Extrahippocampal involvement in human herpesvirus 6 encephalitis depicted at MR imaging

To test the hypothesis that patterns of signal intensity abnormality in human herpesvirus 6 (HHV6)-positive patients would allow distinction from patients who did not test positive for HHV6 encephalitis.

This retrospective study was performed with institutional review board committee approval by using a waiver of informed consent. Sixteen immunocompromised patients (nine males, seven females; age range, 2-39 years) underwent magnetic resonance (MR) imaging and cerebrospinal fluid polymerase chain reaction (PCR) testing for HHV6 DNA on the basis of clinical findings suspicious for encephalitis. MR images acquired during acute illness were examined without knowing PCR results.

Nine patients were HHV6 positive. Seven showed signal intensity abnormalities, with prominent involvement of the hippocampus, and six showed additional involvement of the amygdala. Three HHV6-positive patients showed signal intensity abnormality in extrahippocampal divisions of the olfactory cortex and cortical and subcortical structures that maintain prominent connections with the hippocampal formation. Among the seven HHV6-negative patients, six had abnormalities in the hippocampus but only two showed extrahippocampal involvement, which was restricted to the amygdala.

Most patients with HHV6 encephalitis have signal intensity abnormalities in the hippocampal formation and amygdala and, contrary to prior reports, some also have involvement of limbic structures outside of the medial temporal lobe. The presence of MR signal intensity abnormality in the medial temporal lobe should raise the diagnosis of HHV6 encephalitis in immunosuppressed patients, especially when hyperintense lesions are seen in the insular region and inferior frontal lobe.