A 32-year-old woman from Mumbai with a known diagnosis of SLE (diagnosed 2 years ago) presents with acute onset of severe headache, confusion, and a single episode of generalized tonic-clonic seizure. On examination, she is febrile (38.5°C), alert but disoriented to time and place, with neck stiffness. CSF analysis shows: protein 120 mg/dL, glucose 45 mg/dL (serum glucose 95 mg/dL), WBC 85 cells/μL (70% lymphocytes), negative bacterial culture, negative viral PCR for HSV-1/2 and enterovirus. Brain MRI shows no focal lesions or meningeal enhancement. Her current medications include hydroxychloroquine and prednisolone 10 mg daily. Which of the following is the most likely diagnosis?

Explanation

## Diagnosis: Lupus Cerebritis (CNS Lupus) with Aseptic Meningitis ### Clinical Context This patient has **neuropsychiatric lupus (NPL)**, a manifestation of CNS involvement in SLE that occurs in 20–75% of SLE patients. The acute presentation with seizure, headache, confusion, and meningismus in the setting of known SLE with negative infectious workup is pathognomonic for lupus cerebritis. ### CSF Profile Analysis | Parameter | Finding | Interpretation | |-----------|---------|----------------| | Protein | 120 mg/dL | Mildly elevated (normal <45); consistent with inflammation | | Glucose | 45 mg/dL (serum 95) | **Hypoglycorrhachia** (CSF:serum ratio 0.47); suggests CNS inflammation or immune complex deposition | | WBC | 85 cells/μL, 70% lymphocytes | Lymphocytic pleocytosis; rules out bacterial meningitis (which would be >1000 PMNs) | | Bacterial culture | Negative | Rules out bacterial meningitis | | Viral PCR | Negative | Rules out HSV-1/2 and enteroviral meningitis | | MRI | No focal lesions, no enhancement | Rules out focal infection, abscess, or mass lesion | **Key Point:** The **combination of seizure + headache + confusion + lymphocytic pleocytosis + hypoglycorrhachia + negative infectious workup** in a known SLE patient is diagnostic of lupus cerebritis. This is an **immune-mediated** process, not infectious. ### Pathophysiology of CNS Lupus ```mermaid flowchart TD A[SLE with autoimmune activation]:::outcome --> B[Anti-ribosomal P antibodies<br/>Anti-NR2 antibodies]:::outcome B --> C[Crossing blood-brain barrier<br/>Immune complex deposition in meninges]:::action C --> D[Complement activation<br/>Neuroinflammation]:::action D --> E{Clinical manifestation}:::decision E -->|Seizures| F[Cortical involvement]:::outcome E -->|Headache + meningismus| G[Meningitis/Aseptic meningitis]:::outcome E -->|Confusion| H[Encephalitis/Cognitive dysfunction]:::outcome E -->|Stroke| I[Vasculitis or thrombosis]:::outcome ``` **High-Yield:** Lupus cerebritis is mediated by **anti-ribosomal P and anti-NMDA receptor (NR2) antibodies**, which cross the blood-brain barrier and cause neuroinflammation. These are NOT infectious antibodies. ### Diagnostic Criteria for CNS Lupus According to ACR criteria, CNS lupus includes: - **Aseptic meningitis** (headache, meningismus, CSF pleocytosis with negative cultures) - **Seizures** (generalized or focal) - **Psychosis** (acute behavioral changes, hallucinations) - **Cerebritis** (encephalitis with cognitive decline) - **Transverse myelitis** - **Stroke** (vasculitis or thrombotic) This patient meets criteria for **aseptic meningitis + seizures**. ### Why Hypoglycorrhachia Occurs in Lupus Cerebritis Hypoglycorrhachia (low CSF glucose relative to serum) is typically seen in: - Bacterial meningitis (glucose <40 mg/dL, CSF:serum ratio <0.4) - Tuberculous meningitis - Fungal meningitis - **Lupus cerebritis** (immune complex consumption of glucose in CSF) In this case, the **negative cultures + lymphocytic pleocytosis + known SLE** make lupus cerebritis the diagnosis, not infection. **Clinical Pearl:** A CSF glucose of 45 mg/dL with negative bacterial culture in a SLE patient is lupus cerebritis until proven otherwise. The immune complexes and complement activation consume glucose in the CSF, mimicking bacterial meningitis but with sterile cultures. ### Management Treatment includes: 1. **High-dose corticosteroids** (methylprednisolone 1 g IV daily × 3–5 days, then oral prednisolone taper) 2. **Immunosuppression** (mycophenolate mofetil or cyclophosphamide for severe cases) 3. **Seizure prophylaxis** (levetiracetam or phenytoin) 4. **Exclude infection** (blood cultures, repeat CSF analysis if no improvement in 48–72 hours)