Viral encephalitis - Tips from Other Journals

Date: Nov, 1990

Viral Encephalitis

Approximately 20,000 cases of encephalitis occur annually in the United States. Whitley reviews the diagnosis and treatment of acute and postinfectious encephalitis. Clinical findings commonly seen in patients with acute viral encephalitis include altered level of consciousness, disorientation, behavior and speech disturbances, and neurologic signs, sometimes focal but generally diffuse. These findings are helpful for distinguishing viral encephalitis from viral meningitis, which is characterized by nuchal rigidity, headache, photophobia and fever.

In the United States, herpes simplex virus is the most common cause of nonepidemic, sporadic, acute focal encephalitis. Use of vidarabine or acyclovir has significantly reduced morbidity and mortality from herpes simplex encephalitis. Other causes of acute viral encephalitis include human immunodeficiency virus (HIV), rabies virus and several arthropod-borne viruses.

A distinction must be made between acute viral encephalitis and postinfectious encephalomyelitis, a condition that develops days to weeks after the onset of a viral upper respiratory illness. Postinfectious encephalomyelitis generally develops following a vague viral syndrome, usually of the respiratory tract, and is most common in children. Measles is a major cause of postinfectious encephalomyelitis in countries in which vaccination is not routine. Demyelination is a prominent pathologic finding.

Cerebrospinal fluid findings in viral encephalitis include pleocytosis, a predominance of mononuclear cells and an elevated protein level. Cerebrospinal fluid is normal in approximately 3 to 5 percent of patients with severe viral infections of the central nervous system, such as herpes simplex encephalitis. Cultures of cerebrospinal fluid are of little value.

Neurodiagnostic studies, such as computed tomographic (CT) scanning, electroencephalography and magnetic resonance imaging (MRI), are useful in the evaluation of patients with altered mentation and fever. BRain biopsy is the most sensitive and specific means of diagnosing herpes simplex encephalitis and the diseases that mimic it. This procedure is currently reserved for patients who do not respond to acyclovir therapy and have an undiagnosed abnormality on CT scan or MRI. (New England Journal of Medicine, July 26, 1990, vol. 323, p. 242.)

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