Prevalence of dementia in adults with and without Down syndrome.

Declines in adaptive behavior were examined in a study of dementia in adults with Down syndrome and other forms of mental retardation. No significant differences were found between adults under 50 years of age with and without Down syndrome. In contrast, individuals over 50 who had Down syndrome were more likely to be classified as having dementia over a range of quantitative decision criteria; nevertheless, prevalence estimates of dementia were substantially below the presumed 100% prevalence of neuropathological markers of Alzheimer disease. This apparent discrepancy between functional and neuropathological findings may be associated with variations in risk associated with Down syndrome genotypes and/or a true lack of correspondence between classical neuropathological hallmarks of Alzheimer disease in this population and clinical expression.