Asymmetric bilateral limbic involvement — medial temporal lobes, insula, cingulate — with restricted diffusion and later haemorrhage, sparing the basal ganglia; the report should prompt aciclovir now, not after the PCR.
Orient first
- HSV-1 in adults targets the limbic system; the basal ganglia are spared (unlike MCA infarction).
- DWI is the earliest sensitive sequence; CT is often normal early.
- Autoimmune limbic encephalitis mimics it but is usually bilateral symmetric and non-haemorrhagic.
Acquire the study
- DWI/ADC, FLAIR, SWI, post-gadolinium T1.
The manoeuvre
- DWI: restricted diffusion in the medial temporal lobe, insula and cingulate.
- FLAIR: cortical swelling and hyperintensity; bilateral asymmetric.
- SWI: petechial haemorrhage (subacute).
- Basal ganglia: spared — against infarction.
What confirms it
- Limbic DWI/FLAIR abnormality with CSF HSV PCR positive.
What licenses you to exclude it
- A normal MRI with DWI in the first days makes HSV encephalitis less likely but does not exclude it; PCR decides.
The classic misread
- Calling it an MCA infarct — infarcts follow the arterial territory and involve the lentiform nucleus.