condition. The typical MRI features in HSE are areas of focal
oedema in the temporal lobes and orbital surface of the
frontal lobes as well as the insular cortex and angular gyrus10
(fig 1); abnormal areas may show enhancement with
gadolinium. Midline shift may be present in cases with
significant cerebral oedema. Some other types of viral
encephalitis such as Japanese encephalitis and eastern
equine encephalitis are also associated with particular MRI
abnormalities. Where available, functional imaging such as
SPECT (single photon emission computed tomography) may
yield additional useful information in cases of viral encephalitis—
for example, temporal lobe hyperperfusion may be a
marker of HSE. An EEG should also be performed in all cases
of possible encephalitis, and is particularly useful in the
distinction between encephalitis and metabolic encephalopathy.
The EEG is invariably abnormal in HSE showing either
early non-specific slowing or later ‘‘PLEDS’’ (periodic
lateralising epileptiform discharges), although these are not
themselves diagnostic of HSE.