Acute viral encephalitis - Infections of the nervous system

Acute viral encephalitis

Definition

Inflammation of the brain parenchyma caused by viruses.

Aetiology

This is an uncommon disease, but the most common cause in the United Kingdom is the herpes simplex virus (HSV). Other viruses causing encephalopathy include echovirus, coxsackie virus, other members of the herpesviruses (e.g. varicella zoster, cytomegalovirus, Epstein–Barr virus). Around the world, arthropod-borne viruses cause epidemics and rabies causes an almost invariably fatal encephalitis.

Pathophysiology

HSV tends to cause a temporal encephalitis. Immuno-compromised individuals, children, teenagers and the elderly have an increased risk. Inflammation affects the meninges and parenchyma causing oedema and hence raised intracranial pressure, diffuse and focal neurological dysfunction.

Clinical features

The main triad of symptoms is headache, fever and altered level of consciousness. It is an important differential of bacterial meningitis. In HSV type I encephalitis nausea, vomiting, and meningism (neck stiffness) affect over two-thirds and up to half develop focal symptoms and signs, e.g. hemiparesis or dysphasia. Seizures (particularly temporal lobe seizures) are also a presenting feature.

Macroscopy/microscopy

The meninges are hyperaemic, the brain is swollen, sometimes with evidence of petechial haemorrhage and necrosis. There is cuffing of blood vessels by mononu-clear cells and viral inclusion bodies may be seen.

Investigations

CT scanning may show areas of oedema (a normal scan does not exclude the diagnosis, but is indicated before lumbar puncture (LP) in cases with altered consciousness or focal neurological signs). MRI is more sensitive.

LP – the cerebrospinal fluid commonly shows lymphocytosis and raised protein levels. Glucose is uncommonly reduced (a sign of bacterial infection). CSF may be sent for HSV PCR and antibody tests for HSV, EBV, CMV and VZV. CSF cultures are usually unhelpful.

EEG may be helpful in the diagnosis in over 90% of cases of HSV-I.

In systemic illnesses, serum viral antibody titres can be helpful.

Management

In all cases except herpes simplex encephalitis there is no effective treatment apart from supportive management. Seizures are treated with anticonvulsants. Suspected cases of herpes encephalitis are treated urgently with high dose i.v. acyclovir for 10 days, with up to 5% of cases relapsing after treatment.

Prognosis

Herpes simplex encephalitis has a mortality of 20% despite treatment, with poor prognostic factors including older age, GCS ≤10 at onset of therapy. Persistent neurological deficits occur in 50%, particularly memory impairment, personality change, dysphasia and epilepsy.