Results
Overall, 43% of the Ontario population had a history of at least one of the selected chronic conditions in 2003 and 50.8% had a history of at least one condition in 2009 (Table 1). The proportions varied by age groups, from 24.9% in those younger than 18 years to 92.4% among those aged 90 or more in 2009. The prevalence of multimorbidity was 24.3% in 2009 – an increase of forty percent from 17.4% in 2003 (Table 1). The increasing proportion of people with multiple conditions is a concern for all age groups. For example, 2.2% of people 0 to 17 years and 10.6% of those 18 to 44 years had multimorbidity in 2009 representing an increase of 57.1% and 43.2% respectively compared to 2003 (Table 1). The prevalence of 3 or more conditions also nearly doubled between 2003 and 2009. The increase in multimorbidity is remarkable given that the prevalence of having only one condition remained relatively stable at approximately 26%.
The prevalence of each chronic condition for each year is shown in Table 2. With the exception of depression and AMI, the prevalence of each condition in 2009 was higher than in 2003. The most prevalent conditions in the cohort as of 2009 were osteoarthritis and other arthritis, hypertension, asthma, depression, diabetes and cancer. However the prevalence of each condition varied with age. Asthma represented the most common condition among children less than 18 years whereas hypertension or arthritis represented the most common conditions among those aged 45+ years (Table 2).
Figure 1 compares the trends of multimorbidity across ages between 2003 and 2009. Multimorbidity is highly prevalent in the oldest age groups, where more than 80% of the population had at least two conditions in 2009 (i.e. almost the whole area under the curve for ages 75 or more). While multimorbidity is most frequent in the oldest segments of the population, the prevalence is substantial even among individuals aged 40 or less. Within sub-groups of individuals based on birth year, we observed increasing numbers of individuals with multimorbidity from 2003 to 2009. For example, among individuals who were 56 years old in 2003, 48,101 had two or more conditions; six years later 81,046 of these individuals had two or more conditions. Moreover, in 2003, there was no birth cohort (based on year of birth) that included more than 100,000 people with multimorbidity; but by 2009, there were 20 birth cohorts that included more than 100,000 with multimorbidity (area above the line in Figure 1).
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Figure 1.
Distribution of the number of individuals with multimorbidity in Ontario across ages, by number of common chronic conditions and year.
Figure 2 shows the prevalence of multimorbidity within individuals with each chronic condition in 2003 and 2009. With the exception of asthma, which was common in children younger than 18 years and thus most likely to present as a single condition, other conditions were unlikely to occur alone. In 2009, multimorbidity (at least one other condition) within individual chronic conditions ranged from 44% (asthma) to 99% (AMI). In 2009, the proportions of individuals with 4 or more other conditions, ranged from 7.2% (asthma) to 60.8% (CHF).
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Figure 2.
Distribution of the proportion of individuals with multimorbidity in Ontario, within common chronic conditions and by year.
Table 3 displays the top five most common diseases pairs, triads, quartets, and quintets in 2009. We observed many different combinations of co-occurring conditions within each level of multimorbidity, and as the number of conditions increased, the number of observed clusters increased exponentially. Among those with only two conditions, five possible combinations accounted for 50% of the population with that level of multimorbidity. However, 243 unique combinations of quintets of conditions were required to capture the first fifty percent of individuals with 5 or more conditions. The number of clusters required to include eighty percent of the population increased from 14 (among individuals with two conditions) to 2744 clusters of conditions (among individuals with 5 or more conditions). The five most prevalent clusters accounted for only 6% to 52% of each level of multimorbidity. Even among people with only 2 or 3 conditions, we observed a total of 113 and 443 different clusters represented among the study population (data not shown). Mathematically, with the 16 conditions included in this study, there are 120 and 560 possible combinations of 2 and 3 conditions.