Encephalitis is an inflammation (irritation and swelling with presence of extra immune cells) of the brain, usually caused by infections. See also meningitis .

Causes, incidence, and risk factors

Encephalitis is most often caused by a viral infection and many types of viruses can cause it. Exposure to viruses can occur through insect bites , food or drink contamination, inhalation of respiratory droplets from an infected person, or skin contact. In rural areas, arboviruses 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 extremely rarely -- vaccinations . Once the virus has entered the blood stream, it can localize in the brain causing inflammation of the brain tissue 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. The elderly and infants are more vulnerable and may have a more severe course of the disease.

Signs and tests

Various symptoms resembling meningitis may be present. An examination may show signs of meningeal irritation (especially neck stiffness), increased intracranial pressure or other neurologic symptoms (such as muscle weakness , mental confusion, speech problems, and abnormal reflexes). The patient may have a skin rash, mouth ulcers, and signs of involvement of other organs (e.g., the liver and lungs).

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lumbar puncture test and cerebrospinal fluid (CSF) examination show clear fluid, high pressure, high white-blood cell count and protein levels (indicating inflammation). Blood may be present in the CSF.

Sometimes the virus can be detected in CSF, blood, or urine through a laboratory test called "viral culture." However, this test is cumbersome and rarely useful. In some cases viral "PCR" (which stands for "polymerase chain reaction," a test able to detect very tiny amounts of viral DNA) is used to identify the virus. Health care providers also rely on

serology tests (serologies detect some proteins called antibodies, which are produced in response to an specific virus) to provide evidence of viral infection.

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EEG (a test of the electrical activity of the brain) may provide indirect clues for the diagnosis of encephalitis. Some EEG wave patterns may suggest a seizure disorder, or point to a specific virus as cause of the infection. Certain EEG wave patterns can suggest encephalitis due to herpes, for instance.

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brain MRI , which provides high-quality pictures of the brain, or a CAT scan of the head may be used to determine the presence or absence of internal bleeding or focal areas of brain inflammation .

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. Most of the time, however, no specific antiviral drugs are available to combat the infection. Antibiotics may be prescribed when the infection is caused by some organisms, such as certain bacteria. Anti-seizure medications (such a phenytoin) are used to suppress seizures . On rare occasions, potent anti-inflamatory drugs called "steroids" (such as dexamethasone) are used to reduce brain swelling. Sedatives may be needed to treat irritability or restlessness . Other medications, like acetaminophen, 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 like physical therapy and speech therapy may be necessary after the acute illness is controlled.

Expectations (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 impairments including memory, speech, vision, hearing, muscle control, and sensation difficulties can occur in people who survive severe cases of encephalitis.

Calling your health care provider

Go to the emergency room or call the local emergency number (such as 911) if sudden fever , neurologic changes, and other symptoms suggestive of encephalitis occur.

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 caused by rabies virus. Vaccination is available to prevent a form of viral encephalitis which often affects people living in dorms or in the military.