Encephalitis is a term that theoretically applies for any inflammatory condition involving the brain. A broad distinction is, however, usually made between focal and diffuse parenchymal inflammatory conditions and the term encephalitis is usually reserved for the latter. Viruses are the commonest cause of meningitis. Other infectious agents such as bacteria and fungi are usually responsible for abscesses or focal lesions, sometimes referred to as cerebritis.
Cerebral parenchymal inflammatory conditions can be associated with either meningitis or myelitis.
Various types of encephalitis exist as a result of the variety of possible interactions between the aetiological agent and the infected tissue. Acute necrotizing encephalitis is the result of a direct insult by the infecting agent in immunocompetent patients and it is the most common type of encephalitis. The disease presents with headache, fever and eventually seizures and neurological deficits; it rapidly progresses if untreated to impairment of consciousness and coma. Herpes simplex virus encephalitis is the prototype of this type of infection. Similar infections in the immunocompetent patient tend to present with a geographic predilection, in epidemic forms and with seasonal occurrence. Examples of this include infections transmitted by blood sucking arthropod vectors such as mosquitos and ticks in which the virus multiplies. These types of encephalitis are mostly named after the geographic area in which they develop (St. Louis, California, Japanese encephalitides) or after the mammal with which the human infection is shared (equine encephalitis). Rabies is another acute encephalitic syndrome. This disease, characterized by hyperirritability, laryngal spasm, hydrophobia and coma, is transmitted by the direct bite of dogs and other domestic and wild animals. Common and widespread exanthematous infections of children such as mumps and measles may also be responsible for acute encephalitis and encephalomyelitis by direct insult. An indirect mechanism underlies the development of acute disseminated encephalomyelitis (ADEM) an inflammatory/demyelinating syndrome that develops following exanthematous infections and vaccination (to smallpox and rabies) and which is mediated by immunopathogenetic factors.
In immunocompromized patients the encephalitides follow a chronic course and present with cognitive signs and mental alterations progressing to dementia over months, rather than with focal neurological deficits. The commonest aetiological agents in such patients are HIV, CMV, HSV and VZV. Group B human papovavirus JC is responsible for the extensive destruction of oligodendroglia resulting in a progressive and fatal demyelinating disease (progressive multifocal leukoencephalopathy (PML)).
Some encephalitides follow a chronic course independently of the immune status of the patients. Examples include subacute sclerosing panencephalitis (SSPE), a disease caused by a defective measles virus and occurring several years after measles infection, and progressive rubella panencephalitis a similar disease caused by rubella virus. Unconventional transmissable agents (prions) are believed to be responsible for noninflammatory diseases with spongiform degeneration such as Kuru and Creutzfeldt Jakob disease.
A particular case in the spectrum of interactions between aetiological agents and infected brain is represented by congenital infections whose sequelae reflect both the specific agent involved (one among the so-called TORCH agents group: Toxoplasmosis, Rubella, Cytomegalovirus and Herpes simplex); the timing of the insult relative to brain development often results in extensive encephaloclastic lesions and malformations.
Some clinically defined syndromes are believed to be infectious in origin but a specific agent has not yet been discovered. An example of this is Rasmussen encephalitis in children which is characterized by unremitting motor seizures involving part or all of one side of the body and followed by slow neurological deterioration.
MR has become a first line tool in the assessment of brain lesions related to clinical infectious syndromes. In some cases, such as in Herpes simplex acute necrotizing encephalitis the MR picture, in the appropriate clinical setting, is quite suggestive of the diagnosis and it may help prompting immediate treatment. In the remaining cases the pattern is nonspecific and is characterized by different degrees of white and grey matter abnormalities in different distributions.
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