Abstract
Introduction
ADHD is a common childhood-onset neurobehavioral disorder that affects 5% of school-aged children and is threefold to fourfold more common in males than in females (Polanczyk, de Lima, Horta, Biederman, & Rohde, 2007). The disorder is characterized by inappropriate levels of inattention, hyperactivity, and impulsivity (Wolraich, Hannah, Pinnock, Baumgaertel, & Brown, 1996) and may result in poor lifestyle and health outcomes, as well as psychiatric comorbidities (Nigg, 2013). ADHD can also result in impaired physical and mental health in children or adolescents and can result in disabilities in adults (Brook, Brook, Zhang, Seltzer, & Finch, 2013; Merikangas et al., 2015).
Children and teenagers with ADHD are reported to be at increased risks of engaging in health-threatening behaviors (Rowland, Lesesne, & Abramowitz, 2002), including smoking (Lambert & Hartsough, 1998), substance use (Molina & Pelham, 2003), and risky driving (Barkley, Murphy, & Kwasnik, 1996). Moreover, undesirable lifestyles, such as abnormal sleep patterns, decreased physical activity, and poor eating habits have been reported in children with ADHD, which might also predispose to long-term health problems (Holton & Nigg, 2016).
ADHD can affect the health of children and adolescents profoundly (Schieve et al., 2012) and can result in injury (Hurtig, Ebeling, Jokelainen, Koivumaa-Honkanen, & Taanila, 2016). The prevalence rates of physical diseases, such as asthma (Chen et al., 2013), atopy (Tsai, Chang, Mou, Sung, & Lue, 2013), allergic rhinitis (Chou et al., 2013), ear infection (Adesman, Altshuler, Lipkin, & Walco, 1990), enuresis, stomach problems, and obesity (Jameson et al., 2016; Nigg, 2013) are reportedly higher among children or adolescents with ADHD than among those without ADHD. It is common for ADHD to co-occur with other medical conditions (Schieve et al., 2012). In addition to physical ill health, ADHD is significantly associated with sustaining injuries (DiScala, Lescohier, Barthel, & Li, 1998) through accidents (DiScala et al., 1998; Pless, Taylor, & Arsenault, 1995) or violence committed by others (Unnever & Cornell, 2003).
Children and adolescents with ADHD have an elevated risk of mortality and morbidity due to accidental injuries (Hurtig et al., 2016). This association between ADHD and injury is likely to be mediated in part by factors associated with increased risk-taking behavior, such as taking greater risk when crossing the road on foot or the overestimation of physical abilities due to diminished inhibitory control (Bonander, Beckman, Janson, & Jernbro, 2016). Children with ADHD may also be at higher risk of experiencing victimization or of being a perpetrator of bullying because of symptoms of ADHD such as impulsivity and externalizing behaviors (Twyman et al., 2010).
These pervasive comorbidities could increase medical care use and costs (Leibson, Katusic, Barbaresi, Ransom, & O’Drien, 2001). Previous studies have reported that ADHD is associated with an increased rate of outpatient and emergency department visits and increased use of prescription medication (Boulet, Boyle, & Schieve, 2009; Chan, Zhan, & Homer, 2002; Cuffe, Moore, & McKeown, 2009). The overall costs of care for children with ADHD are significantly greater than those for children in general, so the economic burden of ADHD can be large (Chan et al., 2002). However, there may be significant unmet care needs among children with ADHD, despite the high rate of health care utilization (Bitsko et al., 2009; Cuffe et al., 2009; Schieve et al., 2012).
The relationship between ADHD and high comorbidity rates for physical disease is multifactorial and heterogeneous (Tsai et al., 2013). Indeed, it could be explained by genetic and other neurobiological factors, including neurodevelopmental, immunological, and infectious factors, in addition to the aforementioned health behaviors (Merikangas et al., 2015). The relationships among ADHD, health behaviors, and health outcomes are therefore complex, and these factors could be causal, mediators, or sequelae of each other (Holton & Nigg, 2016).
Understanding patterns of comorbidity, injury, health behaviors, and health care utilization is important when evaluating and treating children and adolescents with ADHD (Jameson et al., 2016; Hurtig et al., 2016; Schieve et al., 2012). This can provide the rationale for early identification of developmental issues and can minimize health, educational, and social sequelae (Merikangas et al., 2015). Therefore, we examined the health behaviors, health care utilization, and risks of various physical health outcomes among children aged 7 to 18 years with ADHD using a representative Korean nationwide sample. We comprehensively investigated the overall health status, including the risk of comorbid health conditions like physical disease and injury, and engagement in risky health behaviors. In addition, we examined whether ADHD was associated with increased or decreased health care utilization among children aged 7 to 18 years, and whether they had unmet health care needs.
Method
Study Sample
This study was based on data obtained from the Korean National Health and Nutrition Examination Survey (KNHANES IV, V, and VI, 2007-2015), a cross-sectional survey conducted to assess health and nutritional statuses in a representative sample of noninstitutionalized Korean civilians aged ≥1 year (Kweon et al., 2014). Data for the survey were collected through a health interview, health examination, and nutrition survey. The survey used a complex, stratified, multistage probability sampling model, with sampling units comprising an average of 60 households. In total, 20 households per sampling unit were sampled by intrastratification based on geographic area, age, and sex.
Of the 73,353 participants in the KNHANES (2007-2015), we included 10,838 children aged 7 to 18 years in the present study based on those who answered questions regarding a diagnosis of ADHD in the health interview (Figure 1). The Institutional Review Board of Korea Centers for Disease Control and Prevention approved the protocol (2007-02CON-04-P, 2008-04EXP-01-C, 2009-01CON-03-2C, 2010-02CON-21-C, 2011-02CON-06-C, 2012-01EXP-01-2C, 2013-07CON-03-4C, 2013-12EXP-03-5C). All participants of the KNHANES provided a written informed consent that extended to this study.

Flow diagram of study participant selection.
Measures
The survey questions were based on proxy reports from either parent of the participants, except for the questions on safety behaviors, which were based on the self-report of children aged 12 to 18 years.
ADHD
This was based on physician diagnosis, with the following question incorporated in the health interview: “Has the participant ever been diagnosed with ADHD by a physician?”
Overall health
Participants were asked the question, “In general, how would you rate the participant’s health?” Possible answers were “very poor,” “poor,” “fair,” “good,” and “very good.” Responses of “very poor” or “poor” were defined as having a bad overall health.
Physical diseases
Body mass index (BMI) was calculated using the weight and height measurements of participants (weight [kg]/height [m2]). In children aged 7 to 18 years, obesity was defined as a BMI at or above the 95th percentile of the Centers for Disease Control sex-specific BMI-for-age growth charts for the year 2000 (Kuczmarski et al., 2002; Ogden & Flegal, 2010). Data on asthma, atopic dermatitis, allergic rhinitis (2013-2015), otitis media (2007-2009, 2015), and pneumonia, as well as data on ADHD were obtained using the health interview by asking, “Has the participant ever been diagnosed with the disease by a physician?”
Injury
Experience of injury was measured as a positive response (“yes”) to the following question in the health interview: “Has the participant ever had an accident or an addiction problem that had to be treated at a hospital or an emergency room?” For participants who reported an experience of injury, additional questions were used to define the cause of injury (unexpected accidents or violence by others) by asking the question “Was this accident or addiction unexpected?” or “Was the accident intentionally caused by yourself or someone else?”
Health behaviors
Health behaviors included physical activity, safety behaviors, and medical service use. Moderate-intensity physical activity was defined as involvement in physical activity requiring a moderately greater amount of effort than usual and causing noticeably rapid breathing for >30 min at a time for more than five times in the past week. Vigorous-intensity physical activity was defined as involvement in physical activity requiring intensely greater amount of effort than usual and causing excessive rapid breathing for >20 min at a time for more than three times in the past week. Physical activity was only surveyed in adolescents aged 12 to 18 years. Vehicle safety was assessed using different questions depending on the age of the participants: “Does the participant never sit in the front seat of a car?” (7-11 years) and “Do you always wear a seat belt when you sit in the front seat of a car?” (12-18 years). In all age groups, bicycle safety was assessed using the question “Has the participant always worn helmets when riding a bicycle in the past 12 months?”
Health care utilization
Health care utilization included information regarding outpatient clinic visits in the preceding 2 weeks and unmet health care needs (i.e., non-utilization of health care services despite a requirement for medical care) during the previous year. Outpatient clinic visits were determined by a “yes” response to the question “In the last 2 weeks, has the participant received treatment in an outpatient clinic?” The existence of unmet health care needs was determined from a “yes” response to the question “In the past year, has the participant decided not to visit a hospital or clinic center (excluding dental clinics or hospitals) despite the need for medical care?”
Statistical Analysis
Weighted values were applied to all analysis procedures given the complex sampling design used. General characteristics, health behaviors, and health care utilization were compared between children with and without ADHD using the chi-square test for categorical data. The prevalence rates of health outcomes (physical diseases, self-perceived health status, and injury) among the children with or without ADHD were calculated for each of the following 3-year age groups: 7 to 9, 10 to 12, 13 to 15, and 16 to 18 years. These rates were directly standardized to the 3-year age groups using the standard population from the Population and Housing Census 2010. Log-binomial regression was used to estimate adjusted prevalence ratios (APRs) and 95% confidence intervals (CIs) after adjusting for sex and age, with the independent variable set as ADHD and the dependent variable set as health outcomes. We used SAS software version 9.4 (SAS institute Inc., Cary, North Carolina, USA) for all analyses.
Results
Table 1 shows the demographic characteristics, health behaviors, and health care utilization of children with and without ADHD. Among the included children (n = 10,838), only 1.16% (n = 118) were diagnosed with ADHD by a physician. The percentage of males was higher among children with ADHD than among those without ADHD (78.04% and 52.56%, respectively; p < .0001). The mean (standard error) age were 12.6 (0.3) years and 12.8 (0.1) years for children with and without ADHD, respectively, which did not significantly differ among children with and without ADHD. Household monthly income was not significantly different between the two groups.
Demographic Characteristics, Health Behaviors, and Health Care Utilization of Children With and Without ADHD.
Note. The sum of numbers in the subgroups does not equal the total number of participants in this study; participants with missing values were excluded. % = population-weighted proportion.
only survey in 12 to 18 years old, with ADHD (N = 63), without ADHD (N = 5,791).
Regarding health behaviors, there were no significant differences between the two groups in physical activity levels (moderate and vigorous intensity) or safety behaviors (vehicle and bicycle safety). Regarding health care utilization, there was no significant difference between the two groups in visits to outpatient clinics, but there was a significant difference in unmet health care needs: Children with ADHD were less likely to receive treatment despite a need when compared with children without ADHD (19.88% vs. 9.64%, p = .0073).
APRs for most health outcomes were significantly higher among children with ADHD than among those without ADHD (Table 2). Compared with children without ADHD, poor overall health was associated with an APR of 1.45 (95% confidence interval [CI] = [1.42, 1.49]) in children with ADHD. Moreover, children with ADHD were significantly more likely to have asthma (APR, 2.02; 95% CI = [2.11, 2.23]), atopic dermatitis (APR, 1.32; 95% CI = [1.30, 1.34]), allergic rhinitis (APR, 1.36; 95% CI = [1.35, 1.38]), otitis media (APR, 1.62; 95% CI = [1.59, 1.65]), and pneumonia (APR, 1.21; 95% CI = [1.19, 1.24]) compared with those without ADHD. Obesity was not found to be significantly associated with ADHD (APR, 0.99; 95% CI = [0.98, 1.01]).
Age-Standardized Prevalence (SE) and PR (95% CI) of Health Outcomes by ADHD Among Korean Children Aged 7 to 18 Years.
Note. PR = prevalence ratio; APR = adjusted prevalence ratio; CI = confidence interval.
adjustment for sex and age.
PR (95% CI), APR (95% CI) of with ADHD versus without ADHD.
with ADHD (N = 50), without ADHD (N = 2,937).
with ADHD (N = 45), without ADHD (N = 5058).
Finally, children with ADHD were more likely to sustain injuries compared with those without ADHD (APR, 1.63; 95% CI = [1.60, 1.66]). Among the children who had experienced injuries, those with ADHD were 1.45 times (95% CI = [1.42, 1.47]) more likely to be injured by an unexpected accident and 4.96 times (95% CI = [4.74, 5.18]) more likely to be injured because of violence by others when compared with children without ADHD.
Discussion
Our findings showed that children aged 7 to 18 years with ADHD were at an increased risk of various health-related problems, including poor overall health, physical disease, and injury. In addition, we showed that children with ADHD were significantly less likely to use health care services when they needed medical care. However, there were no significant associations between most health behaviors and ADHD. Of note, despite there being no significant difference in health behaviors between the two groups, children with ADHD were more likely to have poor health outcomes than those without ADHD.
Our finding that there was no significant difference in the health-related behaviors of ADHD children compared with their peers conflicted with the findings of previous studies (Barkley et al., 1996; Holton & Nigg, 2016; Lambert & Hartsough, 1998; Rowland et al., 2002). However, our finding was consistent with that of a study from the Netherlands, which reported ADHD was not associated with any lifestyle variables in the elderly despite its close association with physical illness and poor self-perceived health (Semeijn et al., 2013). Our findings suggest the association between ADHD and high physical comorbidity may not necessarily be related to health behaviors. Although health behaviors may affect long-term health outcomes (Conner & Norman, 1998; Holton & Nigg, 2016), they may not fully account for the high rate of physical comorbidities in ADHD. However, we concede we examined only a few types of health behaviors, limited to physical activity and safety behaviors. A full understanding of the mediating role of health behaviors in the health problems of ADHD children will require the investigation of other types of health-related behavior, including dietary and sleep habits and thrill-seeking behaviors, which have been reported to be associated with ADHD symptoms (Holton & Nigg, 2016).
Our study also showed that children with ADHD are at an increased risk for sustaining injuries from accidents. This finding is consistent with reports from previous studies of an elevated injury risk among children with ADHD in comparison with their healthy peers (Bonander et al., 2016; DiScala et al., 1998; Hurtig et al., 2016). There could be a close association between injuries sustained by ADHD children and their innate “always-on-the-go” personality (Hurtig et al., 2016) or their inattention and executive dysfunction (Stavrinos et al., 2011). More specifically, ADHD children may be more likely to engage in risky behaviors because of impulsivity, and they have a diminished awareness of risks in dangerous situations and greater difficulties with using prevention strategies, all of which may increase their risk of physical injury (Farmer & Peterson, 1995; Hurtig et al., 2016).
Furthermore, our results show that children with ADHD have an approximately fivefold increased risk of injury due to violence by others. Although few studies have investigated violence-related injuries in children with ADHD, a high incidence of both bullying and victimization has consistently been reported in this group (Taylor, Saylor, Twyman, & Macias, 2010; Twyman et al., 2010; Unnever & Cornell, 2003; Wiener & Mak, 2009). The poor impulse control and reduced frustration control that ADHD children often present may increase the risk of being the target or the perpetrator of bullying (Taylor et al., 2010). For example, in peer-conflicting situations, ADHD children with diminished problem-solving abilities and increased impulsivity may experience more difficulties in managing conflicts in peer interactions. Thus, they may be less well liked by their peers and they can often become the target of bullying. Given the adverse long-term impact on multiple mental health outcomes of bullying victimization experience (Singham et al., 2017; Wolke, Copeland, Angold, & Costello, 2013), as well as the risk of serious physical injury, the violent victimization of ADHD children warrants special clinical and educational attention.
Children with ADHD in this study had increased risks of allergic disorders, such as asthma, atopic dermatitis, and allergic rhinitis. This is consistent with previous studies (Chen et al., 2013; Chou et al., 2013; Tsai et al., 2013), and research evidence that both ADHD and allergic disorders are heritable and influenced by interactions between genetic and environmental factors (Merikangas et al., 2015; Tsai et al., 2013). Moreover, a recent meta-analysis (Miyazaki et al., 2017) described that there were genetic associations between ADHD and asthma. This analysis suggested that an ADHD behavior subtype was linked to a genetic polymorphism of dopamine receptor D5, a type of dopamine D1-like receptor (Mill, Curran, Richards, Taylor, & Asherson, 2004; Ostadali et al., 2004), and that this may be involed in the development of asthma by immunological regulation of T-helper 17 cell differentiation (Nakagome et al., 2011). In contrast, it has been proposed that the symptoms and diagnosis of ADHD may also be triggered by allergic immunopathologies, such as hypersensitivity mechanisms that are activated by exposure to allergens in the environment (Pelsser, Buitelaar, & Savelkoul, 2009). Indeed, Marshall (1989) hypothesized that ADHD could be a side effect of allergic disorders, arguing that allergic reactions engender an imbalance in cholinergic/adrenergic activity in the central nervous system.
The prevalence of infectious diseases, such as otitis media and pneumonia, was higher among children with ADHD than among those without ADHD in our study, which is consistent with previous research (Adesman et al., 1990; Padolsky, 2008). Although less investigated than the association with allergic disorders, some evidence is available regarding the association of viral infections with ADHD (Arpino, Marzio, D’Argenzio, Longo, & Curatolo, 2005; Bekdas et al., 2014; Mick, Biederman, & Faraone, 1996). Several studies have reported that many children with human immunodeficiency virus (Mellins, Brackis-Cott, Dolezal, & Abrams, 2006), enterovirus 71 (Chang et al., 2007), and varicella zoster virus (Dale, Church, & Heyman, 2003) also meet the clinical criteria for ADHD. It has also been reported that ADHD may be predisposed by exposure to viral infections in the first trimester and at the time of birth, notably in the winter months and when mothers have viral infections (Arpino et al., 2005; Mick et al., 1996). This supports the contention that immaturity of the immune system may underlie the increased risks of infection in children with ADHD (Padolsky, 2008).
Despite the higher rate of physical comorbidities in ADHD children, they did not make a significantly higher number of outpatient visits than their peers. Indeed, they were significantly less likely to use health care services when they need medical care. These findings are consistent with those of previous studies, which described reasons for the unmet health care needs of children with ADHD including the inability of their parents to afford needed treatments, as well as delays in care (Bitsko et al., 2009; Schieve et al., 2012). These findings suggest not only do children with ADHD tend to have an increased need for health care for their comorbid medical problems and injuries (Hurtig et al., 2016; Leibson et al., 2001; Schieve et al., 2012) but also that they underutilize health care services relative to their needs. Previous reports have suggested that the primary factor for health care needs being unmet for children with developmental problems was financial, including lack of insurance, poverty, and the high medical cost burden (Chan et al., 2002; Schieve et al., 2012; Weller, Minkovitz, & Anderson, 2003). It is especially notable that the average health care costs for children with ADHD are significantly greater than those for their healthy peers, with increased prescription-related expenses, outpatient visits, and emergency room visits, similar to those of children with chronic conditions such as asthma (Chan et al., 2002; Cuffe et al., 2009). Therefore, although no significant difference in socioeconomic status was observed between the children with and without ADHD, excessive medical costs not fully covered by the national insurance system may act as a barrier for health care utilization in families with ADHD children. In addition to this economic burden, the unmet medical care needs of these children may also be linked with the increased caregiving demands and the aggravation experienced by parents of children with ADHD (Anastopoulos, Guevremont, Shelton, & DuPaul, 1992; Larson, Russ, Kahn, & Halfon, 2011). In particular, parents of ADHD children with a greater number of physical comorbid disorders often report experiencing greater difficulties in caring for their children and feel more bothered and upset by them (Larson et al., 2011). The considerable psychological burden may leave parents less attentive to their children’s medical care needs for comorbid conditions in addition to their ADHD symptoms. However, the present study was limited to assessing and identifying specific reasons behind the increased unmet health care needs among children with ADHD. Given that these unmet health care needs may contribute to poorer health outcomes and a higher prevalence of physical disease among these children (Weissman, Stern, Fielding, & Epstein, 1991), future research is needed to establish the factors that influence the underutilization by children with ADHD of the health care they need.
Strengths and Limitations
The strengths of the present study are the use of a large and nationally representative database and a complex sampling design. To the best of our knowledge, this is the first study to evaluate health conditions comprehensively among children and adolescents with ADHD. However, our study has several limitations. First, the temporal or causal relationship between ADHD and comorbid physical health conditions could not be determined from the results because of the cross-sectional design. Future studies using prospective longitudinal data will be able to expand and confirm our findings. Second, some cases of ADHD might have been misclassified because physician diagnosis of ADHD was based on parent reports that relied on a single question rather than the administration of a structured diagnostic interview or a standard assessment tool. However, diagnosis by the single question (Heliövaara et al., 1993; Kehoe, Wu, Leske, & Chylack, 1994) and by parent report (Visser, Danielson, Bitsko, Perou, & Blumberg, 2013) has been reported to be valid and reliable. Third, this study only investigated a limited spectrum of health behaviors that could potentially lead to the poor health outcomes of children with ADHD, including their dietary and sleep habits and their safety and thrill-seeking behaviors. Future studies that assess a wider range of health behaviors would lead to a broader understanding of the role of health behaviors in influencing the health problems of children with ADHD.
Conclusion
Our study results indicate that care should be taken to manage not only the behavioral symptoms of ADHD but also the diminished overall health and increased risks of physical illness and injury. In particular, allergic and infectious diseases, as well as injuries arising from accidents or the violent behavior of others, should be carefully monitored for children and adolescents with ADHD. In addition, children and adolescents with ADHD may have unmet health care needs; this requires increased efforts to ensure the provision of quality health services for the medical conditions of these vulnerable children as well as an increase in their health care utilization to meet their needs.
Footnotes
Declaration of Conflicting Interests
The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Funding
The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This study was supported by an Intramural Research Grant (No R2017-A) from the National Center for Mental Health, Ministry of Health & Welfare, Republic of Korea.
