What is the prevalence of Alzheimer disease (AD) in Down syndrome (DS)?

Updated: Nov 13, 2019
  • Author: Norberto Alvarez, MD; Chief Editor: Jasvinder Chawla, MD, MBA  more...
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Several studies document that most, if not all, individuals with Down syndrome (DS) develop Alzheimer disease (AD). [16, 17] This is unrelated to the degree of mental retardation. As a result of better clinical management, persons with DS currently often reach age 50 years. In the 1920s, the life span of children born with DS was 9 years, in the 1960s it increased to 30 years, and in 1993 it reached 55 years. [51, 52] In 1996, it was reported that in California the life expectancy of a 1-year-old child with DS and profound mental retardation was 43 years and increased to 55 years in those with mild-to-moderate degree of mental retardation. [53] In the author’s experience at Wrentham Developmental Center, a facility for persons with intellectual and developmental disability, the average age of persons with DS at the time of death was 61 years. (range, 47-70 y). Thus, a strong trend is that the frequency of persons with DS and AD is likely to increase.

Fortunately in the last 2 decades, the special needs of elderly people with developmental delay, and especially those with DS, has gained recognition. Even though early workers in the field presented the issue of aging in developmental delay in the 1960s, [54] the first full session on “The Aging Mentally Retarded” was presented at the 12th Congress on Gerontology, in Germany, in 1981. [55]

Age and the presence of trisomy 21 are the most important factors in disease development. Neuropathologic findings related to AD have been described in all DS individuals older than 35 years. Early clinical signs and symptoms are observed at the end of the fifth decade to the beginning of the sixth decade of life. Mean age at the time of clinical diagnosis is 51 ± 6 years. Most persons with DS may develop AD by age 60-70 years; however, some may remain free of clinical indications of dementia into the late 70s. [5] Several studies described a subset of individuals with full trisomy 21 who do not appear to develop AD, even in old age. [56, 57]

The percentage of people with DS and AD varies in some of the epidemiologic studies presented. A review of these studies showed that 10-25% of patients had AD when aged 40-49 years, 20-50% had AD when aged 50-59 years, and 60-75% had AD when older than 60 years. In one study, all patients with DS who were older than 70 years had AD. [17]

It is not clear if dementia is more common in persons with developmental disability when individuals with DS are excluded. A longitudinal study by Strydom et al [58] evaluated the incidence of dementia in 222 adults older than 60 years (mean age, 68.8 y; standard deviation, SD 7.5; range, 60-94 y) with developmental disabilities but excluding individuals with DS. In this study, the incidence of dementia was 5 times higher than in individuals without developmental disability. Other studies in non-DS developmental disability are also in agreement regarding higher incidence of dementia in person with developmental disability. [59, 60]

However, other reports that also excluded persons with DS do not find differences. [61, 62] In addition, when persons with DS were excluded, [63] 1994 autopsy findings in individuals with intellectual disability confirmed that the incidence of AD is the same as in individuals without intellectual disability.

In summary, these and other epidemiological studies indicate that the high frequency of AD in persons with DS is unique, not related to the developmental disability but related to the cause of the DS and most likely related to the presence of an extra chromosome 21.

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