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Definition

Encephalitis is irritation and swelling (inflammation) of the brain, usually due to infections.

See also: Meningitis

Causes, incidence, and risk factors

Encephalitis is most often caused by a viral infection. Many types of viruses may cause it. Exposure to viruses can occur through:

• Breathing in respiratory droplets from an infected person
• Contaminated food or drink
• Insect bites
• 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
• Echovirus
• Poliovirus

Other viruses that can cause encephalitis include:

• Adenovirus
• Cytomegalovirus
• Eastern Equine Encephalitis Virus
• Epstein-Barr virus
• Herpes simplex infection
• Measles
• Mumps
• Rabies
• Rubella
• Varicella (chickenpox or shingles)
• West Nile virus

AIDS patients and others at high-risk can develop encephalitis due to parasites such as:

• Certain roundworms
• Cysticercosis
• Toxoplasmosis

Although most forms of encephalitis are caused by viruses, the condition may also be caused by bacterial diseases, such as:

• Lyme disease
• Syphilis
• Tuberculosis

Extremely rarely, an allergic reaction to vaccinations can cause encephalitis. Autoimmune disease and the effects of cancer can also cause encephalitis.

Encephalitis is uncommon. The elderly and infants are more vulnerable and may have a more severe case of the disease.

Symptoms

When the virus enters the bloodstream, it may cause 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 destroy nerve cells, cause bleeding in the brain (intracerebral hemorrhage), and brain damage.

Symptoms include:

• Clumsiness, unsteady gait
• Confusion, disorientation
• Drowsiness
• Fever
• Headache
• Irritability or poor temper control
• Light-sensitivity
• Stiff neck and back (occasionally)
• Vomiting

• Loss of consciousness, poor responsiveness, stupor, coma
• Muscle weakness or paralysis
• Seizures
• Sudden change in mental functions:
  • Diminished interest in daily activities
  • "Flat" mood, lack of mood, or mood inappropriate for the situation
  • Impaired judgment
  • Inflexibility, extreme self-centeredness, indecisiveness, or withdrawal from social interaction
  • Memory loss (amnesia), impaired short-term or long-term memory

Reviewer Info: Jatin M. Vyas, PhD, MD, Instructor in Medicine, Harvard Medical School, Assistant in Medicine, Division of Infectious Disease, Massachusetts General Hospital. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc., 09/28/2008