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| Dementia in Alzheimer's disease |
Alzheimer's disease is a primary degenerative cerebral disease of unknown etiology,
with characteristic neuropathological and neurochemical features. It is usually
insidious in onset and develops slowly but steadily over a period of years. This period
can be as short as 2 or 3 years, but can occasionally be considerably longer. The onset
can be in middle adult life or even earlier (Alzheimer's disease with early onset), but
the incidence is higher in later life (Alzheimer's disease with late onset). In cases with
onset before the age of 65-70, there is the likelihood of a family history of a similar
dementia, a more rapid course, and prominence of features of temporal and parietal
lobe damage, including dysphasia or dyspraxia. In cases with a later onset, the course
tends to be slower and to be characterized by more general impairment of higher
cortical functions. Patients with Down's syndrome are at high risk of developing
Alzheimer's disease.
There are characteristic changes in the brain: a marked reduction in the population of
neurons, particularly in the hippocampus, substantia innominata, locus ceruleus, and
temporoparietal and frontal cortex; appearance of neurofibrillary tangles made of
paired helical filaments; neuritic (argentophil) plaques, which consist largely of
amyloid and show a definite progression in their development (although plaques
without amyloid are also known to exist); and granulovacuolar bodies. Neurochemical
changes have also been found, including a marked reduction in the enzyme choline
acetyltransferase, in acetylcholine itself, and in other neurotransmitters and
neuromodulators.
As originally described, the clinical features are accompanied by the above brain
changes. However. it now appears that the two do not always progress in parallel: one
may be indisputably present with only minimal evidence of the other. Nevertheless,
the clinical features of Alzheimer's disease are such that it is often possible to make a
presumptive diagnosis on clinical grounds alone.
