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Herpes simplex encephalitis

Case contributed by Dr Frank Gaillard

Presentation:

History epilepsy. MRI two years ago normal. Now presents with fatigue, and confusion.

Patient Data:

Age: Adult

MRI brain

Modality: MRI

MRI demonstrates extensive oedema in the right temporal lobe with areas of intrinsic high T1 signal, in keeping with haemorrhage.  The changes spare the basal ganglia, a feature which is helpful in distinguishing an MCA infarct with haemorrhagic transformation from herpes simplex encephalitis, the diagnosis in this case.  

The patient went on to have a craniotomy and biopsy. 

MICROSCOPIC DESCRIPTION

These  are  fragments of  cerebral  cortex and  white matter  showing extensive areas of  parenchymal necrosis, with  sheets of foamy macrophages  and egenerating  neutrophils.  Within areas of incipient necrosis there are  numerous red  neurons, some of  which show  intraneuronal  inclusions.  The  adjacent  parenchyma  shows marked  reactive  astrocytosis  and  microglial  activation, with  occasional  microglial  nodules.  There is  a necrotising  small  vessel   vasculitis,  with   occasional   vessels   showing  luminal  microthrombi  and perivascular  haemorrhage.   There is dense  infiltration around, and within the walls of  parenchymal and leptomeningeal vessels, by a mixed inflammatory infiltrate  including lymphocytes (with some activated forms) and plasma cells.

Immunohistochemistry  for HSV-1/2  shows positive staining  in some neurons  and  occasional   glial  cells;   this  confirms   a  diagnosis  of  herpes encephalitis. 

DIAGNOSIS: Right temporal lobe specimens,  including medial structures and uncus: extensively necrotising herpes simplex encephalitis.

Case Discussion:

Herpes simplex encephalitis can be difficult to distinguish from a glioma or a middle cerebral artery infarct. Careful history and a low threshold for treatment are essential. 

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