The core pathology of AD was described by Alois Alzheimer early in 1907: extracellular senile or neuritic plaques made up of an amyloid core, surrounded by cell debris, and intracellular neurofibrillary tangles. More recently Robert Terry has emphasized the importance of neuronal cell loss, and Patrick McGeer has documented a strikingly enhanced cellular immune response in the brain of persons with AD. Peter Whitehouse has demonstrated a relatively selective loss of cholinergic neurons in basal forebrain structures, particularly the nucleus basalis of Meynert. This observation, coupled with the reduction in levels of the acetylcholinesynthesizing enzyme choline acetyltransferase, suggested a neurotransmitter deficiency amenable to pharmacotherapy, similar to dopamine deficiency in Parkinson's disease.
Genetic factors clearly play a major role in AD. Presenilin genes carried on chromosomes 1 and 14, and genes on chromosome 21 modifying beta-amyloid metabolism, cause AD at relatively young ages in a Mendelian dominant pattern. Other genes, such as apolipoprotein E on chromosome 19, increase the risk of AD but do not cause it. Many other genes related to late-onset AD (the most common type) remain to be identified.
Acquired factors over a lifetime can positively or negatively modify the genetic risks. Epidemiological studies have confirmed and found risk factors (see Table 2) and protective factors for AD (see Table 3). Caution should be exerted, since the relative importance of such factors varies between studies. For instance, smoking was considered alternatively a risk and a protective factor; it is now considered neutral as far as AD (but a major risk factor for many other health conditions). High aluminum water content and closed head trauma have been considered risk factors, but the current consensus is that this is not the case. There is currently uncertainty as to the preventive value of hormone replacement therapy (HRT) in postmenopausal women.
Some of these factors clearly make biological sense: systolic hypertension increases the risk of strokes, an additional burden to the aging brain with plaques and tangles; apolipoprotein E4 carriers have a reduced ability to maintain synaptic plasticity (or repair) abilities; NSAIDs suppress the chronic brain inflammatory response associated with neuronal loss; higher education increases the density of synaptic connections; red wine contains a natural antioxidant. Some of these factors interact: higher education and longer HRT (if confirmed to be of value in ongoing randomized studies) will lead to a reduction in the risk of AD associated with female gender. It is hypothesized that a careful weighing of these risk and protective factors for individuals could lead to a preventive strategy in which advice would be proportional to the risk. For example, a person carrying a double apolipoprotein E4 mutation (from both parents) and a positive family history of AD may want to take NSAIDs chronically. Other risk factors can be modified for all individuals, such as systolic hypertension. This strategy needs to be validated in prospective studies but offers hope of delaying onset of symptoms of AD by five to ten years for the population as a whole, thus significantly reducing the prevalence of AD within one generation.