Transmissible spongiform encephalopathies prion dementias

These are rare forms of dementia, caused by accumulation of abnormal prion proteins in the brain. Prion stands for proteinaceous infectious particle, a unique form of infective agent, as it contains no genetic material. Prions are now accepted as the cause of a group of transmissible spongiform encephalopathies including scrapie (in sheep), chronic wasting disease (in deer), and bovine spongiform encephalopathy (BSE) ('mad cow disease').

Human examples of these disorders include Creutzfeld-Jacob Disease (CJD) and kuru (formerly found in Papua New Guinea cannibals; this epidemic peaked in the 1960s and is now more or less extinct, due to the decline of cannibalism). Sporadic CJD has always rarely occurred. Iatrogenic CJD cases involved transmission of the condition by use of pituitary tissue ('harvested' to obtain growth hormone) from brains post-mortem that turned out to be CJD infected. New variant (nvCJD) cases are those linked to BSE. Although at the height of the health scare regarding BSE, huge numbers of cases were predicted, as of August 2006, only 156 had been identified (http://www.cjd.ed.ac.uk/figures.htm).

BSE was described in British cattle in the 1980s and was probably due to use of scrapie-infected sheep products in cattle feed (the temperature at which such products should be treated to render them safe had been reduced by the government for reasons of economy). Other countries such as France had similar epidemics, which were not as well publicized.

Gerstmann-Straussler-Scheinker syndrome and fatal familial insomnia are other rare human examples.

Causes

In some families, these dementias appear to be due to an inherited prion gene mutation, which follows an autosomal dominant pattern. Other cases are infective, due to an abnormal prion protein acquired in various ways. The disorders can be transmitted to experimental animals, and human cases have followed neurosurgery, corneal grafting, or administration of cadaveric growth hormone.

Neuropathological features

Prion protein, a modified cell membrane protein, accumulates within the CNS. There is neuronal loss, astrocytic hyperplasia, and spongiform vacuolation in grey matter, with amyloid plaques.

Clinical features

Dementia, accompanied by myoclonus or ataxia, usually starts in middle life. EEG changes are characteristic, and diagnosis can be confirmed by finding a prion gene mutation in a blood sample. These diseases are always fatal, within a few months typically.

NvCJD is said to differ clinically from 'classical' CJD, in that the symptoms are often initially psychiatric, and progression is slower.

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