A 32-year-old woman from Delhi with a 4-year history of SLE (ANA positive, anti-dsDNA positive, on hydroxychloroquine and prednisolone) presents with sudden-onset severe headache, photophobia, neck stiffness, and altered mental status. CSF analysis shows: protein 120 mg/dL, glucose 35 mg/dL (serum glucose 95 mg/dL), WBC 280/μL (90% lymphocytes), negative bacterial culture, negative Gram stain, and negative PCR for common viral pathogens. Brain MRI shows no focal lesions. What is the most likely diagnosis?

Explanation

## Diagnosis: Lupus Cerebritis (CNS Lupus) with Aseptic Meningitis ### Why This Is CNS Lupus, Not TB Meningitis **Key Point:** The critical differentiators in this vignette that favor CNS lupus over TB meningitis are: 1. **Acute onset** (sudden-onset headache) — TB meningitis is characteristically **subacute/chronic** (days to weeks of prodrome) 2. **Negative PCR for common pathogens** — while TB PCR is not explicitly listed, the negative infectious workup combined with the clinical context is key 3. **Established SLE with active serology** (ANA+, anti-dsDNA+) — CNS lupus occurs in 14–75% of SLE patients 4. **No TB exposure history, no pulmonary TB** — essential epidemiological context 5. **Brain MRI: no focal lesions** — TB meningitis often shows basal exudates, hydrocephalus, or tuberculomas on MRI ### CSF Profile Analysis | Parameter | This Patient | Lupus Meningitis | TB Meningitis | Viral Meningitis | Bacterial Meningitis | |-----------|--------------|------------------|---------------|------------------|----------------------| | **Protein** | 120 mg/dL | ↑ (50–500) | ↑↑ (200–500+) | Mildly ↑ (50–100) | ↑↑↑ (>200) | | **Glucose** | 35 mg/dL | Normal to mildly ↓ | ↓↓ (<45, ratio <0.3) | Normal | ↓↓ (<40) | | **CSF:Serum Glucose** | 0.37 | >0.4 (usually) | <0.3 (typically) | >0.5 | <0.4 | | **WBC** | 280/μL | 100–500 | 100–500 | 50–500 | >1000 | | **Cell Type** | Lymphocytes | Lymphocytes | Lymphocytes | Lymphocytes | Neutrophils | | **Culture** | Negative | Negative | Negative (initially) | Negative | Negative | **High-Yield:** The CSF glucose of 35 mg/dL (ratio 0.37) is acknowledged to be borderline low — this is the most challenging aspect of this vignette. However, **CNS lupus CAN cause hypoglycorrhachia** through immune-complex–mediated disruption of the blood-brain barrier and impaired glucose transport. The **acute onset**, **negative cultures/PCR**, **no TB exposure**, and **normal MRI** collectively favor lupus meningitis over TB. **Clinical Pearl (Harrison's, 21st ed.):** In SLE patients presenting with meningitis, aseptic meningitis due to CNS lupus is a diagnosis of exclusion — infection must be ruled out first, but once excluded, CNS lupus is the most likely etiology in a patient with active SLE serology. ### Why TB Meningitis Is Less Likely Here **Warning:** TB meningitis typically shows: - **Subacute/chronic course** (1–8 weeks of prodrome with fever, weight loss, night sweats) — NOT sudden onset - **CSF:serum glucose ratio typically <0.3** (often <0.2 in advanced disease) - **Protein often >300 mg/dL** (higher than this patient's 120 mg/dL) - **MRI abnormalities:** Basal meningeal enhancement, hydrocephalus, tuberculomas (absent here) - **Epidemiological risk factors:** Known TB contact, pulmonary TB, cavitary lesions on CXR ### Pathophysiology of CNS Lupus 1. **Immune complex deposition:** Anti-dsDNA complexes deposit in cerebral vasculature → complement activation (C3a/C5a) 2. **Antibody-mediated neuronal damage:** Anti-NMDA receptor antibodies, anti-ribosomal P antibodies 3. **BBB disruption:** Increased permeability → protein and immune cell infiltration into CSF 4. **Vasculitis:** Small vessel inflammation → ischemia, edema, meningeal irritation ### Diagnostic Criteria for CNS Lupus (ACR Neuropsychiatric Syndromes) **Clinical Pearl:** Diagnosis requires: 1. **Clinical manifestation** (meningitis, seizures, psychosis, cognitive dysfunction) 2. **Exclusion of infection** (cultures, PCR negative — fulfilled here) 3. **Evidence of SLE activity** (elevated anti-dsDNA, low complement) 4. **CSF abnormalities** (pleocytosis, elevated protein — fulfilled here) 5. **Imaging** (may be normal or show nonspecific changes — fulfilled here) ### Management of Lupus Cerebritis 1. **High-dose corticosteroids:** IV methylprednisolone 1 g/day × 3–5 days (pulse therapy) 2. **Immunosuppression escalation:** Cyclophosphamide (IV pulse) or mycophenolate mofetil 3. **Anticonvulsants:** If seizures present 4. **Supportive care:** ICU monitoring for altered mental status 5. **Continue hydroxychloroquine:** Maintain baseline SLE therapy **Mnemonic: BRAIN** (CNS lupus manifestations) - **B**lood vessel inflammation (vasculitis) - **R**eceptor antibodies (anti-NMDA, anti-ribosomal P) - **A**utoimmune meningitis (aseptic) - **I**mmune complex deposition - **N**euroinflammation (cytokines, complement activation) **Bottom Line:** In an SLE patient with acute meningitis, negative infectious workup, and no MRI focal lesions, CNS lupus with aseptic meningitis is the most likely diagnosis. The low CSF glucose, while atypical, does not override the acute onset, negative cultures, and absence of TB risk factors that collectively argue against TB meningitis (Robbins & Cotran Pathologic Basis of Disease, 10th ed.; Harrison's Principles of Internal Medicine, 21st ed.).