1. VIRAL ENCEPHALITIS
A range of viruses can cause encephalitis but only a minority of patients have a history of recent viral infection. In Europe, the most serious cause of viral encephalitis is herpes simplex), which probably reaches the brain via the olfactory nerves. The development of effective therapy for some forms of encephalitis has increased the importance of clinical diagnosis and virological examination of the CSF.

2. Pathology
Inflammation can occur in the cortex, white matter, basal ganglia and brain stem, and the distribution of lesions varies with the type of virus. In herpes simplex encephalitis, the temporal lobes are usually primarily affected. Inclusion bodies may be present in the neurons and glial cells and there is an infiltration of polymorphonuclear cells in the perivascular space. There is neuronal degeneration and diffuse glial proliferation, often associated with cerebral oedema.

3. Clinical features
Viral encephalitis presents with acute onset of headache, fever, focal neurological signs (aphasia and/or hemiplegia) and seizures. Disturbance of consciousness ranging from drowsiness to deep coma supervenes early and may advance dramatically. Meningism occurs in many patients. Rabies presents a distinct clinical picture

4. Investigations
CT of the head, which should precede lumbar puncture, may show low-density lesions in the temporal lobes. MRI is more sensitive in detecting early abnormalities. The CSF usually contains excess lymphocytes, but polymorphonuclear cells may predominate in the early stages. Occasionally, the CSF is normal. The protein content may be elevated but the glucose is normal. The EEG is usually abnormal in the early stages, especially in herpes simplex encephalitis, with characteristic periodic slow-wave activity in the temporal lobes. Virological investigations of the CSF, including PCR for viral DNA, may reveal the causative organism but the initiation of treatment should not await this

5. Management
Anticonvulsant treatment is often necessary and raised intracranial pressure is treated with dexamethasone 8 mg 12-hourly. Herpes simplex encephalitis responds to aciclovir 10 mg/kg i.v. 8-hourly for 2-3 weeks. This should be given early to all patients suspected of suffering from viral encephalitis. Even with optimum treatment, mortality is 10-30% and significant proportions of survivors have residual epilepsy or cognitive impairment