Inflammation of the brain as a result of a viral infection
Viral encephalitis is a rare condition in which the brain becomes inflamed as a result of a viral infection. Often, the meninges, the membranes surrounding the brain and spinal cord, are also affected. Viral encephalitis varies in severity. An attack can be so mild that it causes almost no symptoms and is barely noticeable. However, occasionally it is serious and potentially life-threatening.
Many different viruses can cause viral encephalitis. Mild cases are sometimes the result of infectious mononucleosis. In addition, viral encephalitis still occurs as a complication of some childhood infections, such as measles and mumps, although routine immunization has made these disorders much less common.
The most common cause of life-threatening viral encephalitis is the herpes simplex virus (see Herpes simplex infections). In tropical countries, viral encephalitis can be caused by mosquito- and tick-borne infections, such as yellow fever.
In the past, the disorder was frequently caused by infection with the polio virus. However, this disease is now rare in developed countries as a result of routine immunization.
Mild cases of viral encephalitis usually develop gradually over several days and may cause only a slight fever and mild headache. However, in severe cases, the symptoms usually develop quickly over 24–72 hours and may include:
Nausea and vomiting.
Problems with speech, such as slurring of words.
Weakness or paralysis in one or more parts of the body.
If the membranes that surround the brain become inflamed (see Meningitis), other symptoms such as a stiff neck and intolerance of bright light may develop. The person affected may have seizures. In some cases, there is drowsiness, which may progress to a gradual loss of consciousness and coma.
If viral encephalitis is suspected, you will be admitted to hospital. Your doctor may arrange for a blood test to look for signs of viral infection. You may also have CT scanning or MRI to look for areas of brain swelling caused by inflammation and to exclude other possible reasons for the symptoms, such as a brain abscess. A sample of the fluid surrounding the brain and spinal cord may be taken (see Lumbar puncture) to look for evidence of infection. You may have an EEG to look for abnormal electrical activity in the brain. Rarely, a brain biopsy is performed, in which a sample of tissue is taken from the brain under general anaesthesia and then examined to confirm the diagnosis.
Viral encephalitis that is caused by the herpes simplex virus can be treated with intravenous doses of aciclovir (see Antiviral drugs) and possibly also with corticosteroids to reduce inflammation of the brain. In severe cases, intravenous aciclovir may be given, even if the cause has not been identified. Anticonvulsant drugs may be prescribed if seizures develop. Severely affected people may need to be treated in an intensive therapy unit.
It is often difficult to predict the outcome of viral encephalitis. People who have mild encephalitis usually make a full recovery over several weeks, but occasional headaches may occur for a few months. However, in severe cases, the condition may be fatal. Encephalitis caused by the herpes simplex virus often produces long-term effects, such as memory problems. In children, herpes simplex viral encephalitis may cause learning difficulties. The effects of this type of viral encephalitis can usually be minimized if treatment is begun early.
From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.