Background. While neuropathological studies indicate a high risk for Alzheimer's disease in adults with Down's syndrome, neuropsychological studies suggest a lower prevalence of dementia. In this study, cognitive deterioration in adults with Down's syndrome was examined prospectively over 4 years to establish rates and profiles of cognitive deterioration.

Methods. Fifty-seven people with Down's syndrome aged 30 years or older were assessed using a battery of neuropsychological tests on five occasions across 50 months. Assessments of domains of cognitive function known to change with the onset of Alzheimer related dementia were employed. These included tests of learning, memory, orientation, agnosia, apraxia and aphasia. The individual growth trajectory methodology was used to analyse change over time.

Results. Severe cognitive deterioration, such as acquired, apraxia and agnosia, was evident in 28·3% of those aged over 30 and a higher prevalence of these impairments was associated with older age. The rate of cognitive deterioration also increased with age and degree of pre-existing cognitive impairment. Additionally, deterioration in memory, learning and orientation preceded the acquisition of aphasia, agnosia and apraxia.

Conclusions. The prevalence of cognitive impairments consistent with the presence of Alzheimer's disease is lower than that suggested by neuropathological studies. The pattern of the acquisition of cognitive impairments in adults with Down's syndrome is similar to that seen in individuals with Alzheimer's disease who do not have Down's syndrome.