Introduction

Encephalitis occupies a position of increasing importance owing to several developments:

1. wider availability of effective pharmacotherapy against several important pathogens;

2. the increasing prevalence of encephalitic disease in the setting of immunosuppressive syndromes;

3. greater recognition of the epidemiological importance of encephalitis in the developing world.

The 'acute encephalitis syndrome' features a triad of fever, head pain, and neurological dysfunction. General pathophysiology

The term encephalitis encompasses a diverse spectrum of infectious and non-infectious illnesses that range in clinical severity from the benign to the rapidly lethal. Inflammation of neural parenchyma is the cardinal pathological feature, and demonstration of pleocytosis on examination of cerebrospinal fluid is an essential step in confirming the diagnosis in the appropriate clinical setting. When encephalitis occurs in concert with inflammation of the coverings of the brain or its myelinated neural tracts, the terms meningoencephalitis and encephalomyelitis respectively are appropriately applied. The neurological damage resulting from encephalitis may result from three general mechanisms: direct cytotoxic effects of specific pathogens, damage resulting from the host's immunological response, and non-specific secondary injury mechanisms such as cerebral edema and vascular obstruction. In large part secondary injury mechanisms exacerbate neurological damage by superimposing hypoxia-ischemia. Most encephalitides are infectious illnesses. As with infectious disease in general, the spectrum of causal agents will depend on the socio-economic and geographic milieu. For example, in the developing world, primary central nervous system infections or systemic infections with prominent central nervous system involvement dominate neurological illness in general, particularly among adolescents and young adults. Parainfectious (or postvaccinial) encephalitis and paraneoplastic encephalitis are rare conditions, characterized by diffuse neurological damage, that are believed to result from unusual manifestations of the immunological response to foreign antigens or other related mechanisms.

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