[encephalo- + -itis]
Inflammation of the white and gray matter of the brain. It is almost always associated with meningoencephalitis and may involve the spinal cord (encephalomyelitis). In the U.S. 20,000 cases are reported annually.
SEE: arbovirus; SEE: herpesviruses; SEE: rabies

Most cases are caused by viruses: there are about 100 different viral agents that may infect the brain. The disease occurs more often in the very young, the very old, and patients with immune-suppressing illnesses. Mosquito-borne equine arboviruses (or, in some cases, a tick-borne virus) are the most common cause of encephalitis in the U.S. Mosquitoes are infected by feeding on infected birds, which then transmit the virus to humans and animals. Viruses may also be transmitted by inhalation (and passed person to person) or by ingestion of infected goat milk. The West Nile virus (WNV) can cause encephalitis and is related to St. Louis encephalitis (SLE). Encephalitis also occurs as a component of rabies, AIDS, and an aftereffect of systemic viral diseases, e.g., herpesvirus, influenza, measles, German measles, and chickenpox. Central nervous system (CNS) involvement occurs in 15% to 20% of patients with AIDS who develop cytomegalovirus infections. Other organisms causing encephalitis in immunosuppressed patients include fungi (such as Candida, Aspergillus, and Cryptococcus) and protozoa (such as Toxoplasma gondii).

Patients present with a wide variety of neurological symptoms, depending on the infected region of the brain and the type and amount of damage the organism has caused. Sudden onset of fever with headache and vomiting may be the first symptoms. These progress to stiff neck and back (meningeal irritation) and to signs of neuronal damage: drowsiness, seizures, tremors, ataxia, cranial nerve paralysis, abnormal reflexes, and muscle weakness and paralysis are common. Personality changes and confusion usually appear before the patient becomes stuporous or comatose. Coma may persist for weeks after the acute phase of illness.

The diagnosis is based on clinical presentation, culture and examination of blood and cerebrospinal fluid, and computerized tomography (CT) scan or magnetic resonance imaging (MRI) results.

Acyclovir is given for herpes simplex virus infection, the only common viral pathogen for which there is effective treatment. Survival and residual neurological deficits appear to be tied to mental status changes before acyclovir therapy begins. Rabies is treated with rabies immune globulin and vaccine. If the infection is bacterial, antibiotics are used. For other viruses, treatment focuses on supportive care and control of increased intracranial pressure (ICP) using osmotic diuretics, e.g., mannitol, corticosteroids, and drainage.

Mortality from encephalitis in the U.S. is approximately 6%.

The acutely ill patient's mental status, level of consciousness, orientation, and motor function are assessed for indications of increasing ICP and documented to monitor changes. The head of the bed is raised slightly to promote venous return; neck flexion is contraindicated. Sedatives help to control restlessness; aspirin or acetaminophen reduces fever and relieves headache. Measures to prevent stimuli that increase ICP are implemented, e.g., preoxygenating with 100% oxygen before suctioning, preventing isometric muscle contraction, using diet and stool softeners to minimize straining at stool, and using turning sheets and head support when turning the patient. Fluid intake should be adequate to prevent dehydration, but overload must be avoided to prevent further cerebral edema. Fluid balance and weight are monitored daily. Adequate nutrition should be maintained with small, frequent meals or enteral or parenteral feeding as necessary. Frequent oral care should be provided. Passive and/or active range-of-motion exercises and resistive exercises to prevent contractures and maintain joint mobility and muscle tone are used as long as they do not increase ICP.

Normal supportive care is provided in a quiet environment, with lights dimmed to ease photophobia, with no shadows, which increase the potential for hallucinations. Emotional support and reassurance should be provided and the patient reoriented if delirium or confusion is present. Behavioral changes that occur with encephalitis usually fade as the acute phase passes, but rehabilitation programs are necessary for the treatment of residual neurological deficits. Public health preventive measures include controlling standing water that provides mosquito breeding sites and insecticide spraying to kill larvae and adult mosquitoes. Public education should focus on reducing outdoor time during early morning and early evening hours, wearing appropriate covering clothing when exposure is unavoidable, and use of insect repellents containing DEET.

acute disseminated encephalitis

SEE: Postinfectious encephalitis.

anti-N-methyl-D-aspartate encephalitis

ABBR: anti-NMDAR encephalitis A relatively common immune-mediated cause of brain dysfunction, typically identified in young women (under age 30) with teratomas. Symptoms include rapid neuropsychiatric deterioration, autonomic instability, movement disorders, and/or seizures. Patients have circulating antibodies to N-methyl-D-aspartate receptors.

Australian encephalitis

SEE: Murray Valley encephalitis.

Bickerstaff brainstem encephalitis

SEE: Bickerstaff brainstem encephalitis

California (La Crosse) virus encephalitis

California-La Crosse virus encephalitisLa Crosse virus encephalitis A viral encephalitis that is the most common mosquito-borne illness in the U.S. It typically affects children in summer or early fall, largely in the Middle Atlantic or midwestern states, causing fever, headache, seizures, and localized muscle paralysis. The primary vector is Aedes triseriatus. A full recovery usually follows the illness.

cortical encephalitis

Encephalitis of only the brain cortex.

Cree encephalitis

SEE: Aicardi-Goutières syndrome.

eastern equine encephalitis

ABBR: EEE Encephalitis caused by the eastern equine alphavirus. It is transmitted to humans by mosquitoes; the incubation period is 1 to 2 weeks. Early symptoms of infection are nonspecific: fevers, headache, nausea, and vomiting, for example. Confusion, rigidity or spasticity, seizures, and coma may result. EEE is fatal in about a third of infected patients. Survivors may have lifelong behavioral, cognitive, or emotional deficits. In the U.S., it occurs on the East Coast, Gulf Coast, and in the Great Lakes region typically from midsummer to early fall. It is transmitted to humans by mosquitoes; the incubation period is 1 to 2 weeks. Early symptoms of infection are nonspecific, e.g., fevers, headache, nausea, and vomiting. Confusion, rigidity or spasticity, confusion, seizures, and coma may result. EEE is fatal in about a third of infected patients. Survivors may have lifelong behavioral, cognitive or emotional deficits. Although this is the least common of the arboviruses, mortality is approx. 25%, and those who survive often have neurological problems.

enteroviral encephalitis

Encephalitis caused by nonpolio enteroviruses such as echoviruses or coxsackie virus. It is more common in adolescents than in adults, but it may afflict people of all ages. It can be diagnosed by polymerase chain reaction (PCR) testing of CSF and/or brain biopsy. PCR tests are sometimes falsely negative.

epidemic encephalitis

Any form of encephalitis that occurs as an epidemic.

equine encephalitis

Encephalitis caused by either the western or the eastern equine arbovirus, which is carried by mosquitoes from horses. The disease ranges from mild to fatal.

granulomatous amebic encephalitis

Encephalitis caused by Acanthamoeba spp. and leptomyxid ameba. This potentially lethal encephalitis typically occurs in the immunosuppressed.

hemorrhagic encephalitis

Herpes encephalitis in which there is hemorrhage with brain inflammation.

herpes simplex encephalitis

SEE: Herpetic encephalitis.

herpetic encephalitis

Encephalitis caused by infection of the brain with herpes simplex virus-1 (or, less often, herpes simplex virus-2). This relatively common form of encephalitis typically involves the inferior surfaces of the temporal lobes and may cause hemorrhagic necrosis of brain tissue. It is fatal in at least one third of all cases. Acyclovir (or an analogue) is used to treat the infection.
SYN: SEE: herpes simplex encephalitis
Descriptive text is not available for this imageFalse-negative tests of cerebrospinal fluid (CSF) for herpes simplex virus are common early in the disease. Polymerase chain reaction tests of CSF should be repeated several days later if a patient with encephalitis does not improve or worsens after several days of supportive care.

immune encephalitis

Antibody-mediated encephalitis, such as encephalitis caused by antibodies against the N-methyl-D-aspartate glutamate receptors (NMDARs) of the brain.

infantile encephalitis

Encephalitis that occurs in infants. The most common agents are arboviruses and herpes simplex virus.

Japanese (B type) encephalitis

ABBR: JE Encephalitis caused by the Japanese B type arbovirus, an infection carried by swine. It occurs sporadically in Japan, Taiwan, China, and Korea and is controlled by vaccine.

lead encephalitis

Encephalitis due to lead poisoning.

encephalitis lethargica

A form of encephalitis that occurred frequently after the influenza pandemic of 1917-1918, but rarely since. Its hallmarks include paralysis of oculomotor function and marked sleepiness or coma. Survivors developed a parkinsonism-like illness.
SYN: SEE: Economo disease

Murray Valley encephalitis

An epidemic viral encephalitis originating in Murray Valley, Australia.
SYN: SEE: Australian encephalitis

neonatal encephalitis

A form of encephalitis occurring within the first several weeks of life.

paraneoplastic limbic encephalitis

ABBR: PNLE A brain disorder occurring in some patients with cancer, characterized by the rapid onset of memory loss, often with temporal lobe disease, seizures, delirium, or disturbances of mood. Patients with PNLE often have antibodies against tumor antigens that also react with nerve cell antigens. In some patients the neurological disorder improves after treatment of the responsible tumor.

encephalitis periaxialis

Inflammation of the white matter of the cerebrum, occurring mainly in the young.

postinfectious encephalitis

SEE: Acute disseminated encephalomyelitis.

postvaccinal encephalitis

SEE: Acute disseminated encephalomyelitis.

purulent encephalitis

Encephalitis characterized by abscesses in the brain.

raccoon roundworm encephalitis

Encephalitis characterized by inflammation of the meninges, eosinophilia, prolonged encephalopathy, retinitis, and delayed recovery with profound neurological deficits. It is transmitted to children (or others) who eat soil contaminated by raccoon feces.

Rasmussen encephalitis

SEE: Rasmussen encephalitis

Russian spring-summer encephalitis

Encephalitis due to a tick-borne virus. Humans may also contract it by drinking goat milk.

St. Louis encephalitis

Encephalitis caused by the St. Louis encephalitis virus (SLEV), which is transmitted by a mosquito. It occurs most often in summer and early fall.

tick-borne encephalitis

A flaviviral infection of the brain transmitted by Ixodes ticks.

toxic encephalitis

Encephalitis resulting from metal poisonings, e.g., lead poisoning.

western equine encephalitis

A mild type of viral encephalitis that has occurred in the western U.S. and Canada.