neuropsychiatric manifestations. 1. **Clinical:** Acute neuropsychiatric symptoms in SLE patient 2. **CSF:** Lymphocytic pleocytosis, elevated protein, low glucose, negative cultures 3. **Imaging:** May show vasculitis, infarcts, or be normal 4. **Serology:** High anti-dsDNA, low complement (active SLE) 5. **Exclusion:** Rule out infection (TB, bacterial, fungal, viral) ### Why This Is NOT Other Diagnoses **Bacterial Meningitis:** Presents with **neutrophilic** pleocytosis (not lymphocytic), markedly elevated protein (>200 mg/dL), very low glucose (<40 mg/dL with CSF:serum <0.4), and positive Gram stain or culture. This patient has lymphocytic predominance and negative cultures. **Tuberculous Meningitis:** Also causes lymphocytic pleocytosis and low glucose, BUT: - TB meningitis typically has **higher protein** (200–500 mg/dL) and **lower glucose** (<20 mg/dL) - Presents subacutely over weeks, not acutely - Requires TB risk factors or pulmonary TB on imaging - AFB smear and TB culture (though slow) would eventually be positive **Viral Meningitis:** Causes lymphocytic pleocytosis but **normal or near-normal glucose** (CSF:serum >0.4). Hypoglycorrhachia is not typical of viral meningitis. **Clinical Pearl:** Hypoglycorrhachia (low CSF glucose with normal serum glucose) is a red flag for either bacterial meningitis, TB meningitis, or **CNS lupus**. In an SLE patient, lupus cerebritis is the most likely diagnosis. **Warning:** Do not assume infection in an immunosuppressed SLE patient — lupus cerebritis is more common than opportunistic infection in this setting. Always exclude infection first, but if cultures are negative and SLE is active, treat for CNS lupus.