Experimental tests of cognitive functions were developed and standardised to detect the onset and progression of the early stage of Alzheimer disease in persons with Down syndrome. The aim was to determine whether or not there was a specific sequence of cognitive changes over a 3-year period for the test measures. When compared with a young group (17-39 years of age at the start), an old group of persons with Down syndrome (40-58 years of age at the start) showed small but statistically significant changes over time suggestive of "pre-clinical signs" of dementia. When the data were sorted into 4 subgroups on the basis of age, a more detailed analysis revealed that the subgroup that was 50 years of age and older at the start showed changes in scores which were of a magnitude more clearly indicative of early dementia on the test measures. Deterioration in learning/memory functions began at a mean age of 54.2 years, followed later by deterioration in movement-related functions (praxis) at a mean age of 56.9 years. Deterioration in scores on an informant-based behaviour rating scale (MOSES) occurred at an intermediate age of 55.0 years. The results provide preliminary support for the hypothesis that persons with Down syndrome who are 50 years of age and older may develop a specific sequence of functional changes during the early stage of dementia. They also illustrate ways in which small sample norms can be effectively used to increase the practical usefulness of tests intended to evaluate dementia in persons with intellectual disabilities.
