Topic 7. CNS Infections: Meningitis & Encephalitis

Clinical Connection 1 — CSF Pattern & Etiology

Acute bacterial infection → intense neutrophilic inflammation → neutrophilic CSF, markedly raised protein and reduced CSF glucose.

Viral meningitis → predominantly lymphocytic response → modest protein rise with usually preserved CSF glucose.

Tuberculous meningitis → chronic granulomatous inflammation → lymphocytic CSF, high protein and low glucose; basal enhancement may be visible on imaging.

Clinical Connection 2 — Neurological Clues & Complications

TB basal exudates → impaired CSF circulation and vessel inflammation → hydrocephalus, cranial nerve palsy or cerebral infarction.

HSV-related temporal lobe inflammation → cortical dysfunction → memory change, altered behavior and focal seizures → temporal MRI changes and HSV CSF PCR support diagnosis.

Clinical Connection 3 — Treatment Timing & Patient Safety

Suspected bacterial meningitis → rapid neurological deterioration risk → immediate empirical IV antibiotics; obtain cultures promptly without delaying treatment.

Suspected HSV encephalitis → potentially reversible viral brain injury → prompt IV acyclovir; monitor renal function and hydration.

⭐ 1. Bacterial CSF and urgent treatment: Marked neutrophilia + high protein + low CSF glucose → pyogenic meningitis is strongly suspected → prompt IV antibiotics are essential.

⭐ 2. Tuberculous meningitis and basal complications: Subacute illness + lymphocytic CSF with low glucose + basal inflammation → consider TB meningitis → monitor for hydrocephalus, vasculitic infarction and cranial nerve palsies.

⭐ 3. Encephalitis and HSV: Fever with altered behavior, impaired consciousness or focal seizures → brain parenchymal involvement → investigate with MRI and CSF HSV PCR; start acyclovir promptly when HSV is suspected.

4. Lumbar puncture safety: CSF analysis helps identify the cause → shock, significant bleeding risk or possible dangerous intracranial pressure gradient may make immediate LP unsafe → stabilize, assess and treat first.

5. Viral versus bacterial disease: Preserved CSF glucose and lymphocytic predominance support viral meningitis → uncomplicated cases often need supportive care, but clinical severity and other tests must exclude dangerous bacterial disease.

6. Pharmacology and follow-up: Antibiotics treat bacterial infection; acyclovir inhibits HSV replication; prolonged combination anti-TB therapy targets tuberculosis → monitor response and important drug adverse effects.

7. Pediatric recognition and recovery: Poor feeding, irritability, lethargy or bulging fontanelle may signal infant meningitis even without neck stiffness → early treatment and hearing/developmental follow-up help reduce long-term disability.

8. Complication-directed care: Cerebral edema or impaired CSF drainage → raised intracranial pressure and neurological deterioration → urgent monitoring, specialist escalation and CSF diversion when clinically indicated.

AIM EXAM TRAP — LP Versus Immediate Treatment

CSF examination is essential for diagnosis, but treatment must not wait for lumbar puncture. When bacterial meningitis is strongly suspected, obtain blood cultures promptly if feasible and begin empirical antibiotics. CT is required before LP only in selected clinical situations, and a normal CT does not guarantee that LP is safe.