Causes and Risks:
Encephalitis is most often caused by a viral infection. The specific viruses involved may vary. Exposure to viruses can occur through insect bites, food or drink, or skin contact. In rural areas, arboviruses that are carried by mosquitoes or ticks, or that are accidentally ingested, are the most common cause.
In urban areas, enteroviruses are most common, including Coxsackie virus, poliovirus, and echovirus. Other causes include herpes simplex infection, varicella (chickenpox or shingles), measles, mumps, rubella, adenovirus, rabies, West Nile virus, and only rarely, vaccinations.
Once the virus has entered the blood stream, it can localize in the brain causing inflammation of the brain cells and surrounding membranes. White blood cells invade the brain tissue as they try to fight off the infection. The brain tissue swells (cerebral edema) and can cause destruction of nerve cells, bleeding within the brain (intracerebral hemorrhage), and brain damage.
Encephalitis is uncommon. It affects approximately 1,500 people per year in the U.S. However, elderly people and infants are more vulnerable to a more severe course of the disease.
Prevention: Public health measures to control mosquitoes (a mosquito bite can transmit some viruses) can reduce the incidence of some types of encephalitis. Animal vaccination is important to prevent encephalitis that occurs because of the rabies virus.
Symptoms:
Emergency symptoms:
Signs and Tests: Various symptoms resembling gastroenteritis or meningitis may be present, or symptoms may mimic subdural hematoma or brain abscess. An examination may show signs of meningeal irritation or increased intracranial pressure, or other neurologic symptoms such as muscle weakness or changes in sensation.
- A lumbar puncture test and cerebrospinal fluid (CSF) examination show clear fluid, high pressure, high white-blood cell and protein levels, and normal glucose. Blood may be present in the CSF.
- Viral cultures of CSF or blood that reveal any type of virus will confirm the diagnosis of encephalitis, however, viral cultures are rarely useful. In some cases viral PCR (polymerase chain reaction) may be used to identify the virus. Health care providers also rely on serology tests to provide evidence of viral infection.
- EEG test results are abnormal.
- A cranial MRI or a CT scan of the head may be used to determine the presence or absence of internal bleeding or focal edema (swelling of a portion of the brain).
Treatment: The goals of treatment are to provide supportive care and relieve symptoms.
Antiviral medications may be prescribed for herpes encephalitis or other severe viral infections. Antibiotics may be prescribed for bacterial infections. Anticonvulsants (such a phenytoin) are used to prevent or treat seizures. Corticosteroids (such as dexamethasone) are used to reduce brain swelling and inflammation. Sedatives may be needed for irritability or restlessness. Over-the-counter medications may be used for fever and headache.
Supportive care (rest, nutrition, fluids) allows the body to fight the infection. Reorientation and emotional support of confused/delirious persons may be helpful.
If brain function is severely affected, interventions -- such as physical therapy and speech therapy -- may be necessary after the acute illness is controlled.
Prognosis: The outcome varies. Some cases are mild, short, and relatively benign with full recovery. Others are severe, and permanent impairment or death is possible.
The acute phase normally lasts for 1 to 2 weeks, with gradual or sudden resolution of fever and neurologic symptoms. Neurologic symptoms may require many months before full recovery.
Complications: Permanent neurologic (such as memory, speech, vision, hearing, muscle control, and sensation) impairments can occur in people who survive severe cases of encephalitis.
Go to the emergency room or call the local emergency number (such as 911) if sudden fever, neurologic changes, and other symptoms of encephalitis are present.
