Meningitis & Encephalitis

The distinction is anatomical. Meningitis is inflammation of the meninges, the protective membranes wrapped around the brain and spinal cord. Encephalitis is inflammation of the brain parenchyma — the working tissue itself. When both are inflamed, it’s meningoencephalitis. The location decides the exam findings: irritated meninges produce headache, fever, and a stiff neck, but the brain underneath still works — in viral meningitis, altered mental status is not a common feature. Encephalitis produces focal or global cerebral dysfunction: confusion and behavior change, seizures, focal neurologic deficits, even coma. Parenchymal dysfunction is the bedside clue that the process is encephalitis, not meningitis.

The flagship encephalitis is herpes simplex encephalitis (HSE). HSV-1 causes about 90% of cases and targets the temporal lobes and limbic system — which is why its signature is behavioral change, altered speech (~40%), seizures (~54%), and focal deficits (~41%, all per StatPearls). It is the most common cause of fatal encephalitis, striking sporadically year-round — and it is the reason empiric IV acyclovir is started the moment encephalitis is suspected, without waiting for confirmation.

Bacteria reach the meninges by three routes: bacteremia crossing the blood–brain barrier (the most instructive route — a colonizing infection seeds the blood), direct extension from otitis media or sinusitis next door, and dural defects — congenital, or acquired through skull fracture or neurosurgery. Which organism arrives depends on age: group B Streptococcus dominates under 2 months (with gram-negative rods like E. coli and Listeria completing the neonatal “big three”); S. pneumoniae is the most common at every other age; N. meningitidis peaks at ages 11–17; Listeria re-enters in adults over 50 and the immunocompromised; H. influenzae persists mainly in the unvaccinated.

Once inside, the damage is largely the host’s own response. Bacterial wall components — lipopolysaccharide from gram-negative walls, teichoic acid from gram-positive — activate the brain’s microglia, launching an inflammatory cascade that makes cortical microvessels leaky. The result is diffuse cerebral edema → increased intracranial pressure, and the rising ICP with falling cerebral perfusion produces the whole clinical picture: headache, fever, altered mental status, seizures, and focal deficits. This is why meningitis care is simultaneously infection care and ICP care — head of bed up, neuro checks, and a low threshold for escalation.

Viral meningitis is the most common form, and it follows a far more benign course — tending to get better on its own within about 7 to 10 days. In one US survey, enteroviruses were the leading identified cause of meningitis at 51%, bacteria only 14%. Nonpolio enteroviruses (coxsackie, echo, parechoviruses) cause at least half of aseptic meningitis at every age, peaking in summer and autumn in temperate climates; herpesviruses, mumps, measles, and arboviruses account for most of the rest. Viruses reach the meninges through the bloodstream, by retrograde spread along nerves, or by reactivating from dormancy inside the nervous system. “Aseptic meningitis” is the umbrella finding: CSF pleocytosis (more than 5 white cells/mm³) with negative bacterial cultures — most of it viral.

Bacterial meningitis is the opposite story: rare but deadly — a medical emergency demanding immediate hospitalization and antibiotics. The two forks look similar at the door (fever, headache, stiff neck), so the bedside rule is brutal and simple: treat every suspected meningitis as bacterial until the CSF proves otherwise. Empiric antibiotics go to everyone; they are discontinued only after CSF analysis excludes bacterial disease. The lumbar puncture is the fork in the road — the CSF profile in the diagnostics table below is what separates a two-week illness from a life-threatening one.

N. meningitidis spreads by respiratory droplets through close, direct contact — which is why it runs through dormitories and households, and why the patient in front of you mandates droplet precautions and their close contacts need prophylaxis. In the bloodstream, the meningococcus attacks the endothelium itself: vessel-lining cells retract and lose integrity, producing capillary hemorrhages alongside vessels occluded by white cells, platelets, and fibrin thrombi — a DIC-type picture of bleeding and clotting at once. On the skin, that is petechiae and purpura: pinpoint-to-patchy purple lesions that do not blanch — they do not fade when pressed with a glass (and may be harder to see on brown or black skin).

The rash appears in 50–60% of meningococcemia (StatPearls) — which cuts both ways. Present, it is an emergency: lesions can coalesce into purpura fulminans, and purpura and shock often develop within hours. Absent, it rules nothing out — 20–30% of children have no rash on presentation, and the teaching for families is explicit: do not wait for a rash before seeking emergency care. Refractory shock in meningococcal disease may reflect an impaired adrenal response — the Waterhouse-Friderichsen picture, treated in children with stress-dose hydrocortisone.