Herpes simplex (HSV) encephalitis is a common topic in the exam. The virus characteristically affects the temporal lobes – questions may give the result of imaging or describe temporal lobe signs e.g. aphasia.

Features

• fever, headache, psychiatric symptoms, seizures, vomiting
• focal features e.g. aphasia
• peripheral lesions (e.g. cold sores) have no relation to the presence of HSV encephalitis

Pathophysiology

• HSV-1 is responsible for 95% of cases in adults
• typically affects temporal and inferior frontal lobes

Investigation

• CSF: lymphocytosis, elevated protein
• PCR for HSV
• CT: medial temporal and inferior frontal changes (e.g. petechial haemorrhages) – normal in one-third of patients
• MRI is better
• EEG pattern: lateralised periodic discharges at 2 Hz

Treatment

• intravenous aciclovir

The prognosis is dependent on whether aciclovir is commenced early. If treatment is started promptly the mortality is 10-20%. Left untreated the mortality approaches 80%