Down syndrome (DS) is characterized by increased mortality rates, both during early and later stages of life, and age-specific mortality risk remains higher in adults with DS compared with the overall population of people with mental retardation and with typically developing populations. Causes of increased mortality rates early in life are primarily due to the increased incidence of congenital heart disease and leukemia, while causes of higher mortality rates later in life may be due to a number of factors, two of which are an increased risk for Alzheimer's disease (AD) and an apparent tendency toward premature aging. In this article, we describe the increase in lifespan for people with DS that has occurred over the past 100 years, as well as advances in the understanding of the occurrence of AD in adults with DS. Aspects of the neurobiology of AD, including the role of amyloid, oxidative stress, Cu/ZN dismutase (SOD-1), as well as advances in neuroimaging are presented. The function of risk factors in the observed heterogeneity in the expression of AD dementia in adults with DS, as well as the need for sensitive and specific biomarkers of the clinical and pathological progressing of AD in adults with DS is considered.

At present, there may be over 210,000 people with Down syndrome (DS) over the age of 55 in the United States (US) who have significant needs for augmented services due to circumstances related to ordinary and/or pathological aging. From 1979 through 2003, the birth prevalence of DS rose from 9.0 to 11.8 (31.1%) per 10,000 live births in 10 representative US regions. This increase, largely due to women conceiving after age 35, portends an ever-growing population of people with DS who may be subject to pathogenic aging. Whereas Trisomy 21 is one of the most widespread genetic causes of intellectual disability (ID), it still is one of the least understood of all genetic ID syndromes. While longevity in people with DS has improved appreciably in as modest a period as 30 years, age-specific risk for mortality still is considerably increased compared both with other people with ID or with the typically developing population. The penetrance of the phenotype is widely distributed, even though a consistent genotype is assumed in 95% of the cases. Some, but not all body systems, exhibit signs of premature or accelerated aging. This may be due to both genetic and epigenetic inheritance. We now know that the long-term outcome for people with DS is not as ominous as once contemplated; a number of people with DS are living into their late 60s and 70s with few if any major signs of pathogenic aging. Alzheimer's disease (AD), a devastating disease that robs a person of their memory, abilities and personality, is particularly common in elder adults with DS, but is not a certainty as originally thought, some 20% to 30% of elder adults with DS might never show any, or at most mild signs of AD. DS has been called a mature well-understood syndrome, not in need of further research or science funding. We are only beginning to understand how epigenetics affects the phenotype and it may be feasible in the future to alter the phenotype through epigenetic interventions. This chapter is divided into two sections. The first section will review typical and atypical aging patterns in somatic issues in elder adults with DS; the second section will review the multifaceted relationship between AD and DS.

Rates of dementia in adults with mental retardation without Down syndrome were equivalent to or lower than would be expected compared to general population rates, whereas prevalence rates of other chronic health concerns varied as a function of condition. Given that individual differences in vulnerability to Alzheimer's disease have been hypothesized to be due to variation in cognitive reserve, adults with mental retardation, who have long-standing intellectual and cognitive impairments, should be at increased risk. This suggests that factors determining intelligence may have little or no direct relationship to risk for dementia and that dementia risk for individuals with mental retardation will be comparable to that of adults without mental retardation unless predisposing risk factors for dementia are also present.

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We interviewed caregivers and reviewed medical records of 278 adults with mental retardation with and without Down syndrome, 45 to 74 years of age. Standardized morbidity ratios were used to compare frequency of medical disorders in these adults to frequency in the general population. In adults with mental retardation, the frequency of common age-related disorders was comparable to that in the general population, but there was an increased frequency of thyroid disorders, nonischemic heart disorders, and sensory impairment. Surveillance of health status and increased access to health care for screening and treatment of age-related disorders that are more frequent in adults with mental retardation would be important to prevent the development or delay the impact of these conditions and to promote healthy aging.