Cranial viral infections in the adult.

Viral infections of the adult are fortunately rare conditions but may carry serious clinical sequelae. Infection is usually acquired by haematogenous spread during a systemic viral illness and may be acute, subacute or chronic. The pathological basis of neuronal degeneration and attempt to repair is common to all illnesses and diagnosis is generally made by analysis of the pattern of disease. Magnetic resonance imaging is now the mainstay of imaging diagnosis. Acute infections include encephalitis due to a wide range of infecting agents and outcome depends on the severity of the acute episode. Subacute and chronic infections, including HIV encephalopathy, most often produce a progressive leucoencephalopathy and ultimately cerebral atrophy. Additionally, disease may also be immune mediated, that most closely associated with viral infection being acute disseminated encephalomyelitis, which is usually a monophasic illness. Finally, prion diseases are characterised by long incubation period and progressive course, leading to death.