Slowing disease accumulation but persistent complexity: population-level multimorbidity dynamics in old age

Most research on multimorbidity has focused on individual disease trajectories, with less attention to how disease accumulation evolves at the population level as cohorts age. Understanding these dynamics is essential for planning care in ageing populations. This study aimed to characterize population-level trajectories of disease accumulation between ages 70 and 100 and quantify the contributions of survivorship, proximity to death and disease composition.

Using nationwide register data from Sweden, we examined disease accumulation between ages 70 and 100 amongst all individuals born in 1920-1922 who survived to age 70, assessing the roles of survivorship, proximity to death, disease composition.

Multimorbidity increased substantially between ages 70 and 100. Disease accumulation accelerated up to age 80, stabilized between ages 80 and 90, and decelerated thereafter, although both disease burden and complexity continued to increase into extreme old age. By age 90, the mean number of diseases was 3.9 in men and 3.5 in women, and more than half of individuals aged ≥95 had five or more diseases. Most accumulation occurred during non-terminal years of life. Cardiovascular disease was the dominant contributor throughout, whilst musculoskeletal and neurosensorial diseases became increasingly important at advanced ages.

Multimorbidity continues to accumulate and diversify well into extreme old age. Although the pace of accumulation slows after age 90, disease burden and complexity continue to increase. These findings highlight that multimorbidity in late life is primarily driven during non-terminal years and increasingly characterized by complex, cross-system disease patterns, with important implications for geriatric care.