Karin Modig is Associate Professor of Epidemiology at Karolinska Institutet and leads the Ageing and Health research group. Their research uses population-based data to understand how health and disease develop as more people survive to advanced ages.
We often study multimorbidity from the perspective of the individual. When does a person develop their first chronic disease? What comes next? And which disease trajectories are associated with disability, healthcare use or mortality? From a clinical perspective, understanding an individual's disease trajectory is essential. For public health, however, we also need to understand how morbidity develops across an ageing population.
The population observed at age 95 is not simply the population observed at age 75 with another 20 years of ageing added. Along the way, people develop diseases, some die, and those who survive become an increasingly selected group. Those reaching very advanced ages are therefore a non-random subset of the original population with respect to their morbidity profiles. This selective survival is likely to depend on both the number and types of diseases people develop and may, in turn, shape the pattern of disease accumulation observed at the population level at older ages.
Taking a population perspective on disease accumulation can help us understand what it means for population health when more and more people survive into their 90s and even to 100. What does this mean for the overall disease burden in the population? When does disease accumulation take off? At what ages does the disease burden grow most rapidly? And does it simply continue to increase at the same pace as people reach their 90s and 100s?
Following a population from 70 to 100
In our study, we used Swedish nationwide register data to follow all individuals born between 1920 and 1922 who were alive and living in Sweden at age 70. We followed disease accumulation as the cohort aged, all the way to age 100.
Diseases were identified from specialist inpatient and outpatient care. This means that the absolute level of disease burden is likely to be somewhat underestimated, as conditions managed exclusively in primary care are not captured.
The pattern was striking. Disease accumulation accelerated up to around age 80, remained at a high and relatively stable rate between 80 and 90, and then began to slow. But slowing down did not mean that disease accumulation stopped. At age 90, men had an average of 3.9 chronic diseases and women 3.5. Among those surviving beyond age 95, around half were living with five or more chronic conditions. And while the pace of accumulation slowed at the highest ages, the combinations of diseases became increasingly complex.
Importantly, most of the increase in disease burden occurred among people who were not in their final two years of life. Longer lives have also been hypothesised to result, at least partly, from postponement or compression of morbidity towards the end of life. We also know that healthcare use is particularly high towards the end of life. Therefore, we might expect disease accumulation at older ages to be driven by people approaching death. This was not what we observed. Instead, much of the accumulation occurred earlier, among people with several years still left to live. Thus, multimorbidity in old age cannot simply be understood as an end-of-life phenomenon. As survival to advanced ages becomes more common, longer survival with multimorbidity may therefore become an increasingly important feature of population ageing, with implications for healthcare and support needs over extended periods of later life.
Why does accumulation slow at very old ages?
There are several possible explanations. One is diagnostic saturation: at very high ages, additional conditions may be less likely to be investigated and diagnosed. New conditions may also increasingly be managed exclusively in primary care and would therefore not be captured in our data. Another is selection. People reaching their 90s and 100s are a highly selected group. Individuals with particularly severe disease profiles are less likely to survive to these ages, meaning that the population still alive becomes increasingly selected towards individuals who have survived with their diseases or who have not developed them. However, even among the highly selected group surviving into their 90s and beyond, diseases continue to accumulate and, importantly, occur in increasingly complex combinations. By ages 90 and 100, combinations spanning four or more disease groups became increasingly common. Multimorbidity at these ages is therefore characterised not simply by accumulating more individual conditions, but by combinations of diseases affecting multiple organ systems.
As more people reach these ages, the challenge will therefore be not only how many diseases people live with, but how we prevent and manage increasingly complex combinations of conditions across a growing older population. Multimorbidity is not simply a feature of the last years of life. Much of the accumulation takes place over decades of older age, among people who still have several years left to live. This means that the growing burden of multimorbidity cannot be addressed solely through better care at the end of life.
Understanding when and how disease burden accumulates across the population is therefore an important part of preparing for a future in which living into the 90s and even to 100 will become increasingly common.
You can read the full on the Age and Ageing journal website: Slowing disease accumulation but persistent complexity: population-level multimorbidity dynamics in old age
