Meningoencephalitis resembles both meningitis, which is an infection or inflammation of the meninges, and encephalitis, which is an infection or inflammation of the brain.

Findings of encephalopathy, meningeal irritation, and inflammation.
The diagnosis of the cause of an meningeal encephalitis is difficult and requires the integration of history, risk factors and multiple testing techniques.
Causes of meningoencephalitis are divided into infectious and not infectious causes.
The most common identifiable cause of acute encephalitis in older adults is herpes simplex virus encephalitis, which typically involves temporal lobes.
The most common type of arboviral meningoencephalitis in the Northeast US is West Nile virus encephalitis with rarer types including Powassan  virus encephalitis and Eastern equine encephalitis.
Approximately 50% of cases have no specific cause identified.

Causative organisms include protozoans, viral and bacterial pathogens.

Bacterial agents include: Listeria monocytogenes, Neisseria meningitidis, Rickettsia prowazekii, Mycoplasma pneumoniae, Tuberculosis, Borrelia (Lyme disease), Leptospirosis, and H. Influenzae.

Viral agents include: Tick-borne meningoencephalitis, West Nile virus, Measles, Epstein-Barr virus, Varicella-zoster virus, Enterovirus, Herpes simplex virus type 1, Herpes simplex virus type 2, mumps, HIV

Mumps is a relatively common cause of meningoencephalitis, but is mild, and usually does not result in death or neurologic sequelae.

Cryptococcus neoformans fungus infection of the CNS can manifest as meningoencephalitis with hydrocephalus.

Ameobic pathogens cause rare and uncommon CNS infections, and ameobic meningoencephalitis can mimic a brain abscess, aseptic or chronic meningitis, or CNS malignancy.

Meningoencephalitis is associated with high rates of mortality and severe morbidity.

The cause of acute meningoencephalitis is not identified in approximately 50% of cases.

Metagenomic next generation sequencing allows a comprehensive spectrum of potential causes from viral, bacterial, fungal, and parasitic identification by a single essay.

Failure to have a timely diagnosis in such a disease contributes to poor patient outcomes, increased anxiety, and a high cost burden to the healthcare system.

Non-infectious causes of meningoencephalitis include paraneoplastic disease with involvement of the limbic system or brain stem with a more generalized encephalomyelitis.
Auto immune encephalitis is rare and most commonly characterized by anti-N-methyl-the-aspertatev (NMDA) receptor encephalitis.
Immune- mediated non-infectious causes of meningoencephalitis can result in subacute neurologic symptoms that affect both the central  and peripheral nervous system.

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