Intellectual disability and dementia

By Shereen Moloney. Published on 13/5/2020

Shereen Moloney, NZ Dementia Foundation Executive Director  discusses the  importance of recognising the growing group of people living with a dual diagnosis of intellectual disability and dementia.

The number of people with dementia in the general population in NZ is increasing, but the number of people with intellectual disability who have dementia is increasing at a much faster rate than the general population.

The estimated number of people with dementia in the general population is currently around 70,000, or 1.1% of the population. This has increased over 18% in the last three years. Research has shown that two out of every three NZers know or have known someone with dementia. For 64% of those people it is a family member and a third have been involved in their direct care and support. The estimated financial cost of dementia in NZ is $1B, but the estimated net value of the impact of dementia is $12.4B. The informal value of carers, who are removed fully or partially from the workforce to care for someone with dementia, was estimated at $37.7 million.

The number of people with dementia is forecast to increase to 170,000 by 2050, making up over 2.6% of the population, and more than triple current numbers. However, this calculation is based on the numbers in the aging population and takes little account of the increasing numbers of people developing dementia at a younger age, through intellectual disability or other reasons. The World Alzheimer Report 2011 revealed that in countries like New Zealand, only 60% of cases are diagnosed/documented. This means that there are potentially a further 40% of people with dementia in New Zealand that have not yet been diagnosed making the numbers above significantly higher.

There are increasing numbers of adults with intellectual disabilities who are surviving into old age. It's predicted that the proportion of people with intellectual disabilities over 65 years of age will have doubled by 2020, with over a third of all people with intellectual disabilities being over 50 years of age by then. With the exception of adults with Down syndrome, population studies show that the rate of occurrence of dementia among people with intellectual disability is about the same as in the general population (or about 6% of people age 60 and older). However, the rate among same-age adults with Down syndrome is much higher - about 25% for adults age 40, and studies suggest that more than 75 percent of those with Down syndrome age 65 and older have dementia, nearly six times the percentage of people in this age group who do not have Down syndrome.

The risk for people with Down syndrome is compounded. Research suggests that people with Down syndrome experience premature aging, likely to be as many as 20 years earlier than other adults. Many are in their mid to late 40s or early 50s when symptoms of dementia first appear, compared to the late 60s for adults with other intellectual disabilities. About 20 to 40 percent of adults with Down syndrome show the behavioural symptoms of dementia, and the progression of the disease takes, on the average, about eight years - less time than among people in the general population.

Studies have shown that the numbers of people with Down's syndrome who have Alzheimer's disease are approximately: 

1 in 50 of those aged 30 to 39 years
1 in 10 of those aged 40 to 49 years
1 in 3 of those aged 50 to 59 years
more than half of those who live to 60 or over.
These numbers indicate a greatly increased risk for dementia compared with the general population.

In the past, dementia has been seen as a condition that affects older people, and the policy focus has traditionally been on aged care. There is a growing awareness among policy makers that dementia affects a wider range of people, and that it also has a significant impact on their family/whanau.

The focus of the NZ Dementia Foundation is to proactively share knowledge and collaborate across the health and disability sector to urgently improve the way the health system responds. The NZ Dementia Foundation is committed to improving the quality of life for all people with dementia and their families/whanau in NZ, and the research above shows that a growing proportion of those people will be people with intellectual disabilities in the future. 


US Department of Health & Human Services National Institute on Ageing:

Alzheimers Disease in People with Down Syndrome

Fact Sheet: Alzheimers Disease in People with Down Syndrome

Alzheimers Disease in Down Syndrome Head, Powell, et al; European Journal Neurodegeneration Disease. 2012 Dec.

Further US National Institute of Ageing information on Down syndrome: Alzheimers and Down syndrome

A Genetic Cause of Alzheimers Disease: Mechanistic Insights from Down Syndrome (Wiseman, Al-Janabi et al; National Review of Neuroscience. 2015)

Sanford - Burnham Prebys Medical Discovery Institute:

Insight Into Why People With Down Syndrome Develop Alzheimers

Alzheimers disease in people with Down syndrome: the prospects for and the challenges of developing preventative treatments (Castro. P. Zaman. S. Journal of Neurology. 2017)

University College London. June 2018: New link between Alzheimers and Down syndrome discovered


About the Author

Shereen has been NZDC Executive Director since 2014. She holds Bachelor and Masters degrees, and has spent 20 years in leadership and management roles in the public and private sectors.

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