meningoencephalitis

Meningoencephalitis represents a spectrum of CNS infection involving both the meningeal layers and underlying brain tissue, distinguishing it from isolated meningitis (meninges only) or encephalitis (parenchyma only). Pathophysiology involves direct pathogen invasion (hematogenous spread, retrograde axonal transport as in HSV via olfactory/trigeminal routes, or direct extension) triggering an inflammatory cascade with cytokine release, blood-brain barrier disruption, cerebral edema, and neuronal injury. Clinically, patients present with fever, headache, nuchal rigidity, photophobia (meningeal signs) combined with altered consciousness, personality changes, seizures, and focal neurologic deficits (encephalitic component). HSV-1 meningoencephalitis classically causes temporal lobe involvement with hemorrhagic necrosis, presenting with bizarre behavior, olfactory hallucinations, and seizures; MRI shows temporal lobe hyperintensities. Arboviral causes (West Nile, St. Louis encephalitis, Eastern/Western equine encephalitis) are transmitted by mosquitoes and vary in severity and mortality. Naegleria fowleri causes primary amoebic meningoencephalitis after freshwater exposure, with rapid, often fatal course. Listeria monocytogenes classically causes meningoencephalitis in neonates, elderly, and immunocompromised patients, with rhombencephalitis (brainstem involvement) in some cases. Diagnostic workup includes lumbar puncture (CSF analysis showing lymphocytic pleocytosis in viral causes, elevated protein, variable glucose), PCR for HSV and other viruses, CSF culture, and neuroimaging (CT/MRI) to assess for edema, hemorrhage, or mass effect before LP if increased ICP is suspected. Empiric treatment often includes IV acyclovir (covers HSV) plus broad-spectrum antibiotics until pathogen is identified, given the high mortality of untreated HSV encephalitis. Prognosis depends on causative organism, age, immune status, and promptness of treatment; HSV encephalitis carries significant mortality/morbidity even with treatment, while arboviral causes range from mild to severe based on specific virus and host factors.