Etiology

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).

Symptoms

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.

Diagnosis

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.

Treatment

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.

Patient care

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.