Definition
Encephalitis is an inflammation (irritation and swelling) 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 may 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 accidentally ingested, are the most common cause.
In urban areas, enteroviruses are most common, including coxsackievirus, poliovirus, and echovirus. Other causes include herpes simplex infection, varicella (chickenpox or shingles), measles, mumps, rubella, adenovirus, rabies, Eastern Equine Encephalitis Virus, West Nile virus, and extremely rarely, allergic reaction to vaccinations.
Once the virus has entered the bloodstream, it may localize in the brain, causing inflammation of 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), which may 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.
Although most forms of encephalitis are caused by viruses, bacteria can also cause this problem. For example, Lyme disease, syphilis, and tuberculosis, all caused by bacteria, can cause encephalitis. AIDS patients and various other high risk individuals throughout the world can develop encephalitis due to parasites such as toxoplasmosis. Auto-immune disease and effects of cancer can also cause encephalitis.
Symptoms
Emergency symptoms:
- Loss of consciousness, poor responsiveness, stupor, coma
- Seizures
- Muscle weakness or paralysis
- Sudden onset of:
- Memory loss (amnesia), impaired short-term memory or impaired long-term memory
- "Flat" mood or lack of discernible mood, or mood inappropriate for the situation
- Diminished interest in daily activities
- Inflexibility, extreme self-centeredness, indecisiveness, or withdrawal from social interaction
- Impaired judgment
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 such as the liver and lungs.
- A lumbar puncture test and cerebrospinal fluid (CSF) examination may show clear fluid, high pressure, high white blood cell count and high protein levels -- indications of 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 (polymerase chain reaction, a test able to detect very tiny amounts of viral DNA) may identify the virus. Health care providers also rely on serology tests to provide evidence of viral infection. Serologies detect proteins called antibodies, which are produced in response to a specific virus or other foreign invader.
- An 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.
- A brain MRI, which provides high-quality pictures of the brain, or a CT scan of the head may be used to determine internal bleeding or specific areas of brain inflammation.
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