Painstaking precision in diagnosis rationalises treatment and improves prognostic certainty

Viral infections of the central nervous system are mostly uncommon complications of common systemic viral infections. Remarkably little is known about the factors that determine why a few patients are affected but most are not. While this uncertainty continues the immunisation of the whole child population is necessary for effective prevention.

Acute encephalopathies of viral (or unknown) origin occur most commonly in the first decade of life, with a peak incidence of 1 in 500–1000 infants in their first six months. 1 2 In practice the diagnosis of encephalitis is often based on an assessment of the clinical features and the exclusion of other possibilities. These include other intracranial infections; Reye's syndrome; hypoxia-ischaemia; haemorrhagic shock and encephalopathy; and metabolic, toxic, vascular, and traumatic disorders. Sometimes the pathogenesis and indeed the diagnosis are left in doubt. Pleocytosis in the cerebrospinal fluid is found in only half of patients with clinically suspected encephalitis, but it helps to narrow the wide differential diagnosis.

No one clinical picture is specific to a single virus or several viruses, and the neurological illness may be triggered by the infecting virus through more than one infective or immune mediated mechanism.3 4 Nevertheless, a careful history that takes account of geographical …