ENCEPHALITIS

plural encephalitides inflammation of the brain; from Greek enkephalos (brain) and itis (inflammation). In some cases, the inflammation affecting the brain may involve adjoining structures as well; there is inflammation of the brain and of the spinal cord in encephalomyelitis and of the brain substance and of the meninges (the membranes covering the brain) in meningoencephalitis. Encephalitis is most often caused by the direct or indirect action of an infective organism and sometimes by such noninfective agents as chemicals. Although encephalitis can be produced by many different types of organisms, such as bacteria, protozoa, and helminths (worms), viruses are the most frequent causal agents. The encephalitis-producing viruses may be divided into two groups: (1) those that invade the body and produce no damage until they are carried by the bloodstream to the nerve cells of the brain, where they lodge and multiply (i.e., the rabies and arthropod-borne viruses); and (2) those that invade the body and first injure non-nervous tissues and then secondarily, and rarely, invade the brain cells (e.g., the viruses causing herpes simplex, herpes zoster, dengue, and yellow fever). The type of brain inflammation known as encephalitis lethargica, or sleeping sickness (to be distinguished from African sleeping sickness, or African trypanosomiasis), occurred in epidemics in Europe and in the United States about the time of World War I but has not been reported since 1930, though individuals with residual symptoms (postencephalitic Parkinsonism) may still be seen in neurological clinics. Although the influenza virus was suspected, the causative agent of sleeping sickness was never established. Among children, a large number of acute encephalitides are of the type known as demyelinating encephalitis, which may develop as a complication of such viral diseases as measles or chicken pox or as a result of vaccination against such viral diseases as smallpox. It is so-called because damage is not done to the nerve cell body but to the insulation (myelin sheath) surrounding the nerve fibres. Multiple sclerosis is the best known of another group of encephalitides in which there is injury to the myelin. Among the chemicals that occasionally produce encephalitis are lead, arsenic, mercury, ethyl alcohol, chlorinated hydrocarbons, morphine, and barbiturates. Symptoms common to most types of encephalitis are fever, headache, drowsiness, lethargy, coma, tremors, and a stiff neck and back. Convulsions may occur in patients of any age but are most common in infants. Characteristic neurological signs include uncoordinated and involuntary movements, weakness of the arms, legs, or other portions of the body, or unusual sensitivity of the skin to various types of stimuli. The symptoms, signs, and an examination of the cerebrospinal fluid can usually establish the presence of encephalitis, but they do not necessarily establish the cause, which often remains unknown. This situation makes specific treatment difficult, and even when the causative virus is known, there may be no drugs effective against it. Generally, treatment aims to relieve the symptoms and ensure quiet rest for the patient. The symptoms remaining after recovery from the acute phase of brain inflammation vary considerably, depending on the type of encephalitis and on the age and general health of the patient. Many individuals are weak and debilitated after an attack but recover with no serious aftereffects. Some encephalitides (i.e., Eastern and Western equine encephalitides, diseases of horses and mules that may be transmitted to humans by the mosquito) may cause irreparable brain damage in about 50 percent of the patients under one year of age. Any form of encephalitis in young children may damage the brain so that it can no longer develop properly.