The prevalence of stroke and depression increased with increasing age. Stroke was more common among men than women, and more common in Sweden than in Finland. Stroke was associated with depression overall, among men, women, Swedish people and Finnish people. In people with stroke, depression was independently associated with dependence in personal ADL and a life crisis the preceding year. In people without stroke, depression was not associated with dependence in personal ADL, but was independently associated with a larger number of external factors.
The age-specific prevalence of stroke appeared to increase with increasing age. Our estimated stroke prevalence figures are in line with previously published self-reported results [12, 13]. The higher stroke prevalence found for men than for women is in line with previous studies [12, 13]. The age-specific prevalence of depression also appeared to increase with increasing age. Our estimated depression prevalence figures are in line with previous pooled estimates of the prevalence of all depressive disorders, although this estimate varies greatly depending on how depression is defined [15].
Being depressed was 1.77 times more common among people with a previous stroke in the full sample, which remained true in all age, sex and country sub-groups, but did not reach significance in 80-year-olds. It is possible that lower power due to fewer participants and a lower participation rate among the 80-year-olds contributed to the non-significant result, but on the other hand both the prevalence of stroke and depression were higher among 80-year-olds than other age groups. It also has to be considered that the factors associated with depression may differ among 80-year-olds from those in elderly people in lower age groups, which could explain our result. Previous studies have found people who have had a stroke to have a higher prevalence of depression than those who have not, both cross-sectionally [1, 2] and after a set interval after suffering a stroke [3, 4].
The association between dependence in personal ADL and depression in people with stroke is in line with previously published results [17]. The association between having a life crisis and depression in people with stroke has not been widely investigated, but social distress was found to be associated with depression in people with stroke in a previous study [27]. Regarding people without stroke, the association between instrumental ADL and depression is in line with disability being associated with depression in a meta-analysis of community dwellers [20], although personal ADL was not independently associated with depression in people without stroke in the present study. A life crisis in the preceding year was also associated with depression in people without stroke, which is in line with previously reported associations between bereavement and depression among community-dwellers [20].
Depression was associated with a larger number of external factors in people without stroke, such as living alone, not having someone to talk to and poor finances, but also diabetes, and having a pain problem. The difference regarding external factors was further supported by interaction analyses of the whole sample, where having had a stroke appeared to weaken the associations between depression and having someone to talk to, poor finances and cancer.
It is possible that among people with stroke, debilitating physical disability denoted by dependence in personal ADL is associated with depression, while external factors are less relevant. One possible explanation is that dependence in personal ADL is likely to be linked to stroke severity, or the extent of brain tissue damage after stroke, which could be a particularly important causative factor for depression in stroke survivors. In support of this, previous studies have shown a clear association between stroke severity and depression [17, 19] and also between ischemic lesions and behavioral symptoms [18]. The underlying biological mechanism may be decreased amine levels due to brain tissue damage [28], and/or possibly inflammation in response to perfusion deficits [29].
However, it is also possible that people with stroke may be particularly sensitive to the social consequences of loss of independence in personal ADL. In the already vulnerable state of having suffered a stroke, dependence in personal ADL may induce feelings of helplessness, reduce self-esteem and ultimately be a pathway to depression. Among people without stroke, external conditions such as their financial situation, living alone and not having someone to confide in, appeared to be more important as factors associated with depression.
The data in the present study is not sufficient to provide grounds for differentiated treatment of depression between people with and without stroke, however the results may indicate that different approaches are needed for treatment and prevention of depression in people with and without stroke. The different characteristics of and risk factors for depression in people with and without stroke should be the subject of further research.
Strengths and limitations
Stroke prevalence was determined by self-reporting using a yes/no question. While some mischaracterized cases are to be expected when using self-reported data, our age-specific stroke prevalence estimates are similar to previous self-reported results [12, 13], and only slightly higher than self-reported cases confirmed by medical documentation [12]. One benefit of using self-reported data is that it made it possible to reach a large number of people representing both urban and rural community-dwellers.
Depression prevalence was determined by a combination of self-reporting with a yes/no question and the use of a depression assessment scale. While depression assessment with a yes/no question has been used successfully in stroke patients [24], depression is known to be underdiagnosed in elderly people [30], meaning that patients who are not diagnosed with depression and do not consider themselves depressed may be overlooked. In addition, our clinical experience suggests that elderly people who are depressed may not be open about it when asked a direct question. We decided to combine self-reporting with a short version of a depression assessment scale to increase the sensitivity for depression. Our estimated depression prevalence was similar to the pooled prevalence of depressive disorders reported in a recent meta-analysis [15].
The investigation of factors associated with depression was limited to items suitable to a postal survey. This precluded the use of any radiological assessment of stroke severity, blood tests or review of medical charts. There was no assessment of stroke severity at the time when the participant had his/her stroke, although long-term functional outcome was assessed with survey items for ADL dependency.
Associations with depression were calculated using log-binomial regression, making it possible to present prevalence ratios. These are easier to interpret than the widely used odds ratios as they are not distorted when analyzing prevalent outcomes. One possible negative aspect of this choice is that it is more difficult to compare with previously published cross-sectional results, as they mainly report associations as odds ratios. Using log-binomial regression is also difficult in most statistical software because of non-convergent optimization procedures in multivariate analyses, and the method used here, involving constrained optimization, is not widely utilized.
In the analytical part of the study, the rural areas were oversampled compared to the urban areas, which might have influenced the results. While survey weights were available, they were used only for the estimates of stroke and depression prevalence that make up the epidemiological part of the study. There is a scientific consensus that descriptive results should be weighted if the survey sampling was unbalanced, but there is no such consensus regarding analytical results [31]. The response rate in the present study was, however, fairly high, including a substantial number of people living both in urban and rural settings.