Meningitis Study Guide

📖 Core Concepts Meningitis: Acute or chronic inflammation of the meninges (pia, arachnoid, dura) that surround the brain and spinal cord. Lumbar puncture (LP): Gold‑standard test; analyzes cerebrospinal fluid (CSF) for pressure, appearance, cell count, glucose, lactate, and microbiology. Empiric therapy: Broad‑spectrum antibiotics + adjunctive dexamethasone started before a definitive pathogen is known. CSF glucose ratio: CSF glucose / serum glucose ≤ 0.4 strongly suggests bacterial meningitis. Classic triad (neck stiffness, fever, altered mental status) is present in < 50 % of bacterial cases—don’t rely on it alone. 📌 Must Remember Mortality: ≤ 15 % with prompt appropriate therapy; overall ≈ 16.7 % worldwide. Empiric antibiotics (US): Ceftriaxone + vancomycin ± ampicillin (≤ 2 mo, > 50 yr, immunocompromised). Corticosteroid timing: Dexamethasone before or with the first antibiotic dose; continue 4 days; stop if non‑pneumococcal pathogen identified. CSF findings: Bacterial → neutrophil‑predominant pleocytosis, ↑ protein, ↓ glucose (≤ 0.4 ratio), ↑ opening pressure. Viral → lymphocyte‑predominant, normal‑to‑slightly ↓ glucose, normal pressure. Cryptococcal → ↑ opening pressure, cloudy CSF, India‑ink positive. Vaccines that prevent meningitis: Hib, PCV/PPV, meningococcal conjugate (A, C, W, Y, B), MMR (mumps), BCG (TB). 🔄 Key Processes Initial assessment Recognize fever + headache + neck stiffness → suspect meningitis. Check for contraindications to LP (mass lesion, high ICP) → obtain head CT first if present. Lumbar puncture Measure opening pressure (6–18 cm H₂O normal). Collect tubes for cell count, chemistry, Gram stain, culture, PCR, antigen tests. Empiric antimicrobial initiation Give ceftriaxone + vancomycin (± ampicillin) immediately after cultures are drawn. Administer dexamethasone ≤ 15 min before the first antibiotic dose. Targeted therapy Narrow antibiotics based on Gram stain, culture, or PCR results. Add rifampicin, ciprofloxacin, or ceftriaxone for close contacts of meningococcal cases (single dose). Adjunctive management Monitor ICP; treat ↑ICP with mannitol/hypertonic saline or external ventricular drain. Treat seizures, correct hyponatremia, provide supportive fluids. 🔍 Key Comparisons Bacterial vs. Viral CSF Cells: Neutrophils ↑ (bacterial) vs. lymphocytes ↑ (viral). Glucose: ≤ 0.4 ratio (bacterial) vs. normal ratio (viral). Opening pressure: Often ↑ in bacterial/cryptococcal, normal‑to‑slightly ↑ in viral. Empiric regimens (US vs. UK) US: Ceftriaxone + vancomycin ± ampicillin. UK: Cefotaxime (or ceftriaxone) ± vancomycin; ampicillin added for high‑risk groups. Corticosteroid benefit Pneumococcal meningitis → clear reduction in hearing loss & mortality. Meningococcal/viral → minimal or no benefit. ⚠️ Common Misunderstandings “Classic triad must be present” → Only 45 % have it; early suspicion based on any combination of fever, headache, neck stiffness is essential. “Normal glucose rules out bacterial” → Early bacterial meningitis may have normal glucose; rely on the full CSF profile. “Antibiotics can wait for culture results” → Delaying empiric antibiotics > 1 h increases mortality; start immediately after cultures. “Corticosteroids are always indicated” → Not recommended for viral meningitis or low‑resource settings without proven benefit. 🧠 Mental Models / Intuition “CSF cellular fingerprint”: Neutrophils → bacterial; lymphocytes → viral; eosinophils → parasitic/fungal. “Time‑is‑brain”: Every hour of delayed antibiotics adds 5 % mortality risk. “Barrier breach pathway”: Bacteria → bloodstream → hematogenous spread → meninges → cytokine‑driven edema → ↑ICP. 🚩 Exceptions & Edge Cases Prior antibiotics → Gram stain sensitivity drops to 40 %; rely more on PCR/antigen tests. Immunocompromised patients → Higher risk for Listeria (add ampicillin) and fungal pathogens. Neonates (≤ 3 mo) → Common pathogens: Group B Strep, E. coli K1, Listeria → require ampicillin + 3rd‑gen cephalosporin. Low‑income pediatric settings → Corticosteroids have not shown benefit; may be omitted. 📍 When to Use Which Empiric antibiotics → All suspected cases after LP (or after CT if needed). Dexamethasone → Adults & high‑income children with suspected bacterial meningitis before first antibiotic; stop if non‑pneumococcal pathogen identified. Ampicillin addition → Age ≤ 2 months, > 50 years, or immunocompromised (covers Listeria). Antiviral (acyclovir) → HSV‑2 or VZV meningitis confirmed or strongly suspected. Antifungal (amphotericin B + flucytosine) → Confirmed cryptococcal meningitis. 👀 Patterns to Recognize Petechial rash + rapid deterioration → Think Neisseria meningitidis. Bulging fontanelle + irritability in infant → Possible meningitis; don’t wait for classic signs. CSF lactate > 35 mg/dL → Strong indicator of bacterial infection, even if glucose is borderline. Seasonal dry‑air dust storms in Sub‑Saharan Africa → High risk for meningococcal epidemics. 🗂️ Exam Traps “If CSF glucose is normal, bacterial meningitis is ruled out.” – Early bacterial cases may still have normal glucose. “All patients need dexamethasone.” – Only indicated for bacterial meningitis in high‑income settings and when given before antibiotics. “Vancomycin alone is sufficient for all adults.” – Must be paired with a 3rd‑gen cephalosporin to cover S. pneumoniae and N. meningitidis. “A negative Gram stain excludes bacterial meningitis.” – Prior antibiotics can produce false‑negative stains; PCR and culture still required. “Meningococcal vaccine eliminates need for prophylaxis of contacts.” – Close contacts still receive rifampicin/cipro/ceftriaxone single dose.