Encephalitis

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, West Nile virus, and extremely rarely, 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.

Symptoms

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

Emergency symptoms:

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

Signs and testsVarious 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.