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The dementias are a group of conditions of which the principal feature is a loss of cognitive capacity, that is the ability to assimilate change in the environment. The conditions are acquired generally in old age, and thereby are distinguished from learning disabilities which are developmental. Dementias are also irreversible and by this feature are distinguished from the confusional states from which recovery may occur if the precipitating cause is removed. Disorientation in time and space is characteristic of the 'organic states' (dementias and confusional states), and memory failure is invariably present and usually progressive in the dementias. Much is known about the pathology of the dementias. In Alzheimer's disease, the commonest, there is accumulation of beta-amyloid protein which forms plaques within brain tissue, and the presennce of neuro-fibrillary tangles within neurones. There are also relatively selective neurotransmitter losses eg of the cholinergic and adrenergic projextions to the cerebral cortex. A number of predisposing genes are established of which by far the most important is ApoE. The presence of homozygocity for the ApoE4 allele substantially increases the individuals risk of suffering from Alzheimer dementia as also to a lesser extent does one E4 allele relative to the E3 and E2 alleles.
Changes resembling the plaques and tangles of Alzheimer dementia have been described in the great apes, but the changes are not nearly so severe as in the human condition and it is unclear whether there is cognitive loss.
Dementias occur in all human populations, probably with the same spectrum of pathology (transmissible dementias of the kuru type and wiith prion-related pathology eg in the Papua-New Guinea population, and the Parkinsonian-dementia syndrome of Guam being unusual exceptions).
The possibility has to be considered that the organic dementias with their age-related and approximately uniform incidence across populations represent sapiens-specific variation that relates to the genetic change implicated in the transition from a hominid precursor eg H heidelbergensis or H antecessor.
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