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