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

See also: Meningitis

Encephalitis is a rare condition. It occurs more often in the first year of life and decreases with age. The very young and the elderly are more likely to have a severe case.

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
• Mosquito, tick, and other insect bites
• Skin contact

Different viruses will occur in different locations. Many cases will tend to cluster in a certain season.

Encephalitis caused by the herpes simplex virus is the leading cause of more severe cases in all ages, including newborns.

A number of viruses for which there is now a vaccine may also cause encephalitis. These include:

• Measles
• Mumps
• Polio
• Rabies
• Rubella
• Varicella (chickenpox)

Other viruses that cause encephalitis include:

• Adenovirus
• Coxsackievirus
• Cytomegalovirus
• Eastern Equine Encephalitis Virus
• Echovirus
• West Nile virus

The virus causes inflammation of brain tissue. The brain tissue swells (cerebral edema), which may destroy nerve cells, cause bleeding in the brain (intracerebral hemorrhage), and brain damage.

Other causes of encephalitis may include:

• An allergic reaction to vaccinations
• Autoimmune disease
• Bacteria, such as Lyme disease,syphilis, and tuberculosis
• Parasites such as roundworms, cysticercosis, and toxoplasmosis in AIDS patients and other people who have a weakened immune system
• The effects of cancer

Some patients may have symptoms of a cold or stomach infection before encephalitis symptoms begin.

When a case of encephalitis is not very severe, the symptoms may be similar to those of other illnesses, including:

• Fever that is not very high
• Mild headache
• Low energy and a poor appetite

Other symptoms include:

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

Symptoms in newborns and younger infants may not be as easy to recognize:

• Body stiffness
• Irritability and crying more often (these symptoms may get worse when the baby is picked up)
• Poor feeding
• Soft spot on the top of the head may bulge out more
• Vomiting

Emergency symptoms:

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

An examination may show:

• Abnormal reflexes
• Increased intracranial pressure
• Mental confusion
• Mouth ulcers
• Muscle weakness
• Neck stiffness
• Signs in other organs, such as the liver and lungs
• Skin rash
• Speech problems

Tests may include:

• Brain MRI
• CT scan of the head
• Culture of cerebrospinal fluid (CSF), blood, or urine (however, this test is rarely useful)
• Electroencephalogram (EEG)
• Lumbar puncture and CSF examination
• Tests that detect antibodies to a virus (serology tests)
• Test that detects tiny amounts of virus DNA (polymerase chain reaction -- PCR)

The goals of treatment are to provide supportive care (rest, nutrition, fluids) to help the body fight the infection, and to relieve symptoms. Reorientation and emotional support for confused or delirious people may be helpful.

Medications may include:

• Antiviral medications, such as acyclovir (Zovirax) and foscarnet (Foscavir) -- to treat herpes encephalitis or other severe viral infections (however, no specific antiviral drugs are available to fight encephalitis)
• Antibiotics -- if the infection is caused by certain bacteria
• Anti-seizure medications (such as phenytoin) -- to prevent seizures
• Steroids (such as dexamethasone) -- to reduce brain swelling (in rare cases)
• Sedatives -- to treat irritability or restlessness
• Acetaminophen -- for fever and headache

If brain function is severely affected, interventions like physical therapy and speech therapy may be needed after the illness is controlled.

The outcome varies. Some cases are mild and short, and the person fully recovers. Other cases are severe, and permanent impairment or death is possible.

The acute phase normally lasts for 1 - 2 weeks. Fever and symptoms gradually or suddenly disappear. Some people may take several months to fully recover.

Permanent brain damage may occur in severe cases of encephalitis. It can affect:

• Hearing
• Memory
• Muscle control
• Sensation
• Speech
• Vision

Go to the emergency room or call the local emergency number (such as 911) if you have:

• Sudden fever
• Other symptoms of encephalitis

Children and adults should avoid contact with anyone who has encephalitis.

Controlling mosquitoes (a mosquito bite can transmit some viruses) may reduce the chance of some infections that can lead to encephalitis.

• Apply an insect repellant containing the chemical, DEET when you go outside (but never use DEET products on infants younger than 2 months).
• Remove any sources of standing water (such as old tires, cans, gutters, and wading pools).
• Wear long-sleeved shirts and pants when outside, particularly at dusk.

Vaccinate animals to prevent encephalitis caused by the rabies virus.

Human vaccinations that are available include:

• A vaccination to prevent a form of viral encephalitis that often affects people living in dorms or in the military
• Herpes zoster
• Measles

Nath A. Berger JR. Acute viral encephalitis. In: Goldman L, Ausiello D, eds. Cecil Medicine. 23rd ed. Philadelphia, Pa: Saunders Elsevier; 2007: chap 439.

Beckham JD, Tyler KL. Encephalitis. In: Mandell GL, Bennett JE, Dolin R, eds. Principles and Practice of Infectious Diseases. 7th ed. Philadelphia, Pa: Elsevier Churchill Livingstone; 2009:chap 87.