Encephalitis means inﬂammation or infection of the brain, usually caused by a virus; it may also be the result of bacterial infection. It occurs throughout the world and aﬀects all racial groups and ages. Rarely it occurs as a complication of common viral disease such as measles, mumps, glandular fever, or chickenpox. It may occur with no evidence of infection elsewhere, such as in HERPES SIMPLEX encephalitis, the most common form seen in Europe and America. RABIES is another form of viral encephalitis, and the HIV virus which causes AIDS invades the brain to cause another form of encephalitis (see AIDS/HIV). In some countries – North and South America, Japan and east Asia and Russia
– there may be epidemics spread by the bite of mosquitoes or ticks.
The clinical features begin with inﬂuenza-like symptoms – aches, temperature and wretchedness; then the patient develops a headache with drowsiness, confusion and neck stiﬀness. Severely ill patients develop changes in behaviour, abnormalities of speech, and deterioration, sometimes with epileptic seizures. Some develop paralysis and memory loss. CT (see COMPUTED TOMOGRAPHY) and MRI brain scans show brain swelling, and damage to the temporal lobes if the herpes virus is involved. ELECTROENCEPHALOGRAPHY (EEG), which records the brainwaves, is abnormal. Diagnosis is possible by an examination of the blood or other body ﬂuids for antibody reaction to the virus, and modern laboratory techniques are very speciﬁc.
In general, drugs are not eﬀective against viruses – antibiotics are of no use. Herpes encephalitis does respond to treatment with the antiviral agent, aciclovir. Treatment is supportive: patients should be given painkillers, and ﬂuid replacement drugs to reduce brain swelling and counter epilepsy if it occurs. Fortunately, most suﬀerers from encephalitis make a complete recovery, but some are left severely disabled with physical defects, personality and memory disturbance, and epileptic ﬁts. Rabies is always fatal and the changes found in patients with AIDS are almost always progressive. Except in very speciﬁc circumstances, it is not possible to be immunised against encephalitis.
Encephalitis lethargica is one, now rare, variety that reached epidemic levels after World War I. It was characterised by drowsiness and headache leading on to COMA. The disease occasionally occurs as a complication after mumps and sometimes aﬀected individuals subsequently develop postencephalitic PARKINSONISM.