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What is Encephalitis?

Updated on April 28, 2011

Encephalitis is an inflammation of the brain occurring in man as well as in other animals. It is often accompanied by inflammation of the spinal cord (encephalomyelitis) or the meninges, the membranes covering the brain (meningoencephalitis). A wide variety of bacteria, spirochetes, fungi, and parasites may invade the central nervous system and cause an inflammation, but the term "encephalitis" is usually restricted to those inflammations caused by viral infections or allergic reactions that may follow viral infections.

The signs and symptoms of encephalitis vary according to the part of the brain or spinal cord involved. They include convulsions, confusion, stupor, or coma; speech difficulties, such as aphasia or mutism; muscular weakness on one side of the body; involuntary movements, muscular incoordination, and jerky motions; nystagmus (rapid involuntary movements of the eyeballs) and facial weakness. Signs of meningeal involvement include fever, headache, stiff neck, and an increase in the number of white blood cells in the spinal fluid.


Types of Encephalitis

A large number of viruses may invade the nervous system, but relatively few occur frequently enough to be of importance. The major group consists of the arthropod-borne viruses, which are transmitted by mosquitoes and other arthropods. These infections tend to occur during specific seasons and in certain geographic locations. Mosquito-borne infections, such as western equine encephalitis, St. Louis encephalitis, Japanese B. encephalitis, and eastern equine encephalitis, occur in late summer and early fall. They are acute, devastating diseases of the cerebrum and are often fatal. Those who survive are usually left with neurological damage. Tick-borne infections, such as Russian spring-summer encephalitis, are very similar to those borne by mosquitoes.

Acute inclusion cell encephalitis, which is caused by the herpes simplex virus (the cold sore virus), occurs throughout the world at all times of the year. It is fatal in about one third of all cases, and many patients who survive are left with seizures, impairment of mental function, and a type of psychosis in which memory is impaired. Subacute inclusion cell encephalitis, also called subacute sclerosing panencephalitis, chiefly affects children and is probably caused by the measles virus. It is characterized by chronic, progressive mental deterioration, muscular incoordination, and jerky movements. Rarer forms of encephalitis are caused by the viruses that cause rabies, mumps, and polio.

Acute disseminated encephalomyelitis is a disorder of the brain and spinal cord characterized by widespread inflammation and other changes in the nerve tissue around small veins. These changes may occur as complications of measles, chicken pox, or smallpox or rabies vaccinations. They are not caused by a direct invasion of the nerve tissue by the viruses but by an altered immune response on the part of the nervous system to the preceding viral invasion.

An epidemic of encephalitis called encephalitis lethargica followed World War I and was prevalent for about the next 10 years. The causative virus was never identified, and the disease seems to have disappeared, but the symptoms caused by this disease are still observed.


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