MI6.1-3 | CNS Infections — Meningitis & Encephalitis — Summary & Reflection

REFLECT

Consider the CSF interpretation algorithm as a clinical decision tree. A patient presents with meningism. The LP is performed. CSF is turbid with 2,000 PMNLs, protein 2.5 g/L, glucose 1.1 mmol/L. Gram stain: no organisms seen (antibiotic already given by the referring centre). How do you proceed? What additional tests would you request? What empiric antibiotic cover is appropriate when no organism is identified? How does your management change if the patient is 60 years old versus 25 years old? This integration of CSF findings, clinical context, and microbiology — without culture confirmation — is the daily reality of infectious disease medicine.

KEY TAKEAWAYS

Meningitis is inflammation of the meninges; encephalitis involves brain parenchyma.

CSF pattern by aetiology:
• Bacterial — turbid, neutrophilic pleocytosis (>1000), very low glucose, very high protein
• Viral — clear, lymphocytic, normal glucose, mildly elevated protein
• TBM — clear/xanthochromic, lymphocytic, very low glucose, very high protein, cobweb clot
• Fungal — clear, lymphocytic, low glucose; India ink or CrAg positive

Gram stain findings → organism → antibiotic:
• Gram-negative diplococci (intracellular) → N. meningitidis → penicillin/ceftriaxone
• Gram-positive lancet-shaped diplococci → S. pneumoniae → ceftriaxone + dexamethasone
• Gram-positive rods → Listeria → ampicillin
• No organism, lymphocytic, low glucose → consider TBM → GeneXpert, ZN stain, ADA

Encephalitis in India:
• Japanese encephalitis (monsoon, Culex, Bihar/UP/Assam) → MAC-ELISA IgM
• Scrub typhus (South India, Orientia tsutsugamushi) → Weil-Felix, PCR
• HSV-1 → temporal lobe; treat with aciclovir urgently
• Rabies → universally fatal once symptomatic; prevention = vaccination

Key rule: Do not delay antibiotics for LP results if signs of raised ICP; blood culture + immediate empiric therapy.