High Risk, Low Prevalence: Meningitis

Bacterial meningitis is uncommon but time-critical, with the classic triad present in only about 40% of cases and mortality still around 10% to 30%. Early antibiotics, selective CT before lumbar puncture, and prompt recognition of elevated intracranial pressure drive the first hours of care.

Recognizing Bacterial Meningitis

• Classic triad limits: Fever, neck stiffness, and altered mental status appear together in only about 40% of bacterial meningitis, so absence of the full triad should not lower your suspicion.
• High-yield symptom pattern: Headache is present in roughly 90% of cases, and about 95% of patients have at least two of the triad features, a more useful bedside frame than waiting for the textbook picture.
• Meningeal sign pitfalls: Kernig and Brudzinski signs are not sensitive, and jolt accentuation performs too inconsistently to rule out disease. We get into the bedside exam nuance in the episode.
• Mimics and tempo: Influenza, COVID, endocarditis, spinal epidural abscess, and even carbon monoxide poisoning can resemble meningitis, while immunocompromised patients may have a slower smoldering course.

Diagnostics and Lumbar Puncture

• CSF over serum labs: Blood tests do little to rule meningitis in or out; cerebrospinal fluid is the key study, ideally obtained within 1 hour when it can be done safely.
• Bacterial CSF profile: Neutrophilic pleocytosis, low glucose, high protein, and elevated opening pressure point toward bacterial meningitis, but a white count under 1000 does not exclude it.
• Rapid confirmatory testing: Gram stain is highly specific but imperfectly sensitive, while CSF PCR microarray stays useful even after antibiotics and returns far faster than culture.
• CSF lactate signal: A CSF lactate above 3.5 mmol/L strongly supports bacterial meningitis, although prior antibiotics can blunt the value and make borderline results less reassuring.
• Selective CT before LP: Head CT before lumbar puncture is reserved for red flags such as elevated ICP, focal deficit, altered mental status, immunocompromise, or new seizure; antibiotics should not wait. We walk through that decision in the chapter.
• LP contraindication checks: Disseminated intravascular coagulation, platelets under 40,000, and anticoagulant use can make lumbar puncture unsafe, whereas aspirin alone is not considered a meaningful bleeding risk.

Early Treatment and ICP Management

• Immediate empiric antibiotics: Every hour of delay worsens outcomes, so empiric therapy starts with ceftriaxone plus vancomycin while cultures and PCR are pending.
• Listeria coverage triggers: Older age, diabetes, cancer, and immunosuppression should prompt added Listeria coverage with ampicillin-class therapy or meropenem, a distinction worth hearing in the episode.
• Adjunctive dexamethasone timing: Dexamethasone should be given just before or with antibiotics, with the clearest benefit seen in pneumococcal meningitis through reduced inflammatory injury.
• When to add acyclovir: Focal deficits, seizures, marked mental status depression, or immunocompromise should raise concern for HSV encephalitis, where early acyclovir is low-risk and potentially lifesaving.
• CPP-focused resuscitation: Suspected meningitis with elevated intracranial pressure is a cerebral perfusion problem as much as an infection problem, so target a higher MAP while lowering ICP.
• First-line ICP lowering: Head-of-bed elevation plus hypertonic saline or mannitol are the headline measures for raised intracranial pressure, with additional escalation paths we cover on the show.

References:

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