Our main findings were that when including all CMPs, the risk was, in general, lower in both foreign-born men and women. A lower risk was found among men and women from Asia, while a higher risk was seen among men from Africa. Regarding age patterns, a lower risk of overall CMP was found among men both for those ≤ 54 years of age, and > 54 years. However, for women it was statistically significant only in the younger age group. There were no significant differences between second-generation immigrants compared to native Swedes.
Studies on other diseases in immigrants in Sweden have found a higher risk in many immigrant groups as regards CHD [17], a higher risk of CHF [18], and also of AF among individuals younger than 45 years of age [19], especially in immigrants from some Middle Eastern countries. Furthermore, the risk of type 2 diabetes is higher in immigrants, particularly those from the Middle East region [17].
The lower risk in general for CMPs could be due to the so-called “healthy migrant effect” [20], i.e. that migrating individuals are healthier than their compatriots in the country of origin and that, in this case, individuals with CMPs, to a higher extent, stayed in their home countries. Thus, groups with increased risks could be more important to identify. Hypertrophic CMPs remain the most common inherited heart disease thus making them of special interest to study among immigrants [5], as there are genetic differences between immigrant groups. The risk was higher in men from African and Asian countries, while not among women, perhaps owing to too low case numbers. Dilated CMPs could also be of familial type [3], but we only found an increased risk among men from the Nordic countries, which was driven by an increased risk among men from Finland, perhaps because of differences in risk factors between population groups.
The correlation of CMPs to CHF is of clinical importance. We found a presence of CHF in the first-generation study of 74% among men and 57% among women, with corresponding rates in the second-generation study of 75% and 56%, respectively. In an earlier Swedish immigrant study conducted among individuals aged 45 years and older, the population attributable fraction (PAF) of CMPs for incident CHF was 4.6% for Swedish-born men and 5.7% for foreign-born men, and it was 2.1% for Swedish-born women and 2.4% for foreign-born women [18].
The correlation between CMPs and atrial fibrillation (AF) is also of clinical importance. We found that the rate of AF in the first-generation study among men was 45% and among women the figure was 29%, and that the corresponding rates in the second-generation study were 43% for men and 23% for women. The risk of AF among most first- and second-generation men and women in Sweden has been found to be lower both in those aged 45 years of age and older [21] and in those < 45 years of age [19]. For the younger individuals, i.e. younger than 45 years of age, the PAFs for CMPs regarding incident AF were 2.9% for Swedish-born men and 4.1% for foreign-born men, 3.1% for Swedish-born women and 1.4% for foreign-born women [19].
Not surprisingly, the rates of CHD, hypertension and, to some extent, also of diabetes, COPD, stroke, and non-rheumatic valvular heart disease, were also high, as was the rate of cancer. When only adjusted for age and cancers, the HRs were lower in general, and also for many of the studied groups.
There are certain limitations of this study. We used three large groups, thus not being able to follow the proposal from the ESC [2]. Dilated and hypertrophic CMPs are the most important and prevalent types of CMPs. However, we included them separately and the other types are less prevalent, hence why we merged them into one group for practical and analytical reasons in order to obtain more statistical power. We used diagnoses from the NPR, based on clinical diagnoses from patient records, with no possibility to check diagnostic criteria. However, the diagnoses were obtained from hospital diagnoses, where most patients are seen by specialists at least once. As most individuals with CMPs are examined in hospitals, the coverage of patients with these diagnoses could be expected to be high, even if some cases may be undiagnosed. We did not include ischemic cardiomyopathy (I25.5), and we cannot rule out that some diagnoses are misclassified, but we have no possibility to check for this in the nationwide data that we used. In total, 7191 men and 1943 women were registered with a diagnosis of ischemic cardiomyopathy, and of these 1042 men (14.5%) and 195 women (10.0%) also were registered with another CMP diagnosis. In addition, diagnoses from primary care were not included and most patients with comorbidities, such as hypertension and diabetes, are treated in primary care.
Our study also has several strengths. In Sweden, personal identity numbers allow linkage between different national Swedish registers [22] thus enabling adjustments for many potential confounding factors. Furthermore, many Swedish registers have been shown to have good quality [23, 24].
In conclusion, we found a generally lower risk of CMPs among foreign-born men and women in Sweden, possibly owing to the “healthy migrant effect”, but a higher risk in some specific groups, i.e. for dilated CMP among men from Finland, and for hypertrophic CMP among men from Africa and Asian countries. Hereditary forms of CMPs seem to be of little importance on a population level, as there were no significant findings when we studied second-generation immigrants.
