In this longitudinal study, data gathered from the KHPS included 3983 Korea elderly people with potential to receive care for depression and/or anxiety symptoms within an 8 year follow up period. To examine the tendency to suffer these two mental health problems simultaneously, which may change over time, GBDTM was applied to identify trajectories of co-occurring depression and anxiety. Among the four groups recognized for their differing probabilities of depression, a large majority showed no depression and were generally unlikely to experience anxiety concomitantly. However, slightly more than 10% did experience depression during the follow up period, with most of these individuals showing a gradual increase in depression probability. Among individuals following this trajectory, 20% also experienced a moderately increase in anxiety risk over time. Regarding anxiety, also recognized to follow four trajectories, the vast majority of respondents did not experience this condition and were also free of depression, although 5% again saw a slow increase in anxiety propensity over time. This was accompanied by an increasing depression tendency in just under half the cases. In general, female sex, not taking income generating activity, and membership in a trajectory suggesting risk for the alternate condition independently predicted a more vulnerable risk trajectory than “low-flat” for both depression and anxiety.
Our presentation of four trajectory groups aligns with other depression trajectories studies focused on the elderly [30, 31, 32, 33, 35, 51]. However, researchers have only infrequently identified older adult anxiety trajectories: a six-year cohort study of depression and anxiety trajectories in older adults by Holmes et al, which only involved two trajectories (a stable anxiety trajectory (82%) and an elevated anxiety trajectory (18%)) [36] and two studies recognizing three anxiety trajectory groups in older adults [37, 38]. In our study, four anxiety trajectory groups were identified. The anxiety trajectory shapes of these latter studies are different than our study as one focused on depressed older patients and the other on musculoskeletal pain patients.
Among our four depression trajectory groups, no decreasing trajectory was found for depression. The high-stable depression group is thought to have less likelihood of recovery in our older population as this life stage is more likely to include reduced life-satisfaction, low income and living quality, and poor health conditions [52, 53, 54]. The low-to-high depression group had an intense increase in depression occurrence from 2009 to 2013 but only contained 31 older adults. The markedly increased probability may have been precipitated by sudden serious events, such as the loss of spouse, physical incapacity, etc. However, among anxiety trajectories, a declining trajectory and a curved shape trajectory showed evidence of a decreasing risk. One explanation may be a reduction in stressful income generating activity as time going by. A possible second explanation for this observe decline is that individuals adapt or cope to their anxious feeling and no longer seek treatment. A third explanation may be that other more pressing medical conditions emerge, eclipsing anxiety management; as such anxiety may still have been present but not identified [55]. The high-curve anxiety trajectory only involves 13 elderly, which is extremely small. However, since the constant high anxiety probability of these subjects, this group is important and cannot be replaced by other trajectory groups.
The association between depression and anxiety were clearly identified from the conditional probabilities of the trajectories and the logistic regression odds ratios. The current finding that the low-to-high and low-to-middle depression group also had risk of being in the low-to-middle anxiety group suggests that older adults with an increasing trend of depression over time also have a greater chance of increasing anxiety, consistent other research [56, 57]. Moreover, low-to-middle depression group members made up a high proportion in the high-curved anxiety group, suggesting that older patients who had severe anxiety may suffer mild depression as well. High-stable depression group members were more likely to have anxiety risk following the high-to-low and, less frequently, the high-curved anxiety trajectory; individuals in this particular overlap have serious mental health conditions and require more attention [2]. The association between depression and anxiety status is also supported by the inverse of these findings; individuals in this study who did not have one of the study conditions tended not to have the other also.
Our evaluation of demographic risk factors shows varying alignment with the literature. In the majority of the depression and anxiety studies, sex does have an association with these conditions, suggesting older females generally at greater risk [58, 59]. Our study’s findings in this regard are consistent with results from other trajectory studies [34, 35, 36, 60]. Nevertheless, other studies have found no sex-specific differences when investigating depression and anxiety [38, 61]. This inconsistency may be related to different economic circumstances, social cultural factors, psychosocial gender roles, or other population differences. In our study, age was a significant univariate influence of depression only, accordant with [36]. Level of education was not a significant predictor of either outcome, which is consistent with some studies [36, 62, 63], but not others [30, 31, 32, 33, 34, 35, 38, 64]. In our study, this lack of relationship may be attributable to the relatively low education level in our respondents overall.
Social factors are also known to influence mental health. Some studies suggest that older adults living along or those having no partner within an isolated social environment have a higher risk of depression or anxiety [60, 65, 66, 67, 68, 69]. However, living alone and marital status did not relate to the outcomes in our study, which is consistent with other studies [32, 33, 34, 37, 70]. The risk behaviors of smoking or excessive drinking might also increasing the risk of depression and anxiety [31, 32, 67, 71]. Nevertheless, this association was not identified in our study or in the work of others [35, 65]. Studies show that homeownership reduces the risk of depression and anxiety [65, 72], but this association did not remain in the multivariate analysis. Income generating activity, however, did predict both depression and anxiety trajectory groups, suggesting that people in later life who were still working and having financial security may have better mental health.
Chronic diseases (heart disease, stroke, diabetes asthma, cancer, arthritis, osteoporosis etc.) are understandably difficult challenges for the elderly that may impact mental health. In studying the relationship between depression, anxiety, and chronic disease, Clarke and Kay reviewed 159 papers between 1995 and 2007, finding that depression was correlated with nearly all chronic diseases [73]. However, anxiety only associated with heart disease, stroke, and diabetes mellitus. Moreover, the depression and anxiety patients who were diagnosed with heart disease, stroke, cancer, and arthritis were difficult to treat [73]. In our study, the older adults who have more than three chronic disease were more likely to develop depression. However, in anxiety trajectory groups, chronic disease was only significant in “high-to-low” group from the univariate analysis, but not in the multivariate analysis. Studies have showed that older adults with physical illness or disability usually positive correlate with depression and anxiety [65, 74, 75, 76], but physical/mental disability were not observed to predict these outcomes in the multivariate analysis.
Several limitations should be considered in this study. First, these outcomes were collected from medical expenses and prescription drug receipts or from medical institutions/pharmacies, potentially leading to inadequate recognition of outcomes in our sample. This is particularly true in the context of other chronic disease conditions [77]. Another limitation was the low prevalence of anxiety across the survey period; this limited predictor evaluation, particularly in the poorly populated trajectories such as the “high-curved” group (n = 13). Third, although the current study employed data from a large elderly subsample of the KHPS dataset, around 35% of the outcome measurements were missing, which might result in bias even though the maximum likelihood estimation and GBDTM were used. Fourth, the variables included in this study did not contain all the potentially important health and psychosocial aspects that may be associated with depression and anxiety, such as stressful life events and social/family support information. And lastly, these observations are all made within the specific cultural context of Korean elderly, which may not be generalizable to all contexts.
In summary, four trajectory groups of both depression and anxiety were generated among the elderly of the KHPS dataset. The large majority of older adults belong to the low-flat trajectory group for both depression (87.0%) and anxiety (92.5%), which suggests that most older adults do not identify depression and anxiety as problems. However, among those who do, an interrelationship between these diagnoses, particularly in those with anxiety, is evident from GBDTM. Female sex, the presence of 3 or more chronic diseases, and involvement in income generating activity are additional predictors for a concerning depression trajectory group, and with the exception of chronic diseases, for the anxiety trajectories as well. The findings of this study can be used to assist health policy decision-makers in identifying individuals at risk for comorbid depression and anxiety and aid in devising supports for older individuals at risk of deteriorating mental health.