Prevalence and incidence rates in centenarians
Research on the very old, often called the 'oldest-old,' indicates that while advanced age is the single greatest risk factor for dementia, the story is complex. The exact percentage of centenarians with dementia varies among studies, largely due to differences in sample size, location, research methods, and diagnostic criteria. Some studies from the 2010s showed prevalence figures exceeding 50% or even 80%, while a more recent 2022 study in the UK using electronic health records found dementia was recorded in only 11% of centenarians, though this likely represents a significant underreporting of true prevalence.
The 90+ Study: A key insight
A notable contribution comes from 'The 90+ Study,' a longitudinal study of aging and dementia in individuals aged 90 and older. Findings from this study have shown an exponential increase in dementia incidence with age, even beyond 90. One finding reported that the incidence rate of all-cause dementia was 40.7% per year in the 100+ age group. This rate continued to increase exponentially with age, doubling approximately every 5.5 years.
Why do studies show different percentages?
Several factors contribute to the variation in reported dementia statistics for centenarians:
- Methodological challenges: Diagnosing dementia in individuals over 100 can be difficult due to high rates of other comorbidities, including physical and sensory impairments that can mimic or obscure cognitive decline. Many standard cognitive tests were not developed for this age group.
- Subject bias: Studies relying on volunteers may suffer from a 'healthy volunteer' effect, potentially underestimating the true prevalence of dementia by missing those in advanced stages of decline or those in long-term care facilities.
- Diagnostic criteria: Different studies use varying diagnostic criteria, which can lead to different prevalence rates. Some rely solely on cognitive test scores, while others combine test results with information from caregivers and clinical interviews, which may provide a more accurate picture.
- Cohort effects: Later-born cohorts of centenarians have sometimes shown better mental and physical function, suggesting that improvements in healthcare, education, and nutrition over time might lead to different outcomes for subsequent generations reaching extreme old age.
The complex pathology behind centenarian dementia
Autopsy studies of centenarians have revealed a high prevalence of various brain pathologies, often finding more than one type of pathology in a single brain. These include Alzheimer's disease neuropathology, vascular pathology, and Lewy body disease.
Multiple pathologies are common
Researchers from The 90+ Study, analyzing autopsied brains, found that the accumulation of multiple neuropathological changes was strongly associated with a higher risk of dementia. The odds of dementia increased with the increasing number of pathologies present. This suggests that in the very old, dementia is more often a result of a mix of underlying brain changes rather than a single cause.
Weakening link between AD pathology and dementia
Interestingly, the link between Alzheimer's disease (AD) neuropathology and clinical dementia appears to weaken with advancing age. This means a centenarian could have significant brain changes characteristic of AD yet show minimal or no cognitive impairment, a phenomenon known as cognitive resilience. This highlights that many factors beyond AD pathology contribute to the clinical expression of dementia in this age group.
Cognitive resilience: A protective factor
Despite the high prevalence of dementia and neuropathological changes, a notable percentage of centenarians remain cognitively healthy. This concept of cognitive resilience is a key area of study in gerontology. Factors associated with resilience include:
- Higher levels of education.
- Maintenance of physical and mental activity.
- Lower burden of other medical conditions.
- Genetics, including a lower frequency of the ApoE e4 allele, which is a known risk factor for Alzheimer's disease.
These findings offer hope that preventative strategies and healthy lifestyle choices may play a role in delaying or mitigating the onset of cognitive decline, even in the presence of neuropathology.
The public health implications
The demographic shift towards an older population, particularly the oldest-old, has significant public health implications. As the number of centenarians continues to grow, so does the public health burden of dementia. Research like that from the University of California, Irvine, in The 90+ Study underscores the need for:
- More targeted research to understand the specific causes and protective factors for dementia in the very old.
- Improved diagnostic tools that account for the unique health profile and comorbidities of centenarians.
- Healthcare planning to address the rising needs of this demographic.
- Emphasis on lifestyle interventions that may bolster cognitive resilience and delay the onset of dementia for as long as possible.
Conclusion: A growing puzzle
Understanding the question of how many people over 100 have dementia? is a crucial step toward better senior care. While the prevalence is high, it is not an inevitable outcome of extreme old age. The existence of cognitively healthy centenarians demonstrates that resilience is possible, driven by a complex interplay of lifestyle factors, genetics, and unknown biological mechanisms. By continuing to research this unique population, scientists hope to unlock the secrets of successful cognitive aging and provide better care for future generations.
For more in-depth information on the research, explore the Wiley Online Library article titled "Study of neuropathological changes and dementia in 100 centenarians and comparison with nonagenarians in The 90+ Study".
