Infectious Disease · Central Nervous System Infections
The facts most likely to be tested
The classic triad of acute bacterial meningitis consists of fever, nuchal rigidity, and altered mental status, though all three are present in less than half of patients.
Lumbar puncture is the diagnostic gold standard, but empiric antibiotics must be administered immediately if there is any delay in performing the procedure.
Head CT must be performed prior to lumbar puncture only in patients with focal neurologic deficits, new-onset seizures, papilledema, or immunocompromised state to rule out mass effect.
Streptococcus pneumoniae is the most common cause of community-acquired bacterial meningitis in adults, while Neisseria meningitidis is associated with petechial rash.
Listeria monocytogenes is a critical pathogen to consider in patients over age 50 or those who are immunocompromised, requiring the addition of ampicillin to the standard regimen.
Dexamethasone should be administered with or just before the first dose of antibiotics to reduce the risk of neurologic sequelae in cases of suspected pneumococcal meningitis.
Cerebrospinal fluid analysis in bacterial meningitis typically reveals elevated opening pressure, high protein, low glucose, and neutrophilic pleocytosis.
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A 62-year-old male is brought to the emergency department by his wife due to two days of worsening headache and confusion. On examination, the patient is febrile at 102.4°F (39.1°C) and exhibits significant nuchal rigidity. He has no focal neurologic deficits, no papilledema, and no history of seizures. His past medical history is significant for chronic lymphocytic leukemia. The patient is currently lethargic but follows simple commands.
What is the most appropriate next step in management?
Administer empiric antibiotics and dexamethasone, then perform lumbar puncture.
This patient has suspected bacterial meningitis and is immunocompromised, but lacks contraindications for immediate lumbar puncture; however, empiric therapy must never be delayed for imaging or the procedure itself.
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High yield triage
Etiology / Epidemiology
Streptococcus pneumoniae is the most common cause in adults; Neisseria meningitidis is common in adolescents/dorms.
Clinical Manifestations
Classic triad: fever, nuchal rigidity, and altered mental status. Look for Kernig's and Brudzinski's signs.
Diagnosis
Lumbar puncture is the gold standard; CSF shows elevated opening pressure, low glucose, and high protein.
Treatment
Empiric therapy is ceftriaxone plus vancomycin; add dexamethasone to reduce neurological sequelae.
Prognosis
High mortality if untreated; sensorineural hearing loss is the most common long-term complication.
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Epidemiology & Etiology
Adults are primarily affected by S. pneumoniae, while N. meningitidis is associated with outbreaks in crowded settings. Listeria monocytogenes must be considered in patients >50 or immunocompromised. Group B Streptococcus is the leading cause in neonates.
Pertinent Anatomy
The subarachnoid space contains the CSF and is the primary site of inflammation. Increased intracranial pressure can lead to herniation through the foramen magnum if a mass effect is present.
Pathophysiology
Bacteria colonize the nasopharynx and invade the bloodstream, crossing the blood-brain barrier. This triggers a massive inflammatory response, leading to cerebral edema and increased intracranial pressure. The resulting exudate can obstruct CSF flow, causing hydrocephalus.
Clinical Manifestations
Patients present with the classic triad of fever, nuchal rigidity, and altered mental status. Kernig's sign (pain with knee extension) and Brudzinski's sign (neck flexion causing hip flexion) are highly specific. Papilledema or focal neurological deficits are red flags for increased intracranial pressure requiring imaging before LP.
Diagnosis
Lumbar puncture is the gold standard for diagnosis. CSF analysis typically reveals WBC >1000/µL (predominantly neutrophils), glucose <40 mg/dL, and protein >200 mg/dL. Do not delay antibiotics for imaging if the patient is stable; obtain blood cultures immediately.
Treatment
Initiate ceftriaxone plus vancomycin immediately after blood cultures. Add ampicillin if Listeria is suspected (age >50 or immunocompromised). Administer dexamethasone with or just before the first antibiotic dose to prevent sensorineural hearing loss. Contraindications for immediate LP include focal deficits or signs of herniation.
Prognosis
Early recognition is critical to prevent permanent neurological deficits. Common complications include seizures, hydrocephalus, and hearing loss. Monitor for signs of SIADH during the acute phase.
Differential Diagnosis
Viral Meningitis: CSF glucose is typically normal
Fungal Meningitis: associated with immunocompromised states and subacute onset
Brain Abscess: focal neurological deficits are more prominent
Subarachnoid Hemorrhage: sudden onset thunderclap headache
Encephalitis: altered mental status is more severe than meningeal signs