Diagnosis

Diagnosis rests on imaging studies using magnetic resonance imaging (MRI) or CT if MRI is not reasonably available, electroencephalography (EEG), and LP. Imaging not only excludes other potential lesions, such as brain abscess, but also may display findings that are highly suggestive of HSV encephalitis, a treatable infection, with involvement of medial temporal and inferior frontal gray matter. MRI is more sensitive than CT in this regard. EEG is quite useful in establishing the broad diagnosis as well. Almost by definition, the EEG findings are abnormal in encephalitis, in contrast to isolated viral meningitis or to a primary psychiatric disorder. Furthermore, HSV produces an almost pathognomonic picture in the setting of acute febrile encephalopathy, with the EEG showing periodic, usually asymmetric sharp waves. These findings can be present before any abnormality is visible on MRI. Realistically, LP is the most useful diagnostic procedure in the emergency department once imaging studies, if clinically indicated and available, rule out the risk of uncal herniation. Findings of aseptic meningitis are typical on CSF examination. It is at least theoretically possible to have encephalitis without meningitis, but it is quite rare.

The differential diagnosis depends on the nature of the presentation. When fever and meningeal symptoms predominate, bacterial meningitis is suspected. In less fulminant meningeal cases, Lyme disease; tuberculous, fungal, and neoplastic meningitis; and subacute subarachnoid hemorrhage are part of the differential diagnosis. When parenchymal features are prominent, brain abscess, bacterial endocarditis, postinfectious encephalomyelitis, and toxic or metabolic encephalopathies should be considered.

The clear diagnostic imperative in the emergency department is to exclude the most immediately life-threatening alternative processes requiring immediate treatment. The two most important are bacterial meningitis and acute subarachnoid hemorrhage. Once that is satisfactorily accomplished, the mandate is less definite. Of the viruses causing encephalitis, only HSV has been shown by clinical trial to be responsive to antiviral therapy. hSv can be isolated in as many as 50 percent of neonatal infections but is rarely found in older children and adults. The polymerase chain reaction (PCR) analysis of CSF has 95 percent sensitivity and 100 percent specificity in the diagnosis of HSV infection with respect to brain biopsy. Regardless of diagnostic approach, because current antiviral treatment is relatively risk and side effect free, empiric therapy is recommended in cases of clinical encephalitis.16 Of the other viral encephalitides, CSF culture results are positive for 50 to 70 percent of patients with enteroviral meningitis but for a much smaller percentage of those with isolated enteroviral encephalitis. The use of PCR techniques for the enteroviruses is also showing promise for central nervous system diagnosis in the future.

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