Encephalitis is inflammation of the brain. The inflammation may involve the whole brain, or just parts of the brain.

Viral infection of the central nervous system can be asymptomatic, present with mild symptoms, or cause meningitis and/or encephalitis. Most cases of encephalitis are caused by a viral infection. Encephalitis may be sporadic or epidemic. In the United States, the most common cause of sporadic encephalitis is the herpes simplex virus. Epidemic encephalitis are usually mosquito- or tick-borne, which may be dependant on the geography and season.

The most common viruses that cause encephalitis include:

• Viruses carried by mosquitoes such as West Nile virus
• Chickenpox virus
• Herpes simplex virus
• Polio virus
• Epstein-Barr virus
• Measles virus
• Mumps virus
• Rotavirus
• Influenza virus
• Rabies
• Enterovirus

A risk factor is something that increases your chance of getting a disease or condition. Risk factors for encephalitis include:

• Living, working, or playing in an area where mosquito-borne viruses are common
• Not being immunized against diseases such as:
  • Measles
  • Mumps
  • Chickenpox
  • Polio
• Having cancer
• Taking immunosuppressive medicines after organ transplant
• Having AIDS

Newborns of mothers who have genital herpes simplex are at risk for herpes simplex encephalitis.

The symptoms may range from mild, such as fever and headache, to severe, such as seizures, loss of consciousness, and permanent neurological damage. Death may also result.

Symptoms include:

• Fever
• Weakness, severe fatigue
• Headache
• Sensitivity to light
• Stiff neck and back
• Vomiting
• Changes in consciousness
• Muscle aches
• Rash
• Personality changes
• Confusion
• Irritability
• Seizures
• Partial or complete paralysis
• Progressive drowsiness
• Yawning
• Trouble walking
• Trouble speaking
• Trouble swallowing

The doctor will ask about your symptoms and medical history, and do a physical exam.

Tests may include:

• Blood tests—to look for signs of infection
• Spinal tap—to test spinal fluid for signs of infection
• CT and/or MRI scans of the head—to look for abnormal areas of enhancement, hemorrhage, or edema in the brain
• Electroencephalogram (EEG)— to look for abnormal electrical activity in the brain
• Brain biopsy—removal of a small sample of brain tissue to test for signs of infection

Treatment is mostly supportive. It may include:

• Antiviral drugs (eg, intravenous acyclovir for herpes simplex encephalitis)—to potentially help shorten the duration of the illness
• Steroid medicines—to decrease brain swelling
• Diuretics such as mannitol—to decrease elevated intracranial pressure
• Intubation with hyperventilation—to decrease elevated intracranial pressure and to maintain respiration and ventilation
• Anticonvulsant medicines—to prevent and/or treat seizures