Viral encephalitis is a viral infection of brain parenchyma producing an inflammatory response. It is distinct from, although often coexists with, viral meningitis. In North America, viruses that cause encephalitis are the arboviruses (including the West Nile virus), herpes simplex virus (HSV), herpes zoster virus (HZV), Epstein-Barr virus, cytomegalovirus (CMV), and rabies.
Encephalitis should be considered in patients presenting with any or all of the following features: new psychiatric symptoms, cognitive deficits (aphasia, amnestic syndrome, acute confusional state), seizures, and movement disorders. Headache, photophobia, fever, and meningeal irritation may be present. Assessment for neurologic findings and cognitive deficits is crucial. Motor and sensory deficits are not typical. Encephalitides may show special regional trophism. HSV involves limbic structures of the temporal and frontal lobes, with prominent psychiatric features, memory disturbance, and aphasia. Some arboviruses predominantly affect the basal ganglia, causing chorea-athetosis and Parkinsonism. Involvement of the brainstem nuclei leads to hydrophobic choking characteristic of rabies encephalitis.
Symptoms of West Nile virus infection include fever, headache, muscle weakness, and lymphadenopathy. Most infections are mild and last only a few days. More severe symptoms and signs consist of high fever, neck stiffness, altered mental status, tremors, and seizures. In rare cases (mostly involving the elderly), the infection can lead to encephalitis and death.
Diagnosis and Differential
Findings on CT or magnetic resonance imaging (MRI) and LP aid in the ED diagnosis of encephalitis. Neuroimaging, particularly MRI, not only excludes other potential lesions, such as brain abscess, but may display findings highly suggestive of HSV encephalitis if the medial temporal and inferior frontal gray matter is involved. Findings of aseptic meningitis are typically found on CSF examination. For the West Nile virus, the most widely used screening test is the IgM ELISA assay for detecting acute antibody.
The differential diagnosis includes brain abscess; Lyme disease; subacute subarachnoid hemorrhage; bacterial, tuberculous, fungal, or neoplastic meningitis; bacterial endocarditis; postinfectious encephalomyelitis; toxic or metabolic encephalopathies; and primary psychiatric disorders.