Encephalitis is an inflammatory disease of the brain. It is caused by a virus that either has invaded the brain, or a virus encountered elsewhere in the body that has caused a sensitivity reaction in the brain. Most cases of encephalitis are considered secondary to a virus infection that stimulates an immune reaction.
An infection that involves the membranes associated with the spinal cord is called meningitis. This is a less-serious condition than encephalitis in that it usually has few long-term effects. Encephalitis is an infection of the brain tissue itself, not the surrounding protective covering (meninges), so the consequences are more serious.
Among the many forms of encephalitis are those that occur seasonally (Russian spring-summer encephalitis, for example), those that affect animals (bovine, fox, and equine), and a form that is carried by a mosquito. Viruses that have been directly implicated in causing encephalitis include the arbovirus, echovirus, poliovirus, and the herpes virus. Encephalitis occurs as a complication of, for example, chickenpox, polio, and vaccinia, which is a cowpox virus used in smallpox vaccinations, as well as the common flu virus. The herpes simplex virus, responsible for the common cold sore, eczema, and genital herpes; the measles (rubeola) virus; some of the 31 types of echo-viruses that also cause a paralytic disease or an infection of the heart muscle; the coxsackievirus responsible for infections of the heart and its covering and paralysis; the mumps virus; the arboviruses that normally infect animals and can be spread by mosquito to humans—all have been implicated as causal agents in human encephalitis.
The virus responsible for the infection can invade the cranium and infect the brain via the circulatory system. The blood-brain barrier, a system that serves to protect the brain from certain drugs and other toxins, is ineffective against viruses. Once it has gained entrance into the brain the virus infects the brain tissue. The immediate reaction is an inflammation causing the brain to swell and activating the immune system. The tightly closed vault of the cranium leaves little room for the brain to enlarge, so when it does expand it is squeezed against the bony skull. This, with the active immune system can result in loss of brain cells (neurons), which can result in permanent postinfection damage, depending upon the location of the damage.
The individual who is developing encephalitis will have a fever, headache, and other symptoms that depend upon the affected area of the brain. He may fade in and out of consciousness and have seizures resembling epileptic seizures. He may also have rigidity in the back of the neck. Nausea, vomiting, weakness, and sore throat are common. Certain viruses may cause symptoms out of the nervous system as well. The mumps virus will cause inflammation of the parotid gland (parotitis), the spleen, and the pancreas as well as of the brain, for example. An infection by the herpes virus can cause hallucinations and bizarre behavior.
Treatment of encephalitis is difficult. It is important that the type of virus causing the infection be identified. Drugs are available to treat a herpes virus infection, but not others. Mortality (the death rate) can be as high as 50% among patients whose encephalitis is caused by the herpes virus. Infection by other viruses, such as the arbovirus, may have a mortality rate as low as 1%. Treatment is supportive of the patient. Reduction of fever, as well as treatment for nausea and headache are needed. Unfortunately, even those who survive viral encephalitis may have remaining neurologic defects and seizures.
Reye's syndrome is a special form of encephalitis coupled with liver dysfunction seen in young children and those in their early teens.
Invariably, the individual who develops Reye's syndrome has had an earlier viral infection from which they seemingly have recovered. Hours or days later they will begin to develop symptoms such as vomiting, convulsions, delirium, and coma. A virus such as the influenza virus, varicella (measles), and coxsackie virus are responsible. For reasons unknown, giving a child aspirin tablets to reduce fever accompanying a cold or flu can trigger Reye's syndrome.
At the time the nervous system begins to show signs of infection, the liver is also being affected. Fatty deposits begin to replace functional liver tissue. Similar fatty tissue can be found in the heart muscle and the kidneys. The relationship between the viral effects on the brain and the parallel liver damage is not known.
Treatment is not specific to the virus, but is directed at relieving pressure on the brain and reducing symptoms. The head of the bed can be elevated and the room left very cool. Care is taken to maintain blood sugar level at normal and not let it drop. Other blood factors such as sodium and potassium also fall quickly and must be corrected.
The mortality rate for Reye's syndrome can be as high as 25-50%. Early diagnosis and initiation of treatment play an important part in keeping the mortality low. Other factors, including age and severity of symptoms, affect the outcome. Some children who survive Reye's syndrome will show signs of brain damage such as impaired mental capacity or seizures.
Thus, it is important that children who contract one of the common childhood diseases of viral origin, such as mumps, measles, or chickenpox be watched closely to insure they do not develop symptoms of a brain infection from the same virus.
Adams, R.M. "Meningitis and Encephalitis: Diseases that Attack the Brain." Current Health 21 (October, 1994): 27-29.