Geographical variation in treated psychotic and other mental disorders in Finland by region and urbanicity

The prevalence of psychotic and other mental disorders varies globally and locally [1‐4], with urban–rural differences being a particularly important factor in Northern Europe [5‐9]. The underlying mechanisms for these variations are not well understood and are thought to be influenced by a combination of neighbourhood and individual-level social-environmental factors, including pollution, lack of green space, social stress or selective migration, among other things [5]. Some combined analyses have shown gene-environment synergism in the risk profiles [10‐14].

In Finland, there is a well-documented pattern of higher prevalence of schizophrenia and other psychotic disorders in the east and of mood and anxiety disorders in the south [15‐20]. In schizophrenia, regional differences have been more significant than urban–rural variations, and this geographical east–west pattern in schizophrenia prevalence coincides with schizophrenia polygenic risk scores, leading to the hypothesis that population genetics may play a role (Supplementary Fig. S1a) [16, 17, 21, 22]. However, social determinants of mental health, such as the proportion of low-income earners (Supplementary Fig. S1b), level of education, unemployment, migration, or household structure also vary across the country with less favourable compositions often seen in the eastern parts of the country. Urban areas, on the other hand, are more common in southern and western regions (Supplementary Fig. S1c). It is not known to what extent regional and urban–rural variations interact, and to what extent the geographical variations are confounded by socioeconomic factors.

We aimed to evaluate regional and urban–rural variation in psychotic and all mental disorders, their interaction, and the impact of socioeconomic adjustments on these geographical differences. To facilitate comparisons of geographical differences in prevalence of schizophrenia with different adjustments and schizophrenia polygenic risk scores that have previously been reported, we grouped the administrative regions of Finland into three aggregate regions and aimed to present detailed maps of the geographical prevalence distributions (Fig. 1). We hypothesized that much of the variability in prevalence of mental disorders would be explained by demographic and socioeconomic factors.

We conducted a population-based register study including all individuals living in Finland from 2011 to 2017. Using individual-level population and health care registers, we calculated the prevalence of people with a history of mental health-related contact with primary care or psychiatric secondary inpatient or outpatient care on the last day of each of the study years. In addition, all individuals living in Finland between 1996 and 2017 were followed up in the registers to identify the incidence of the first psychiatric inpatient admissions. These time limits were based on the coverage of the national health care registers.

The Research Ethics Committee of the Finnish Institute for Health and Welfare approved the study protocol (decision #10/2016§751). Data were linked with permission from Statistics Finland (TK-53–1696-16) and the Finnish Institute of Health and Welfare. Informed consent is not required for register-based studies in Finland.

During the years 2011 to 2017, a total of 5,898,180 individuals contributed to the study population. Altogether, 1,197,690 individuals of the total of 5,512,745 at the end of 2017 had a history of some medical contact in primary or secondary care mental health services. This resulted in a crude prevalence rate of 21.73% (24.07% in women and 19.32% in men). Prevalences stratified by the covariates are reported in the Supplementary Table.

In this nationwide register-based study of over 5 million Finnish persons, we found that the prevalence of all mental disorders and psychotic disorders treated in both primary or secondary care was higher in the eastern and northern regions compared to coastal regions. After adjusting for socioeconomic factors, however, this geographical difference was no longer evident. By contrast, the urban–rural differences, as measured using a detailed seven-level classification of current residency, persisted after the adjustments and were consistent with previous findings from other Nordic countries. Urban effect was evident across the country and diagnostic categories, although regional differences in some diagnostic subgroups, such as schizophrenia and bipolar disorders, were observed. Taken together, our results demonstrate the significant impact of social determinants on the mental health of the population and have important national implications.

To the best of our knowledge, this is the first comprehensive study demonstrating the associations between the within-country distribution of socioeconomic and demographic factors and the prevalence of mental disorders treated with either primary or secondary care. Epidemiological studies in Finland have investigated regional and urban–rural variations in mental disorders since the 1930s but have usually included only inpatient register data [15‐20, 29]. Including outpatient and primary care data can be seen as a main strength of the current study, as including this data substantially changed the prevalence ratios in eastern and norther parts of the country, and there are some variations in the overall inpatient care across the regions [30]. The east–west differences have been consistently observed, but one recent study found significant regional variation in mental disorder disability pensions that did not follow the traditional east–west health differences [31]. Thus, within-country geographical differences in mental health are sensitive to a variety of social determinants and draw a more complex picture than reported in previous studies.

Previous findings on the association between urbanicity and mental disorders in Finland have been mixed, with the earliest studies showing an association between living in cities and schizophrenia [29], but more recent studies suggestive of urban effects but yielding inconsistent results [15‐17]. Current results align with previous studies in Northern Europe, demonstrating an association between variety of psychotic disorders and urbanicity [5, 7, 32]. Finland no longer appears to be an exception in this respect. Structural changes in demography, employment and services have affected particularly eastern rural parts of the country in recent decades and probably affect the temporal differences in the link between urbanicity and psychotic disorders in Finland [17, 33].

Household income was a particularly strong cofactor in the models. This is not surprising, as income inequality and individual level low income and mental disorders have been strongly linked with complex bi-directional pathways [34‐38]. In the current study, we did not explore the causal pathways behind the mental disorders and income distribution. Nevertheless, income was a relevant cofactor, as it is unlikely that the within-country distribution of income was determined by the regional prevalence of mental disorders.

Contrary to regional differences, urban–rural variation did not disappear after socioeconomic adjustments. Urban environments in a sparsely populated country such as Finland may vary greatly within the country in terms of potential urban risk attributes such as nature spaces, migration, social stress, or demographical and socioeconomic composition. Our analysis of the urbanicity-region interaction with socioeconomic adjustments showed that urbanicity is a relevant factor for mental health in all regions of the country, regardless of the size of the regional urban centre, from Kajaani with a population of 36,000 to Helsinki with a population of 665,000. We evaluated regional differences and urbanicity based on current residency, while controlling for living in the birth region. This approach enabled accounting for within-country migration. However, we did not have data on individual histories of urban residency or changes in geographical distribution of urbanicity. Selective migration can affect regional composition and socioeconomic contexts, and also affects the associations between urbanicity and psychotic disorders [5, 10, 39, 40]. In Finland, however, it has been suggested that individuals with mental disorders are not particularly likely to move to the most urban centers [41], and accessibility of Finnish primary health care is mostly at good level, although in a recent study, travel time in rural areas negatively associated with primary care mental health service use [42, 43].

The relatively high prevalence of bipolar disorder with psychotic features in southern urban areas and the comparatively high prevalence of schizophrenia spectrum diagnoses in eastern and northern areas emphasize the importance of considering different register-based diagnoses side by side. Although the Finnish registers show good consistence [44], a tendency towards a narrow definition of schizophrenia in clinical practice in Finland has been recognized [45]. Whether there are differences in diagnostic practices in primary or secondary care mental health services across the country has not been evaluated recently. In Finland, there is a relatively high number of specialists in psychiatry and general practitioners are trained in psychiatry as well [46]. With the observed differences in certain diagnostic categories in mind, future assessment of the real-world diagnostic consistency and reliability might be useful in terms of both scientific and clinical accuracy.

The study of population genetics in Finland has attracted a great deal of interest, and there is a well-documented north–south and east–west genetic differentiation within the population [22, 47, 48]. Although the use of polygenic risk scores for explaining geographic differences in phenotypes is not currently recommended due to methodological limitations, the striking similarity between schizophrenia prevalence and polygenic scores has been suggested as an example of the potential of polygenic risk scores to explain geographic health differences [21]. Our results showed that after adjusting for socioeconomic factors, the prevalence of all psychotic disorders did not display statistically significant east–west differences, and did not align with the geographical gradient of schizophrenia polygenic scores. A diagnosis of schizophrenia was slightly more prevalent in eastern parts of the country, but did not follow a gradient that was comparable to that of schizophrenia polygenic scores. Mental disorders are highly polygenic and pleiotrophic, and most of their genetic common variant architecture has not been identified [49]. Schizophrenia polygenic risk scores are associated with a variety of traits, adding complexity to the concept [50‐54]. In the present study, however, genetics were not evaluated. Thus, accounting for neighbourhood contextual factors and socioeconomic composition and individual level social determinants, together with genetic information, may be beneficial in future studies of geographical differences in mental health in Finland.

Urbanicity and socioeconomic position are important determinants of geographical variations in population mental health. In this study, the previously well documented east–west gradient in psychotic disorders that coincides with the geographical distribution of schizophrenia polygenic risk scores, was no longer observed after detailed adjustments. Our current findings align with previous studies in Northern Europe, demonstrating a solid association between psychotic disorders and urbanicity also in Finland, which has previously been uncertain. At the national level, acknowledging these geographical patterns and their correlations with societal factors may enhance understanding of population health. While the utilization of primary care registers represents a noteworthy strength for Finnish register-based epidemiology, their diagnostic accuracy regarding mental disorders remains to be evaluated. Further study is needed to provide better understanding of the geographical patterns of mental health.