Down syndrome, which is the most common form of mental retardation, appears in 1.0 to 1.2 per 1,000 live births' and 15 percent of the mentally retarded population.2 In the late 1930's and 1940's, the average life expectancy ofpersons with Down syndrome did not extend beyond the middle 20s. However, with advances in medical technology and improvements in personal care procedures, approximately 70 percent of persons with Down syndrome now live beyond their 50th birthday.3According to recent longitudinal studies, all of these individuals will have the neuropathology of Alzheimer's disease by age 35 with an average age of clinical dementia onset between 51 and 54 years of age.4
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References
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4. Lai F, Williams RS (1989): A prospective study of Alzheimer's disease in Down Syndrome. Arch Neruol46: 849-853.
