Encephalitis
Overview
Encephalitis is irritation and swelling (inflammation) of the brain. It often coexists with inflammation of the covering of the brain and spinal cord (meningitis) and most cases are caused by viral infection. Encephalitis ranges from mild to severe and may result in permanent neurological damage and death.
Types
Primary Encephalitis-This type results from viral infection of the brain and spinal cord. Primary encephalitis may occur in isolated cases (sporadic) or occur in many people at the same time in the same area (epidemic).
The most common type of sporadic infection is herpes simplex encephalitis, which is caused by the herpesvirus. This type carries a high risk for serious neurological damage and death and can occur in newborns if the virus is passed from the mother to the infant during birth.
Arthropod-borne viruses (transmitted through the bite of insects and ticks) may cause arboviral encephalitis. Mosquitoes are the most common agents of transmission and most cases occur during warmer weather, when the insects are more active. Arboviral encephalitis and rabies encephalitis (usually transmitted through the bite of an infected animal) may be sporadic or epidemic.
In the United States, the most common types of arboviral encephalitis are St. Louis, La Crosse, western equine, and eastern equine. Recent outbreaks of West Nile encephalitis (transmitted by mosquitoes, commonly infects birds) have occurred in eastern, southeastern, and midwestern regions of the United States, following bird migration.
Other types of arboviral encephalitis include the following:
- Japanese (widespread in Asia)
- Murray Valley (endemic in Australia)
- Powassan (transmitted by ticks; occurs in Canada and the northern United States)
- Tick-borne (occurs throughout Europe; vaccine available)
- Venezuelan equine (common in Central and South America)
Secondary Encephalitis-This type develops as a complication of a viral infection or reactivation of a latent virus. Viruses can become reactive when the immune system is suppressed by other conditions (e.g., malnutrition, stress, disease). Infections that may cause secondary encephalitis include influenza, chickenpox (varicella-zoster), measles (rubeola), mumps, and German measles (rubella).
Secondary encephalitis that develops as a result of a variola virus infection following smallpox vaccination or reactivation of another viral infection (called acute disseminated encephalitis) is often fatal.
Incidence and Prevalence
Incidence of encephalitis throughout the world is difficult to determine because the disease is often underreported. Approximately 150 to 3000 cases, most of which are mild, may occur each year in the United States. Herpesvirus accounts for most cases of encephalitis in the United States.
Arboviral encephalitis is more prevalent in warm climates and incidence varies considerably from area to area and from year to year. St. Louis encephalitis is the most prevalent type of arboviral encephalitis in the United States, and Japanese encephalitis is the most prevalent type in other parts of the world.
Encephalitis is more common in children and young adults.
Causes and Risk Factors
Encephalitis is caused by several types of viral infections. Herpesvirus is the most common cause and encephalitis can also result from infection following smallpox vaccination and reactivation of viral infection such as influenza, chickenpox, measles, mumps, and rabies (usually in undeveloped countries).
Arthropod-borne viruses, which are usually transmitted by mosquitoes, cause arboviral encephalitis. People who live in warm, moist climates are at higher risk for this type.
Signs and Symptoms
Primary symptoms of encephalitis include sudden fever, stiff neck, malaise, sensitivity to light (photosensitivity), and headache. Infants may develop bulging of the soft spots (fontanels) of the skull.
Other early symptoms include the following:
- Abnormal sleep patterns
- Behavioral changes (e.g., lethargy, confusion)
- Exhaustion
- Nausea
- Muscle stiffness
- Sore throat
- Upper respiratory tract infection (coughing, sneezing, congestion)
Neurological complications that may be permanent or improve as the infection runs its course include the following:
- Altered mental state (e.g., disorientation, personality changes)
- Convulsions
- Drooping eyelids (ptosis), double vision (diplopia), crossed eyes (strabismus)
- Hyperactive deep tendon reflexes
- Increased intracranial pressure
- Loss of consciousness
- Mental retardation
- Motor dysfunction
- Partial paralysis (paresis) of the extremities
- Projectile vomiting
- Pupil irregularities
- Restlessness
- Seizures
- Tremor
Most people infected with an arthropod-borne virus do not develop encephalitis. Infection is usually does not produce symptoms (called asymptomatic) or causes flu-like symptoms such as fever, headache, and malaise.
Diagnosis
Diagnosis of encephalitis is based on the following:
- Medical history (including recent exposure to insects, travel, personality changes, and contact with unusual animals or illnesses)
- Neurological examination
- Blood and urine tests
- Imaging tests (e.g., CT scan, MRI scan, EEG)
- Spinal tap
A neurological exam is performed to evaluate mental status, detect neurological problems such as motor dysfunction and seizures, and help determine which area of the brain is affected.
Blood and urine tests are used to isolate and identify viruses. Enzyme-linked immunosorbent assays (ELISA), including IgM-capture ELISA (MAC-ELISA) and IgG ELISA, can identify viruses that cause encephalitis soon after infection. Polymerase chain reaction (PCR) can identify small amounts of viral DNA.
Computed tomography (CT scan) and magnetic resonance imaging (MRI scan) produce computer images of the brain and are used to detect abnormalities such as swelling (edema) and bleeding (hemorrhage). MRI is able to detect abnormalities earlier in the course of the infection.
Electroencephalogram (EEG) involves placing electrodes on the scalp to record and analyze electrical activity in the brain. Wave patterns can suggest seizure disorder or a specific viral infection, such as herpesvirus.
Spinal tap, or lumbar puncture, is performed to detect signs of infection in cerebrospinal fluid and help make a diagnosis. In this procedure, a needle is inserted between two lower spine (lumbar) vertebrae, cerebrospinal fluid is collected, and the fluid is analyzed for elevated white blood cell counts, blood, and the presence of virus.
Treatment
Treatment for encephalitis depends on the cause. Some cases of viral encephalitis can be treated successfully if medication is started as soon as possible.
If herpes simplex encephalitis is suspected, antiviral medication such as acyclovir (Zovirax®) or ribavirin (Virazole®) is often administered immediately to improve chances for recovery and prevent complications. Side effects of these medications include nausea, vomiting, and headache. Treatment for viral encephalitis also includes palliative care.
There is no cure for arboviral encephalitis and the goal of treatment is to relieve symptoms (palliative). Palliative care may include intravenous fluids (to prevent dehydration), antibiotics (to prevent secondary infections), and other medications (to prevent complications). Diuretics (e.g., furosemide, mannitol) may be administered to reduce intracranial pressure and benzodiazepines (e.g., lorazepam [Ativan®]) may be administered to prevent seizures.
Prognosis
Prognosis depends on the type of encephalitis, the patient's age, overall health, and status of the immune system. Encephalitis caused by rabies, eastern equine encephalitis, Japanese encephalitis, and untreated viral encephalitis caused by herpesvirus carry a high risk for serious neurological damage and death. The prognosis is worse in very young patients, elderly patients, and patients with compromised immune systems.
Acute disseminated encephalitis and encephalitis caused by rabies infection is often fatal. Rabies is transmitted through the bite of an infected animal and there is no cure once symptoms have developed.
Prevention
In areas where arboviral encephalitis is prevalent, insecticide spraying may be used to control outbreaks. Wearing insect repellent and avoiding outdoor activities when mosquitoes are active may also be helpful.
A vaccine for Japanese encephalitis is available in the United States. People traveling to areas of the world where this disease is prevalent should be vaccinated. A vaccine for tick-borne encephalitis is available in Europe.
Administering antiviral medication (e.g., acyclovir) as soon as possible when encephalitis caused by herpes virus is suspected may prevent serious neurological complications.