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HHV-6 Encephalitis


KEY FACTS

Terminology

  • Encephalitis caused by human herpes virus 6 (HHV-6)

Imaging

  • Immunocompromised patient with abnormal signal medial temporal lobe(s)

    • Limbic system: Hippocampus, amygdala, parahippocampal gyrus

    • Insular region, inferior frontal lobe involvement less common than herpes simplex encephalitis

  • Atypical pattern in infants/children (basal ganglia, thalami, cerebellum, brainstem)

  • Best imaging: MR (coronal T2/FLAIR, DWI, T1 C+)

Top Differential Diagnoses

  • Herpes simplex virus (HSV) encephalitis

    • Early HHV-6 exclusive involvement of mesial temporal lobes

    • Early extrahippocampal involvement common in HSV

  • Paraneoplastic limbic encephalitis

  • Status epilepticus

Pathology

  • HHV-6: DNA virus belongs to herpes virus family

  • 2 variants: HHV-6A and HHV-6B

  • Reactivation in immunocompromised patients

Clinical Issues

  • Mental status changes, short-term memory loss, fever, seizure, headache

  • Febrile seizures in infants

  • Post hemopoietic stem cell transplant, lung/liver transplant

  • Antiviral drugs: Ganciclovir and foscarnet

  • Mortality of HHV-6 encephalitis > 50%

Diagnostic Checklist

  • Consider HHV-6 encephalitis in immunocompromised patients with CNS symptoms + unilateral/bilateral medial temporal lobe signal changes

Axial DWI MR in a 42-year-old patient post stem cell transplant who presented with altered sensorium demonstrates restricted diffusion in the hippocampi bilaterally
, more pronounced on the right.

Axial FLAIR MR in the same patient shows corresponding high signal in the hippocampi bilaterally
, again more pronounced on the right. In this patient with immunosuppression, imaging findings are very typical for human herpes virus-6 (HHV-6) encephalitis

Coronal T2WI MR in a 37-year-old man with immunosuppression, fever, and seizures shows hyperintense signal in the medial temporal lobes
, basal ganglia
, and subinsular white matter
.

Axial T2WI MR in the same patient shows asymmetric hyperintense signal in the subinsular white matter
. Extrahippocampal involvement in HHV-6 encephalitis is less common than herpes simplex encephalitis.

TERMINOLOGY

Abbreviations

  • Human herpes virus 6 (HHV-6) encephalitis

Definitions

  • Encephalitis caused by HHV-6

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