Clinical Pearls & Morning Reports
Each year in the United States, approximately 7 patients are hospitalized for encephalitis per 100,000 population. The cause is unknown in approximately half these cases. Of the cases with a known cause, 20 to 50% are attributed to viruses. Read the latest NEJM Review Article here.
Q: Herpes simplex encephalitis accounts for what percentage of cases of viral encephalitis?
A: Herpes simplex virus (HSV) accounts for 50 to 75% of identified viral cases, with varicella–zoster virus (VZV), enteroviruses, and arboviruses accounting for the majority of the remainder. Initial diagnostic efforts focus on distinguishing viral from autoimmune encephalitis and on differentiating HSV encephalitis from other viral causes.
Q: What features may help distinguish between the different causes of encephalitis?
A: In a review of cases of adult encephalitis that were characterized by abnormalities in the temporal lobes on magnetic resonance imaging (MRI), features favoring HSV over other causes included older age, acute clinical presentation (in 88% of patients with HSV encephalitis vs. 64% of patients with encephalitis from other causes), fever (80% vs. 49%), gastrointestinal symptoms (37% vs. 19%), and lower incidences of ataxia (18% vs. 33%) and rash (2% vs. 15%). Patients with HSV encephalitis were more likely than those with autoimmune encephalitis to be men (50% vs. 14%) and were less likely to have psychosis (5% vs. 20%) or rash (2% vs. 21%). Most neurologic symptoms, including impaired consciousness, confusion, aphasia, hallucinations, and movement disorders, did not differ among the various types of encephalitis.
A: Most available viral diagnostic methods test for a single organism and are ordered individually from diagnostic laboratories. It is possible to perform a comprehensive analysis of a large panel of antiviral antibodies against all known human viruses, known as systemic viral epitope scanning, although this procedure is not yet commercially available. Simpler and less sophisticated multiplex diagnostic panels are entering clinical practice. For example, the Food and Drug Administration has approved a multiplex diagnostic panel that allows for rapid PCR-based detection of multiple pathogens associated with meningitis and encephalitis in CSF specimens, including seven viruses (HSV-1, HSV-2, varicella zoster virus, enterovirus, cytomegalovirus, human herpesvirus 6, and human parechovirus). Available multiplex assays have an overall sensitivity of 86 to 100% and a specificity of more than 99.5%. However, additional studies in broad populations and various settings are needed to confirm their sensitivity and specificity.
A: The outcomes of acute viral encephalitis remain generally poor. Predictors of a poor outcome include the presence of an immunocompromised state, a Glasgow Coma Scale score of 8 or less (on a scale from 3 to 15, with lower scores indicating greater neurologic deficits), the need for admission to an intensive care unit, and an age of more than 65 years. In HSV encephalitis, the outcome of which has been more extensively studied than that of other viral encephalitides, factors affecting the outcome 6 to 12 months after hospital discharge, in approximate order of importance, are coma, restricted diffusion on MRI, more than a 24-hour delay in the initiation of acyclovir therapy after admission, and older age. Other MRI or EEG features and CSF test results have not been predictive of outcomes. Prognostic factors in arbovirus encephalitis have been identified with less certainty, but in West Nile virus disease, older age, membership in certain ethnic groups, female sex, and coma at presentation have been indicators of a poor prognosis.