Frequency, distribution, and risk factors for Alzheimer's disease.

Both prevalence and incidence of AD increase steeply with advancing age in all populations investigated thus far. In general, women have higher prevalence and incidence; however, there are exceptions to this pattern. When considering only methodologically comparable surveys, and taking age into account, there are no major geographic differences in either prevalence or incidence. Data on incidence are unfortunately limited worldwide. There are no major time trends in either prevalence or incidence. The only definite risk factors for AD are age and familial aggregation. Putative risk factors are familial aggregation of Down's syndrome, familial aggregation of Parkinson's disease, late maternal age, head trauma, history of depression, and history of hypothyroidism. Cigarette smoking was found to be less common in the history of patients with AD than in that of controls. This association is probably spurious, however. Unfortunately, current knowledge about risk factors for AD does not justify the conduct of preventive trials or the introduction of large-scale interventions.