Dementia-related disability in the population aged 90 years and over: differences over time and the role of comorbidity in the vitality 90 + study

BACKGROUND: The burden of dementia, multimorbidity, and disability is high in the oldest old. However, the contribution of dementia and comorbidities to functional ability in this age group remains unclear. We examined the combined effects of dementia and comorbidities on ADL and mobility disability and differences between dementia-related disability between 2001, 2010, and 2018. METHODS: Our data came from three repeated cross-sectional surveys in the population aged 90 + in the Finnish Vitality 90 + Study. The associations of dementia with disability and the combined effects of dementia and comorbidity on disability adjusted for age, gender, occupational class, number of chronic conditions, and study year were determined by generalized estimating equations. An interaction term was calculated to assess differences in the effects of dementia on disability over time. RESULTS: In people with dementia, the odds of ADL disability were almost five-fold compared to people with three other diseases but no dementia. Among those with dementia, comorbidities did not increase ADL disability but did increase mobility disability. Differences in disability between people with and without dementia were greater in 2010 and 2018 than in 2001. CONCLUSION: We found a widening gap in disability between people with and without dementia over time as functional ability improved mainly in people without dementia. Dementia was the main driver of disability and among those with dementia, comorbidities were associated with mobility disability but not with ADL disability. These results imply the need for strategies to maintain functioning and for clinical updates, rehabilitative services, care planning, and capacity building among care providers. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1186/s12877-023-03980-5.