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Herpes Encephalitis


KEY FACTS

Terminology

  • Brain parenchyma infection caused by herpes simplex virus type 1 (HSV1)

  • Typically reactivation in immunocompetent patients

Imaging

  • Best imaging clue: T2/FLAIR hyperintensity of limbic system (medial temporal and inferior frontal cortex) with DWI restriction

    • Typically bilateral disease, but asymmetric

    • Deep gray nuclei usually spared

  • CT often normal early

  • MR with DWI most sensitive for early diagnosis

  • T2/FLAIR: Cortical, subcortical hyperintensity with relative white matter sparing

  • GRE: If hemorrhagic, hypointensity “blooms” within edematous brain

  • DWI: Restricted diffusion in limbic system

  • T1WI C+: May see mild, patchy enhancement early

    • Gyriform enhancement usually seen 1 week after initial symptoms

Top Differential Diagnoses

  • Acute cerebral ischemia-infarction

  • Status epilepticus

  • Limbic encephalitis

  • Infiltrating neoplasm

Clinical Issues

  • Common presentation: Fever, headache, seizures, ± viral prodrome

  • Children often present with nonspecific symptoms

  • PCR of cerebrospinal fluid most accurate diagnosis

  • HSV1 causes 95% of all herpetic encephalitis

  • Start IV acyclovir immediately if herpes simplex encephalitis suspected

Coronal graphic shows the classic features of herpes encephalitis with bilateral but asymmetric involvement of the limbic system. There is inflammation involving the temporal lobes
, cingulate gyri
, and insular cortices
.

NECT in a 52-year-old man with herpes simplex encephalitis shows hypodensity
and mild mass effect in the right anteromedial temporal lobe. Hemorrhage and contrast enhancement appear 2-3 days later in the disease course.

Axial FLAIR MR shows classic bilateral but asymmetric abnormal hyperintense signal in the medial temporal lobes of this patient with herpes encephalitis. Note involvement of the hippocampi
.

Axial DWI MR in the same patient shows diffusion restriction as a bright signal in the medial temporal lobes
and hippocampi
. DWI and FLAIR are the most sensitive sequences for detection of encephalitis. CT is often normal early in the course of herpes encephalitis.

TERMINOLOGY

Abbreviations

  • Herpes simplex encephalitis (HSE)

Definitions

  • Brain parenchyma infection caused by herpes simplex virus type 1 (HSV1)

  • Typically reactivation in immunocompetent patients

IMAGING

General Features

  • Best diagnostic clue

    • Abnormal signal in medial temporal and inferior frontal cortex with DWI restriction

    • Involvement of cingulate gyrus and contralateral temporal lobe highly suggestive

  • Location

    • Limbic system: Temporal lobes, insula, subfrontal area, cingulate gyri typical

    • Cerebral convexity, posterior occipital cortex may become involved

    • Typically bilateral disease, but asymmetric

    • Basal ganglia usually spared

    • Atypical patterns seen in infants and children (may be caused by HSV1 or HSV2)

      • –

        May primarily affect cerebral hemispheres

    • Rarely affects midbrain and pons (mesenrhombencephalitis)

CT Findings

  • NECT

    • CT often normal early

      • –

        Low attenuation, mild mass effect in medial temporal lobes, insula

      • –

        Hemorrhage typically late feature

    • Predilection for limbic system; basal ganglia spared

    • Earliest CT findings at 3 days after symptom onset

  • CECT

    • Patchy or gyriform enhancement of temporal lobes (late acute/subacute feature)

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