Nationwide trends in the prevalence and incidence of depressive disorders and their correlates among adults with disabilities in Korea from 2006 to 2017

Introduction

Depression has devastating impacts not only on individuals and families, but also on economies and societies (OECD, 2018). Depression, in particular, is a major risk factor for mortality due to suicide (Bolton et al., 2015). Ample evidence indicates that people with disabilities, whether it’s physical or mental, are more likely to suffer from depression or other psychological distress (Meins, 1993; Menolascino, 1990; Nezu et al., 1995).

One of the theories explaining the co-occurrence of depression and disabilities is the cognitive model, which contends that stressors generate strain or potential harm that exceeds the coping resources of a person (Lazarus & Folkman, 1984). People with physical disabilities may experience acute stressors such as getting the initial diagnosis, undergoing medical procedures, and/or disease progression, while chronic stressors include the threat of death, ongoing self-management, discrimination and loss of independence, among other things (Heijmans et al., 2004; Moss-Morris, 2013). People with mental disabilities may experience from social comparison, a cognitive process through which we assess ourselves by comparing to others in terms of achievement/rank, social attractiveness and group belonging (Dagnan & Sandhu, 1999). Nezu et al. (1995) also suggested that adults with intellectual disabilities experience depression more frequently because they are more stressed out by surrounding social relationship (Nezu et al., 1995). Additionally, socioeconomic status (SES) and social capital, which have been identified as protective factors in mitigating mental health problems, have been shown to be limited among those with both physical (Kutner, 1987) and intellectual disabilities (Lippold & Burns, 2009). It was demonstrated that reduced access to resources caused by a disability are more closely associated with depressive symptoms than the presence of health conditions or impairment per se (Emerson et al., 2012).

Evidence on disabilities and depression have focused on specific types of disability, particularly intellectual (Hermans et al., 2013; Turygin et al., 2013) and physical disabilities (Jensen et al., 2014; Tariq et al., 2019; Turner et al., 2006), or specific age groups such as the elderly, with only a small sample size (Hermans et al., 2013; Renaud & Bédard, 2013; Tariq et al., 2019). No studies have comprehensively evaluated the distribution of depression overall as well as by different types and severity of disability. In addition, no nationwide studies of longitudinal trends in depressive disorders among the disabled have been conducted. Thus, our study was performed with two aims to fill these gaps. First, we investigated longitudinal trends in the prevalence and incidence of depressive disorders in people with and without disabilities, as well as by disability type and severity, between 2006 and 2017. Although the concept of disability is not only limited to functional losses and clinical impairment but also encompasses negative aspects of the interaction of individual health status and contextual factors (gender, age, social perception, etc) (Ustün et al., 2003; WHO, 2006), the definition of disability in our study is based on medical criteria due to the nature of the data source. We excluded mental disabilities (intellectual/psychiatric disabilities) from the main analyses for several reasons. First, the number of registered persons with mild intellectual or psychiatric disabilities was too small (ranging between 87 and 115 depending on the year with decreasing trend) to reliably estimate the prevalence and incidence of depressive disorder. Incidence was even zero in many years between 2006 and 2017 resulting in unstable fluctuation in incidence rate. Separately, multiple studies consistently support the view that diagnostic overshadowing affects the accurate diagnosis of mental illness in persons with intellectual disabilities. Diagnostic overshadowing is a common clinical bias, which refers to linking the mental illness to the intellectual disability rather than diagnosing it as an illness (Jopp & Keys, 2001; Reiss et al., 1982; Seay, 1991; Spengler et al., 1990; White et al., 1995). Additionally, because psychiatric disabilities are defined as persistent psychological or psychiatric disorders, including depression, they overlap with the outcome. All of these issues renders make the estimation unreliable. However, we conducted a sensitivity analysis where we included intellectual/psychiatric disabilities in the model. Second, because there is evidence that poorer health (including mental health) among people with disabilities may be due at least partly to socioeconomic disadvantages, we attempted to identify the association between each type and each severity level of disability and the incidence and prevalence of depressive disorders after adjusting for SES and other comorbidities in order to determine the true effect of disability status on depressive disorders.

Methods

Results

Long-term trends in the age-standardised prevalence and incidence of depressive disorders between 2006 and 2017

The age-standardised prevalence of depressive disorders consistently increased for both the disabled and the non-disabled, and both men and women across all study years (Figure 1a and b) (Supplemental Table S3). The prevalence was more than two times higher among the disabled than among the non-disabled in both men and women during all study years. Not only was the absolute prevalence of depressive disorder higher among the disabled, but the rate of increase was also higher among the disabled than the non-disabled both for men and women (average annual percent change [AAPC]: 1.135% and 1.128% for disabled and non-disabled men and 1.119% and 1.113% for disabled and non-disabled women, respectively). Although depressive disorders were more prevalent in women than in men, both for the disabled and non-disabled, the gap in prevalence between the disabled and non-disabled was greater in men than in women (Figure 1a and b) (Supplemental Table S3).

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Figure 1.

Prevalence of depressive disorders (a) by disability status, in men, (b) by disability status in women, (c) by disability severity in men, (d) by disability severity in women (e) by disability type in men, and (f) by disability type in women.

The age-standardised incidence of depressive disorders was also higher among the disabled than among the non-disabled across all years and in both sexes. The long-term time trend, however, was not the same as the prevalence. The incidence in disabled men slightly increased between 2006 and 2017. The incidence among disabled women, in contrast, declined during the same period (Figure 2a and b) (Supplemental Table S4).

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Figure 2.

Incidence of depressive disorders (a) by disability status in men, (b) by disability status in women, (c) by disability severity in men, (d) by disability severity in women, (e) by disability type in men, and (f) by disability type in women.

Risk factors for the prevalence and incidence of depressive disorders among men and women

The sex-stratified unadjusted regression analyses of prevalence revealed that individuals with disabilities of all grades and types were more likely to have depressive disorders than those without disabilities (Table 1). All categories of disability severity and type continued to show strongly significant associations after adjusting for age, income, insurance coverage scheme, residence, and comorbidities, although the associations were weakened (Table 1). In model 3 of the adjusted analysis for prevalence, the odds of having depressive disorders were not necessarily gradual according to disability severity (model 2 in Table 1). People with brain injuries were the most likely to have depressive disorders (OR = 3.71 and 2.89, 95% CI = [3.67, 3.76] and [2.89, 2.97] in men and women, respectively) while people with disabilities in major internal organs were the least likely to have depressive disorders both in men and women (OR = 1.40 and 1.24, 95% CI = [1.38, 1.42] and [1.21, 1.26] in men and women, respectively) (model 4 in Table 1). Overall, men with severe brain injuries were most likely to have depressive disorders (OR = 3.86, 95% CI = [3.80, 3.92]) (model 5 in Table 1).

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Table 1.

The results from unadjusted and adjusted regression analyses for depressive symptom prevalence.

		Male		Female
		Unadjusted	Adjusted	Unadjusted	Adjusted
Disability
M1	Yes (vs. No)	3.71 [3.70, 3.73]	1.83 [1.82, 1.84]	3.88 [3.87, 3.89]	1.79 [1.78, 1.80]
By severity
M2	Severe (vs. No)	4.96 [4.94, 4.99]	2.04 [2.02, 2.05]	4.09 [4.07, 4.11]	1.75 [1.73, 1.76]
	Mild (vs. No)	3.23 [3.22, 3.24]	1.74 [1.73, 1.75]	3.80 [3.79, 3.82]	1.80 [1.79, 1.81]
By severity grade
M3	Grade 1 (vs. No)	5.79 [5.74, 5.85]	2.38 [2.34, 2.41]	4.30 [4.25, 4.34]	1.91 [1.87, 1.94]
	Grade 2 (vs. No)	5.18 [5.14, 5.22]	1.874 [1.85, 1.90]	3.98 [3.95, 4.02]	1.56 [1.54, 1.59]
	Grade 3 (vs. No)	4.40 [4.37, 4.43]	2.01 [1.99, 2.04]	4.06 [4.02, 4.09]	1.84 [1.82, 1.86]
	Grade 4 (vs. No)	3.85 [3.82, 3.88]	1.84 [1.82, 1.86]	4.11 [4.08, 4.13]	1.80 [1.78, 1.82]
	Grade 5 (vs. No)	3.59 [3.57, 3.61]	1.83 [1.81, 1.84]	3.99 [3.97, 4.02]	1.84 [1.83, 1.86]
	Grade 6 (vs. No)	2.72 [2.71, 2.74]	1.63 [1.62, 1.65]	3.39 [3.37, 3.41]	1.77 [1.75, 1.78]
By disability type
M4	Physical (vs. No)	3.00 [2.99, 3.01]	1.68 [1.67, 1.69]	3.81 [3.79, 3.82]	1.83 [1.82, 1.84]
	Brain injury (vs. No)	10.23 [10.15, 10.39]	3.71 [3.67, 3.76]	7.67 [7.60, 7.74]	2.93 [2.89, 2.97]
	Communication (vs. No)	3.35 [3.33, 3.37]	1.69 [1.68, 1.70]	3.04 [3.03, 3.06]	1.50 [1.48, 1.51]
	Major internal organ (vs. No)	4.75 [4.70, 4.80]	1.40 [1.38, 1.42]	3.63 [3.58, 3.67]	1.24 [1.21, 1.26]
By disability type and severe
M5	Physical
	Severe (vs. No)	3.53 [3.50, 3.56]	1.72 [1.70, 1.74]	3.25 [3.22, 3.28]	1.58 [1.56, 1.61]
	Mild (vs. No)	2.85 [2.84, 2.86]	1.67 [1.66, 1.68]	3.93 [3.91, 3.95]	1.880 [1.87, 1.89]
	Brain injury
	Severe (vs. No)	10.83 [10.73, 10.94]	3.86 [3.80, 3.92]	7.69 [7.61, 7.77]	2.88 [2.83, 2.92]
	Mild (vs. No)	9.38 [9.27, 9.49]	3.51 [3.45, 3.58]	7.62 [7.50, 7.74]	3.05 [2.98, 3.13]
	Communication
	Severe (vs. No)	3.72 [3.69, 3.77]	1.72 [1.69, 1.75]	2.79 [2.76, 2.82]	1.39 [1.37, 1.41]
	Mild (vs. No)	3.23 [3.21, 3.25]	1.68 [1.66, 1.70]	3.16 [3.13, 3.18]	1.54 [1.53, 1.56]
	Major internal organ
	Severe (vs. No)	5.84 [5.76, 5.91]	1.53 [1.50, 1.56]	4.32 [4.26, 4.39]	1.29 [1.26, 1.32]
	Mild (vs. No)	3.16 [3.10, 3.22]	1.16 [1.13, 1.19]	2.58 [2.53, 2.64]	1.13 [1.10, 1.17]

The results for incidence revealed a different pattern. Although the odds of developing depressive disorder were higher in all types of disability compared to non-disability, except for severe brain injuries in women (OR = 0.90, 95% CI = [0.86, 0.95]), statistical significance was lost, or the association was reversed in many of those categories after adjusting for covariates. Men with grade 1 and 2 disabilities and women with grade 1, 2, and 3 disabilities were less likely to develop depressive disorders than men and women without disabilities in the adjusted associations (model 3 in Table 2). People with brain injuries and major internal organ disabilities showed lower risks of developing depressive disorders than the non-disabled in both men and women (model 4 in Table 2). Among all combined categories of disability severity and type, compared to individuals without disabilities, the incidence was higher only in men with mild and severe physical disabilities (OR = 1.21 and 1.06, 95% CI = [1.18, 1.24] and [1.06, 1.11], respectively), men with mild and severe communication disabilities (OR = 1.08 and 1.23, 95% CI = [1.02, 1.16] and [1.18, 1.27], respectively), and women with mild physical disabilities and mild communication disabilities (OR = 1.08 and 1.13, 95% CI = [1.06, 1.11] and [1.09, 1.16], respectively) (model 5 in Table 2).

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Table 2.

The results from unadjusted and adjusted regression analyses for depressive symptom incidence.

		Male		Female
		Unadjusted	Adjusted	Unadjusted	Adjusted
Disability
M1	Yes (vs. No)	1.61 [1.59, 1.62]	1.10 [1.08, 1.12]	1.39 [1.38, 1.41]	0.99 [0.97, 1.01]
By severity
M2	Severe (vs. No)	1.56 [1.53, 1.59]	0.93 [0.90, 0.96]	1.18 [1.15, 1.20]	0.81 [0.78, 0.84]
	Mild (vs. No)	1.63 [1.60, 1.65]	1.18 [1.16, 1.20]	1.48 [1.46, 1.50]	1.07 [1.05, 1.09]
By severity grade
M3	Grade 1 (vs. No)	1.35 [1.27, 1.41]	0.78 [0.72, 0.83]	0.99 [0.94, 1.03]	0.68 [0.63, 0.73]
	Grade 2 (vs. No)	1.59 [1.53, 1.65]	0.87 [0.82, 0.91]	1.18 [1.13, 1.22]	0.78 [0.74, 0.82]
	Grade 3 (vs. No)	1.65 [1.60, 1.71]	1.07 [1.02, 1.12]	1.30 [1.25, 1.35]	0.93 [0.88, 0.97]
	Grade 4 (vs. No)	1.69 [1.64, 1.74]	1.12 [1.08, 1.17]	1.46 [1.43, 1.50]	1.04 [1.00, 1.08]
	Grade 5 (vs. No)	1.70 [1.66, 1.74]	1.22 [1.18, 1.26]	1.48 [1.45, 1.51]	1.04 [1.01, 1.07]
	Grade 6 (vs. No)	1.54 [1.51, 1.58]	1.18 [1.15, 1.22]	1.48 [1.45, 1.53]	1.12 [1.09, 1.16]
By disability type
M4	Physical (vs. No)	1.56 [1.54, 1.59]	1.17 [1.14, 1.19]	1.46 [1.44, 1.48]	1.06 [1.03, 1.07]
	Brain injury (vs. No)	1.53 [1.48, 1.59]	0.81 [0.77, 0.86]	1.03 [0.99, 1.07]	0.64 [0.61, 0.68]
	Communication (vs. No)	1.69 [1.66, 1.73]	1.18 [1.14, 1.22]	1.42 [1.39, 1.45]	1.08 [1.04, 1.11]
	Major internal organ (vs. No)	1.80 [1.73, 1.88]	0.85 [0.80, 0.90]	1.26 [1.20, 1.33]	0.75 [0.79, 0.81]
By disability severity and type
M5	Physical
	Severe (vs. No)	1.55 [1.50, 1.60]	1.06 [1.01, 1.11]	1.28 [1.23, 1.33]	0.94 [0.89, 0.99]
	Mild (vs. No)	1.57 [1.54, 1.59]	1.21 [1.18, 1.24]	1.50 [1.48, 1.52]	1.08 [1.06, 1.11]
	Brain injury
	Severe (vs. No)	1.32 [1.25, 1.38]	0.67 [0.63, 0.73]	0.90 [0.86, 0.95]	0.57 [0.53, 0.61]
	Mild (vs. No)	1.86 [1.77, 1.96]	1.05 [0.97, 1.12]	1.31 [1.23, 1.40]	0.81 [0.74, 0.89]
	Communication
	Severe (vs. No)	1.60 [1.53, 1.67]	1.08 [1.02, 1.16]	1.25 [1.19, 1.30]	0.96 [0.90, 1.02]
	Mild (vs. No)	1.73 [1.69, 1.78]	1.23 [1.18, 1.27]	1.49 [1.45, 1.53]	1.13 [1.09, 1.17]
	Major internal organ
	Severe (vs. No)	1.93 [1.84, 2.03]	0.86 [0.80, 0.93]	1.33 [1.25, 1.42]	0.77 [0.71, 0.84]
	Mild (vs. No)	1.57 [1.46, 1.69]	0.84 [0.76, 0.94]	1.14 [1.04, 1.25]	0.70 [0.62, 0.80]

When intellectual or psychiatric disabilities were included in the analyses, the prevalence of depressive disorder was the highest and the second highest of all types of disabilities in men and women, respectively (Supplemental Table S6). On the other hand, incidence of the depressive disorder was lower in intellectual or psychiatric disabilities than in physical and communication disabilities (Supplemental Table S7).

The odds of depressive disorder incidence were lower, and the odds of prevalence were higher in the 30 to 49 and over-50-year age groups than in the age groups of 20 to 29 years. The poor, Medical Aid enrollees, rural dwellers, and those with more comorbidities had higher odds of incidence and prevalence than their counterparts (Supplemental Table S8).

Discussion

This is the first study to perform a comprehensive investigation of the prevalence, incidence, and risk level of depressive disorders according to the severity and type of disabilities. Other strengths of our study are the fact that it investigated long-term trends spanning 12 years, from 2006 to 2017, and that it is based on data from the entire South Korean population.

We observed several notable findings. First, as predicted, the prevalence and incidence of depressive disorders were higher among the disabled than among the non-disabled. The incidence was 1.45 and 1.34 times higher among the disabled compared to among the non-disabled in 2017 in men and women, respectively. The gap in the prevalence between the disabled and the non-disabled was even bigger (2.2 and 1.95 times in men and women, respectively). This suggests that when people with disabilities develop a depressive disorder, they are less quickly cured and are affected by the disease for a longer period of time than non-disabled people. Several studies can explain this. First, SES is strong correlate with depression. Melchior et al. (2013) have demonstrated that the long-term depression trajectory followed a socioeconomic gradient (Melchior et al., 2013). People with disabilities, meanwhile, are more likely to have a lower SES which sustains depression. Furthermore, social capital, which is not only protective against developing depressive disorders but is also beneficial for recovery from depression, is also low in people with disabilities (Webber et al., 2014). According to the study by Mithen et al. (2015), all types of social capital, including financial and emotional support, as well as informal (e.g. social contact) and formal networks (e.g. group membership), were lower among the disabled than the non-disabled (Mithen et al., 2015). The government’s policies integrating the disabled into society and supporting their self-reliance would be critical.

There has been significant progress in Korea’s welfare system supporting people with disabilities over the past few decades. In 1976, Korea adopted a mandatory quota system for the employment of people with disability, like Japan, Germany, or France, and have since expanded the scope of benefits. As a result, all disabled people including those with intellectual disabilities were subject to this system, unlike Japan where it only targeted those who fit specific criteria (Shim et al., 2017). However, Korea still ranks at the bottom in almost all indicators related to the welfare budget, income, employment, and social service policy for the disabled among OECD countries. The share of the welfare budget for the disabled of the total welfare budget was the lowest among the OECD countries (0.2% compared to the OECD average of 2.1%). The SES gap between the disabled and the non-disabled was bigger in Korea than in other OECD countries: Income for the disabled was approximately 20% lower compared to the non-disabled in Korea (vs. 15% OECD average). Consequently, the gap in the poverty rate was also higher in Korea than the OECD average (35%vs. 22%). The Budget for homecare services for the disabled, which is one of the important social support, was one-third of the OECD average (S. Y. Kim et al., 2011). The government’s input in policies for the disabled is necessary at least to narrow the gap with the OECD average.

Meanwhile, a positive sign was observed in the trend of the incidence of depressive disorders. Although the incidence of depressive disorders in men with disabilities increased during the study period, the rate of increase was lower than in people without disabilities (AAPC: 1.011% vs. 1.029%). Among women, the incidence of depressive disorders even decreased in people with disabilities, while it increased in people without disabilities. This finding could be one manifestation of improved living conditions for the disabled in Korea. Specifically, there have been efforts to protect people with disabilities from disadvantages since Korea enacted the Anti-Discrimination and Remedies for Persons with Disabilities Act in 2017. Although these steps are still far from enough, the percentage of disabled people who were satisfied with their general life and with culture/leisure activities increased from 53.2% to 58.6% and from 39.5% to 49.3%, respectively according to the National Survey for the Disabled. The proportion of disabled people who answered that there is no discrimination against the disabled also increased from 27.4% in 2014 to 36.5% in 2017 (KIHASA, 2021).

Second, regression analyses revealed that adjusting for SES and comorbidities reduced the odds ratios significantly, particularly for incidence. In adjusted regression, the likelihood of developing depressive disorders among male people with disabilities was only 1.01 times higher than that of non-disabled people, and among women with disabilities, it was not significantly different from that of non-disabled people, indicating that the depressive disorders among the disabled are most likely due to collateral difficulties of poor economic status or other comorbidities caused by disabilities.

Third, unadjusted and adjusted regression showed that depressive disorder does not necessarily occur at a proportionally higher rate depending on the severity of disabilities. Rather, a more severe disability is associated with a lower odds ratio of depressive disorder incidence. The likelihood of depressive disorders was lower to a statistically significant degree even in the groups with disabilities below grade 4 than in those without disabilities, which is an unexpected finding. This finding is noteworthy, but it should be interpreted with caution. One possible explanation for this could be less exposure to stressful working environments. People with disabilities often rely on government subsidies for living because it is difficult for them to engage in economic activities. As a result, they are less likely to face competition and do not need to strive to avoid inferiority and missing out in the workplace, which is an important factor triggering depression and anxiety (Gilbert et al., 2009). However, we should not rule out another possibility. Since our data on depressive disorder include only people who were diagnosed at health facilities, those who had depressive disorders, but did not or could not access health care, were not considered. The proportion of undetected cases is expected to be higher in groups with more severe disabilities.

Fourth, men with mild to severe physical and communication disabilities and women with mild physical and communication disabilities were more likely to develop depressive disorder than their non-disabled counterparts. High-level evidence has found a link between impairment in communication and depression, many of which discussed the higher incidence of depression in people with impaired communication ability should be discussed in light of alienation, social isolation, and loneliness (Brunes & Heir, 2020; Choi et al., 2018; Chou & Chi, 2004; Hogan et al., 2009; S. Y. Kim et al., 2011; Li et al., 2014; C. L. Nollett et al., 2016; Parravano et al., 2021). While social interaction is an integral part of life, communication difficulties can impede social activities and integration. Those who are ‘socially isolated’ due to communication disability lack social contact and a sense of belonging, which leads to depression (Dalton et al., 2003; Jiang et al., 2022). A higher incidence among people with mild communication disabilities than among those with severe disabilities was found in a previous study, explaining that those with more severe hearing impairment are more likely to be identified, and as a result, to obtain earlier interventions such as hearing aids and alternative listening devices than those with mild impairment (Li et al., 2014) (Meyer & Hickson, 2012). Prior studies on visual impairment found no relationship between severity and depression (Garin et al., 2014; C. Nollett et al., 2019; Shmuely-Dulitzki et al., 1995). This evidence collectively points out that equal or more attention should be paid to people with mild communication disabilities, not only those with severe disabilities.

Depression in people with physical disabilities has been shown to be directly associated with the physical disabilities themselves, other secondary health conditions, and functional impairment such as pain, fatigue, and social functioning (Battalio et al., 2018, 2019). Because no relevant studies were found, it is unclear why people with mild physical disabilities had a higher incidence than those with severe disabilities. Again, we cannot rule out the possibility of more hidden cases of depressive disorders among people with severe physical disabilities. More in-depth studies on this topic are urgent.

Finally, it is notable that the adjusted odds of developing depressive disorders in people with brain injuries and disabilities in major internal organs were lower than in non-disabled individuals. This contradicts a finding from a previous study, which reported an increased risk of depression among adults with cerebral palsy, one of the most common brain disorders causing disability (Smith et al., 2019). Although more research is needed, it is likely that the lower odds of depression among people with brain injury were due to diagnostic overshadowing, which means that clinicians frequently overlook depressive symptoms in people with intellectual dysfunction (Mason & Scior, 2004). Even though we excluded people with intellectual disabilities from the analyses, people with brain disorders are also more likely to have a decline in intellectual ability that is not severe enough to be labeled as intellectual disability (Carr et al., 2005). This could have masked their depressive symptoms from clinicians. The lower odds of depressive disorder among people with disabilities in major internal organs than in the non-disabled is also poorly understood. Previous research has reported higher odds of developing depression among those with disabilities in major internal organs, such as renal dialysis patients (Craven et al., 1987) and heart transplant recipients (Brown et al., 2004; McCartney et al., 2017). Future research should look into the mechanisms connecting each type of disability to a lower or higher incidence of depressive disorders.

Conclusion

People with disabilities were found to be more likely to develop depressive disorders and are less likely to recover, continuing to be affected by the disease for longer than their non-disabled counterparts. A significant proportion of the depressive disorders in the disabled are caused by financial difficulties or comorbidities, rather than the disabilities themselves. We need to pay special attention to those who are unable to access health care due to disabilities, and to those whose depressive disorders are misdiagnosed just as an impairment of intellectual ability. More research is needed to elucidate the causal mechanisms of depressive disorders in people with various types and severities of disabilities in order to develop tailored interventions.

Supplemental Material