Influence of Relative Age on Diagnosis and Treatment of Attention-Deficit Hyperactivity Disorder in Taiwanese Children

Quantifying Potential Bias Resulting From Child Age on Screening for Hyperactive/Impulsive Presentations of ADHD

OBJECTIVE

This study aims to quantify the potential age bias in screening of hyperactive/impulsive presentations of ADHD in children ages 5 to 12 through comparison of age-based and overall percentiles in screening.

METHOD

A referred clinical sample of 307 children ages 5 to 12 with behavioral concerns completed the Vanderbilt Attention Deficit-Hyperactivity Disorder Diagnostic Parent Rating Scale (VADPRS) and were formally evaluated for ADHD with a diagnostic interview. Analysis utilizing logistic regression and receiver operating characteristic (ROC) curves was performed to compare the screening performance of agebased and overall percentiles.

RESULTS

The age-based percentiles demonstrated no improvement in the analyzed models compared to overall percentiles in hyperactive presentation ADHD screening. This finding was present in the overall sample and in the sub analysis of the 5 to 6 year old children.

CONCLUSIONS

This study identifies no improvement in modeling of hyperactive/impulsive ADHD screening when considering a child's age using age-based percentiles.

Children's Relative Age and Medicine Treatment for Attention-Deficit/Hyperactivity Disorder Across Australian Jurisdictions with Different School Enrolment Policies

Background: Children who are relatively young for their school grade are more likely to receive treatment for attention-deficit/hyperactivity disorder (ADHD). It is unclear whether the phenomenon also exists across Australia or is impacted by the school enrolment policy in place. Objective: We evaluated the association between children's relative age and initiation of ADHD medicines across Australian jurisdictions with different school enrolment policies and rates of delayed school entry. Methods: We used Australia-wide dispensing data for a 15% random sample of children 4-9 years of age in 2013-2017 to create a nationwide cohort. Due to high rates of delayed school entry in New South Wales (NSW), we used linked prescribing and education data for a cohort of NSW residents starting school in 2009 and 2012. We estimated incidence rate ratios (IRRs) for ADHD medicine across children's birth month, sex, and jurisdiction. We used asthma medicines as a negative control. Results: For girls, we observed a relative age effect in three out of five jurisdictions, with an IRR ranging from 1.3 to 2.8, comparing the youngest versus oldest birth month thirds. We observed more modest effects among boys, ranging from null to 1.5-fold. In NSW, the relatively youngest boys were less likely to initiate stimulant medicines than the oldest (IRR = 0.5, 95% confidence interval 0.29-0.78). We did not observe a relative age effect for initiation of asthma medicines. Conclusions: In jurisdictions with low rates of delayed entry, relatively young children were more likely to initiate ADHD medicines than their older classmates. We observed the inverse association in NSW where delayed entry was highest, likely reflecting the characteristics and needs of children who delay school entry for 1 year and become the oldest children in the grade. Increased awareness around children's maturity differences and school readiness may enhance appropriate diagnosis and treatment of ADHD.

Relative Age Effect in Attention Deficit/Hyperactivity Disorder at Various Stages of the Medicalization Process

Diagnosis and pharmacological treatment of ADHD are more common among the youngest children in a classroom, born in the months immediately preceding the school entry cutoff date. The mechanisms behind this phenomenon, called the relative age effect (RAE), are not yet well understood. Nearly all hypotheses involve the school system, various teachers’ actions, and concern children’s immaturity. However, most previous studies have been based on reports of health professionals’ diagnoses and prescriptions found in official databases rather than on reports of teachers’ behavior identification or suspicion of ADHD, despite their being at the first stages of the medicalization process. Our study overcomes this limitation by using reports of parents’ and teachers’ behavior identification or suspicion of ADHD within a three-level multilevel survey design, comprising 1294 children, 130 teachers, and 17 elementary public schools. The goal of our study was to investigate whether RAE stems (1) from adults’ judgement of the child’s expression of immaturity or (2) from the consequences associated with the child’s double burden of being immature and exhibiting ADHD behaviors. Our multilevel analyses put forward the first hypothesis only, supporting the medicalization of immaturity. RAE in ADHD seems mostly initiated by teachers’ pre-diagnostic actions toward younger children.

Attention deficit hyperactivity disorder among children related to maternal job stress during pregnancy in Taiwan: a prospective cohort study

OBJECTIVE

Attention deficit hyperactivity disorder (ADHD) is the most common neurobehavioral disorders. Although studies have suggested relationships between ADHD in children and maternal psychosocial stress during pregnancy, little is known about the effects of work-related mental stress. Considering the increasing number of pregnant women who continue to work during the gestation period, this study investigated whether work-related stress during pregnancy is related to offspring ADHD.

METHODS

The Taiwan Birth Cohort Study followed selected representative mother-infant pairs in a face-to-face interview since a child was 6 months old. A total of 10,556 working pregnant women who completed follow-up 8 years later were included. Whether the 8-year-old child had ever received a diagnosis of ADHD were inquired. Self-reported job stress during pregnant period was obtained 6 months after delivery. Factors including perinatal and socioeconomic factors as well as the mother's job conditions were further analyzed with logistic regression.

RESULTS

Among those who continued working during pregnancy, 3850 (36.5%) mothers reported having job stress during pregnancy, and 210 (2.0%) of the children were diagnosed as having ADHD before 8 years of age. Compared with mothers who reported no job stress, the adjusted odds ratio of child ADHD was 1.91 (95% CI 1.21-3.07) for mothers with "very stressful" jobs during pregnancy and 1.53 (95% CI 1.04-2.25) for mothers with "rather stressful" jobs.

CONCLUSION

Among pregnant female workers, higher levels of job stress were related to the higher occurrence of ADHD in their children.

Age at school entry and reported symptoms of attention-deficit/hyperactivity in first graders: results of the prospective cohort study ikidS

Young age at school entry (ASE) is related to attention-deficit/hyperactivity disorder in higher grades. The reason for this association is unclear, but medical oversupply and stress-related factors are discussed. We aimed to investigate whether ASE is associated with reported symptoms of attention-deficit/hyperactivity (ADH) already in first grade. Data of a population-based prospective cohort study (N = 2003; Mainz-Bingen region; Rhineland-Palatinate; Germany) with baseline assessments prior to school entry and two follow-ups during first grade were analysed. ADH symptoms were assessed by parent and teacher versions of the Strengths and Difficulties Questionnaire. Associations between ASE and scores of the hyperactivity/inattention subscale (range 0-10) were investigated by regression analysis and adjusted for potential confounders and baseline symptoms prior to school entry. In total, 1633 children (52% boys, mean ASE 6.5 years) were included. There were no relationships between ASE and parent-reported scores of the hyperactivity/inattention subscale prior to school entry and 3 months thereafter. However, at the end of first grade, ASE was negatively associated with the hyperactivity/inattention subscale in parent (- 0.7 subscale points per year ASE, standard error = 0.16, p < 0.0001) and teacher reports (- 1.2 subscale points per year ASE, standard error = 0.25, p < 0.0001). This ASE effect appeared more pronounced in girls than in boys. Young ASE is related to more reported symptoms of ADH at the end of first grade, but not before. The evolvement of this effect during first grade may be a clue to ASE-related stress factors.

Effect of relative age on childhood mental health: A cohort of 9,548,393 children and adolescents

BACKGROUND

The effect of relative age on the diagnoses of attention deficit hyperactivity disorder (ADHD), disruptive behavior disorder (DD), anxiety disorder, and depressive disorder and the prescription for ADHD and antidepressant medications remains unclear.

AIM

To clarify the impact of relative age in a school year with the diagnoses of ADHD, DD, anxiety disorder, and depressive disorder and the prescription for ADHD and antidepressant medications.

METHODS

The annual cutoff birthdate for entry to school in Taiwan is August 31. The Taiwan National Health Insurance Research Database was used to enroll 9,548,393 children and adolescents aged 3-17 years during the study period (September 1, 2001, to August 31, 2011). The Poisson regression model was performed to examine the likelihood of receiving diagnoses of ADHD, DD, anxiety disorder, and depressive disorder, as well as the prescription of ADHD and antidepressant medications among children born in August (the youngest) and September (the oldest).

RESULTS

Both boys and girls born in August had a higher risk of being diagnosed as having ADHD (odds ratio [OR] = boys: 1.65, girls: 1.80), DD (1.29, 1.45), anxiety disorder (1.49, 1.33), and depressive disorder (1.10, 1.10). Furthermore, children born in August were more likely to be prescribed ADHD medication (1.71, 1.72) and antidepressants (1.18, 1.09) compared with those born in September.

DISCUSSION

Relative age, as an indicator of neurocognitive maturity, is a critical factor for the likelihood of being diagnosed as having ADHD, DD, anxiety disorder, and depressive disorder among children and adolescents.

Impact of the School Environment on Medical Treatment of Attention Deficit Hyperactivity Disorder: A Population-Wide Register Data Study of School-Wide Positive Behavioral Interventions and Supports

Although attention deficit hyperactivity disorder (ADHD) is among the most heritable psychiatric childhood disorders, social and gene-environment interactions seemingly play an important role in the etiology of ADHD. Consistent with this, this study finds that School-Wide Positive Behavioral Interventions and Supports (SWPBIS) reduced the likelihood of pharmacotherapeutic treatment for ADHD at age 14-16 by 12%, using population-wide Norwegian register data and a difference-in-difference design (N = 698,364, birth cohorts 1990-2002, 48.7% girls, 5.7% immigrant background). At-risk students in schools with high fidelity of implementation are driving these intervention effects. Overall, the findings indicate that children with a genetic disposition for ADHD are more likely to avoid medical treatment in an organized and predictable school setting with a focus on positive reinforcement.

Children's Relative Age and ADHD Medication Use: A Finnish Population-Based Study

OBJECTIVES

The youngest children in a classroom are at increased risk of being medicated for attention-deficit/hyperactivity disorder (ADHD). We examined the association between children's birth month and ADHD medication rates in Finland.

METHODS

Using a population-based study, we analyzed ADHD medication use among children born in 2005 to 2007. Cases (n = 7054) were identified from the first purchase of medication for ADHD. Cox proportional hazard models and hazard ratios (HRs) were examined by birth month and sex. Finnish children start first grade in the year of their seventh birthday. The cutoff date is December 31.

RESULTS

Risk of ADHD medication use increased throughout the year by birth month (ie, January through April to May through August to September through December). Among boys born in September to December, the association remained stable across cohorts (HR: 1.3; 95% confidence interval [CI]: 1.1-1.5). Among girls born in September to December, the HR in the 2005 cohort was 1.4 (95% CI: 1.1-1.8), whereas in the 2007 cohort it was 1.7 (95% CI: 1.3-2.2). In a restricted follow-up, which ended at the end of the year of the children's eighth birthday, the HRs for boys and girls born in September to December 2007 were 1.5 (95% CI: 1.3-1.7) and 2.0 (95% CI: 1.5-2.8), respectively.

CONCLUSIONS

Relative immaturity increases the likelihood of ADHD medication use in Finland. The association was more pronounced during the first school years. Increased awareness of this association is needed among clinicians and teachers.

Hyperactivity disorder in children related to traffic-based air pollution during pregnancy

BACKGROUND

Attention-deficit hyperactivity disorder (ADHD) is one of the most common neurobehavioral disorders in childhood globally. Between the two components of ADHD, hyperactivity disorder is more prevalent than inattention during early childhood. Although some investigations have implied a relationship between childhood ADHD and gestational exposure to air pollution, the evidence is limited. The aim of this study was to investigate the association between gestational exposure to air pollution exposure and hyperactivity disorder in childhood in a population-based birth cohort.

METHODS

The Taiwan Birth Cohort Study started from all deliveries of Taiwan in 2005 by the birth registry, and recruited representative 12% of all mother-infant pairs by two-stage stratified sampling. At age of 8 years in each child, their main caretaker was inquired whether the child had ever received a hyperactivity diagnosis from a physician or other specialist, like special needs educator. Exposure to air pollutants during gestation was estimated through ordinary kriging based on data from air monitoring stations of Environmental Protection Administration, Taiwan. Logistic regression was used to determine adjusted odds ratios (aORs) of hyperactivity disorder in relation to air pollutants.

RESULTS

A total of 16,376 mother-infant pairs were included in the final analysis; 374 (2.3%) of the children had received a diagnosis of hyperactivity before 8 years of age. The occurrence of hyperactivity was significantly related to prenatal nitrogen oxide (NO_x), but not to particulate matter 10 μm or less in diameter or sulfur dioxide. Further analysis to separate effects by nitrogen dioxide (NO₂) and/or nitric oxide (NO) showed that only NO was significantly related to hyperactivity [aOR per interquartile range (3.14 ppb): 1.26, 95% confidence interval: 1.09-1.46].

CONCLUSIONS

In conclusion, our study found childhood hyperactivity disorder to be positively associated with prenatal NO exposure. Further confirmation on potential hazardous effects of NO and investigation on potential mechanisms are warranted.

Relative Age and Attention-Deficit/Hyperactivity Disorder: Data From Three Epidemiological Cohorts and a Meta-analysis

OBJECTIVE

To investigate the effect of relatively younger age on attention-deficit/hyperactivity disorder (ADHD) symptoms and diagnosis through three population-based cohorts and a meta-analysis.

METHOD

This study included participants of three community-based cohorts in Brazil: 1993 Pelotas Cohort (N = 5,249), 2004 Pelotas Cohort (N = 4,231), and Brazilian High-Risk Study for Psychiatric Disorders (HRC study) (N = 2,511). We analyzed the effect of relatively younger age on ADHD symptoms and diagnosis. For the meta-analysis, we searched MEDLINE, PsycINFO, and Web of Science from inception through December 25, 2018. We selected studies that reported measures of association between relative immaturity and an ADHD diagnosis. We followed the Meta-analysis Of Observational Studies in Epidemiology guidelines. The protocol for meta-analysis is available on PROSPERO (CRD42018099966).

RESULTS

In the meta-analysis, we identified 1,799 potentially eligible records, from which 25 studies including 8,076,570 subjects (164,049 ADHD cases) were analyzed with their effect estimates. The summarized relative risk of an ADHD diagnosis was 1.34 (95% CI, 1.26-1.43, p < .001) for children born in the first 4 months of the school year (relatively younger). Heterogeneity was high (I² = 96.7%). Relative younger age was associated with higher levels of ADHD symptoms in the 1993 Pelotas Cohort (p = .003), 2004 Pelotas Cohort (p = .046), and HRC study (p = .010).

CONCLUSION

Children and adolescents who are relatively younger compared with their classmates have a higher risk of receiving an ADHD diagnosis. Clinicians should consider the developmental level of young children when evaluating ADHD symptoms.

Learning a Phenotypic-Attribute Attentional Brain Connectivity Embedding for ADHD Classification using rs-fMRI

Automated diagnosis of Attention Deficit/Hyperactivity Disorder (ADHD) from brain's functional imaging has gained more interest due to its high prevalence rates among children. While phenotypic information, such as age and gender, is known to be important in diagnosing ADHD and critically affects the representation derived from fMRI brain images, limited studies have integrated phenotypic information when learning discriminative embedding from brain imaging for such an automatic classification task. In this work, we propose to integrate age and gender attributes through attention mechanism that is jointly optimized when learning a brain connectivity embedding using convolutional variational autoencoder derived from resting state functional magnetic resonance imaging (rs-fMRI) data. Our proposed framework achieves a state-of-the-art average of 86.22% accuracy in ADHD vs. typical develop control (TDC) binary classification task evaluated across five public ADHD-200 competition datasets. Furthermore, our analysis points out that there are insufficient linked connections to the brain region of precuneus in the ADHD group.

What Differentiates Children with ADHD Symptoms Who Do and Do Not Receive a Formal Diagnosis? Results from a Prospective Longitudinal Cohort Study

ADHD diagnoses are increasing worldwide, in patterns involving both overdiagnosis of some groups and underdiagnosis of others. The current study uses data from a national longitudinal study of Irish children (N = 8568) to examine the sociodemographic, clinical and psychological variables that differentiate children with high hyperactivity/inattention symptoms, who had and had not received a diagnosis of ADHD. Analysis identified no significant differences in the demographic characteristics or socio-emotional wellbeing of 9-year-olds with hyperactivity/inattention who had and who had not received a diagnosis of ADHD. However, by age 13, those who had held a diagnosis at 9 years showed more emotional and peer relationship problems, worse prosocial behaviour, and poorer self-concept. Further research is required to clarify the developmental pathways responsible for these effects.

Attention deficit hyperactivity disorder late birthdate effect common in both high and low prescribing international jurisdictions: a systematic review

BACKGROUND

Multiple studies have found that the youngest children in a classroom are at elevated risk of being diagnosed with, or medicated for, ADHD. This systematic review was conducted to investigate whether this late birthdate effect is the norm and whether the strength of effect is related to the absolute risk of being diagnosed/medicated.

METHODS

A literature search of the PubMed and ERIC databases and snowball and grey literature searching were conducted.

RESULTS

A total of 19 studies in 13 countries covering over 15.4 million children investigating this relationship were identified. Three other studies exploring related topics were identified. The diversity of methodologies prevented a meta-analysis. Instead a systematic review of the 22 studies was conducted. A total of 17 of the 19 studies found that the youngest children in a school year were considerably more likely to be diagnosed and/or medicated than their older classmates. Two Danish studies found either a weak or no late birth date effect. There was no consistent relationship between per-capita diagnosis or medication rates and the strength of the relative age effect, with strong effects reported in most jurisdictions with comparatively low rates.

CONCLUSIONS

It is the norm internationally for the youngest children in a classroom to be at increased risk of being medicated for ADHD, even in jurisdictions with relatively low prescribing rates. A lack of a strong effect in Denmark may be accounted for by the common practice of academic 'redshirting', where children judged by parents as immature have a delayed school start. Redshirting may prevent and/or disguise late birthdate effects and further research is warranted. The evidence of strong late birthdate effects in jurisdictions with comparatively low diagnosis/medication rates challenges the notion that low rates indicate sound diagnostic practices.

Relative age and ADHD symptoms, diagnosis and medication: a systematic review

There is a growing international literature investigating the relationship between attention-deficit/hyperactivity disorder (ADHD) and younger relative age within the school year, but results have been mixed. There are no published systematic reviews on this topic. This study aimed to systematically review the published studies on the relative age effect in ADHD. Systematic database searches of: Medline, Embase, PsycINFO, Web of Science, ERIC, Psychology and Behavioral Sciences Collection and The Cochrane Library were conducted. Studies were selected which investigated the relative age effect in ADHD in children and adolescents. Twenty papers were included in the review. Sixteen (of 20) papers reported a significantly higher proportion of relatively younger children being diagnosed with ADHD and/or receiving medication for this. Meta-analyses involving 17 of these 20 papers revealed a modest relative age effect in countries with higher prescribing rates, risk ratio = 1.27 (95% CI 1.19-1.35) for receipt of medication. The relative age effect is well demonstrated in countries with known higher prescribing rates. Amongst other countries, there is also increasing evidence for the relative age effect, however, there is high heterogeneity amongst studies. Further research is needed to understand the possible reasons under-pinning the relative age effect and to inform attempts to reduce it.

Preliminary Validity of the Eyberg Child Behavior Inventory with Taiwanese Clinic-Referred Children

The Eyberg Child Behavior Inventory (ECBI) has previously been shown to be a psychometrically sound instrument used to assess disruptive behaviors in children in the United States and in other cultures/countries but not in Taiwan. The purpose of this study was to examine the factor structure and to establish the discriminative validity of the ECBI with two groups of Taiwanese children: 70 clinic-referred children with clinically elevated externalizing behavior problems and 70 community-based matched comparison children. Exploratory factor analyses resulted in a six-factor model for the clinic-referred sample and a five-factStrengths and Difficultieor model for the matched comparison sample, indicating that the ECBI is not unidimensional. Adequate convergent and divergent validity also were established between the ECBI Intensity and Problem Scales and another measure of child externalizing (for assessing convergent validity) and internalizing (for determining divergent validity) behavior. The results of the present study suggest that the ECBI is a valid measure of assessing externalizing behavior problems in Taiwanese children. Future research may seek to refine the factor structure of the ECBI in a Taiwanese sample. Future studies are also needed to examine other psychometrics of the ECBI, replicate this study with a larger sample, and establish its normative data in Taiwan.

Teachers' perceptions of behavioral problems in Dutch primary education pupils: The role of relative age

A growing number of studies suggest that relatively young behavior of pupils gives them a much greater likelihood of being diagnosed with a disorder such as ADHD. This 'relative age effect' has also been demonstrated for special educational needs, learning difficulties, being bullied, and so on. The current study investigated the relationship between relative age of pupils in primary education and teachers' perception of their behavior. The study sample included 1973 pupils, aged between 6 and 12. Six linear mixed models were carried out with birth day in a year as predictor variable and 'total problem score', 'problems with hyperactivity', 'behavioral problems', 'emotional problems', 'problems with peers' and 'pro-social behavior' as dependent variables. Random intercepts were added for school and teacher level. Cluster-mean centering disaggregated between-school effects and within-school effects. We found no associations between relative age of pupils and teacher perceptions of their behavior. Several explanations are postulated to account for these findings which contradict prior studies on relative age effects.

Young age at school entry and attention-deficit hyperactivity disorder-related symptoms during primary school: results of a prospective cohort study conducted at German Rudolf Steiner Schools

OBJECTIVES

Young age at school entry (ASE) for students has been related to their impaired mental health in higher grades. To avoid the negative health consequences of young ASE, preschool examinations and individual school entry deferral for young children are routinely performed by some school authorities. We aimed to investigate whether ASE was associated with attention-deficit hyperactivity disorder (ADHD)-related symptoms in pupils attending schools using a selective school enrolment procedure.

DESIGN

Prospective open cohort study with baseline assessments at school entry and two follow-ups in the second and fourth grades.

SETTING

Up to 128 Rudolf Steiner Schools (Waldorf Schools) located within Germany.

PARTICIPANTS

Of the 3079 children from whom data were gathered in the second or fourth grade, 2671 children born between 1 July 2001 and 31 October 2002 (age at baseline: mean 6.7, min 5.91, max 7.24 years, 50% girls) were selected for analysis to avoid bias introduced by individuals at the edges of the ASE distribution.

MAIN OUTCOME MEASURES

ADHD-related symptoms were assessed at school entry and second and fourth grades by parent-reported and teacher-reported versions of the Strengths and Difficulties Questionnaire (Hyperactivity-Inattention Subscale).

RESULTS

The agreement between parent-reported and teacher-reported symptoms was poor (intra-class correlation: 0.41 and 0.44 in second and fourth grade assessments, respectively). Regarding teacher reports, ASE was negatively associated with ADHD-related symptoms in the second grade (regression coefficient β=-0.66 per year, P=0.0006) and fourth grade (β=-0.56, P=0.0014). Associations remained after adjusting for potential confounders and pre-existing symptoms at baseline. Regarding parent reports, associations were markedly weaker in both grades (second grade: β=-0.22, P=0.12; fourth grade: β=-0.09, P=0.48).

CONCLUSIONS

Using a prospective study design and comprehensive adjustment for confounding and baseline symptoms, we confirmed prior evidence of the association between young ASE and teacher-reported ADHD symptoms in primary school.

The gift of time? School starting age and mental health

Using linked Danish survey and register data, we estimate the causal effect of age at kindergarten entry on mental health. Danish children are supposed to enter kindergarten in the calendar year in which they turn 6 years. In a "fuzzy" regression-discontinuity design based on this rule and exact dates of birth, we find that a 1-year delay in kindergarten entry dramatically reduces inattention/hyperactivity at age 7 (effect size = -0.73), a measure of self-regulation with strong negative links to student achievement. The effect is primarily identified for girls but persists at age 11.

How evolutionary thinking can help us to understand ADHD

We argue that current debates about attention-deficit hyperactivity disorder (ADHD) can be considered afresh using an evolutionary lens. We show how the symptoms of ADHD can often be considered adaptive to their specific environment. We suggest that, from an evolutionary point of view, ADHD symptoms might be understood to result from an ‘evolutionary mismatch’, in which current environmental demands do not fit with what evolution has prepared us to cope with. For example, in our ancestral environment of evolutionary adaptedness (EEA), children were not expected to sit still and concentrate on academic tasks for many hours a day. Understanding ADHD in terms of such a ‘mismatch’ raises significant issues regarding the management of childhood ADHD, including ethical ones. An approach based on the concept of mismatch could provide an alternative to current debates on whether ADHD results from nature or nurture and whether it is under- or over-diagnosed. It would allow clinicians and policy makers to take both the child and the environment into account and consider what might be desirable and feasible, both in society and for specific children, to lessen the mismatch.

LEARNING OBJECTIVES

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Relative age within the school year and diagnosis of attention-deficit hyperactivity disorder: a nationwide population-based study

BACKGROUND

Findings are mixed on the relationship between attention-deficit hyperactivity disorder (ADHD) and younger relative age in the school year. We aimed to investigate whether relative age is associated with ADHD diagnosis in a country where prescribing rates are low and whether any such association has changed over time or relates to comorbid disorders (eg, conduct disorder [CD], oppositional defiant disorder [ODD], or learning disorder [LD]).

METHODS

We used nationwide population-based registers to identify all Finnish children born between Jan 1, 1991, and Dec 31, 2004, who were diagnosed with ADHD from age 7 years onwards (age of starting school). We calculated incidence ratios to assess the inter-relations between relative age within the school year, age at ADHD diagnosis, and year of diagnosis (1998-2003 vs 2004-11).

FINDINGS

Between Jan 1, 1998, and Dec 31, 2011, 6136 children with ADHD were identified. Compared with the oldest children in the school year (ie, those born between January and April), the cumulative incidence of an ADHD diagnosis was greatest for the youngest children (ie, those born between September and December); for boys the incidence ratio was 1·26 (95% CI 1·18-1·35; p<0·0001) and for girls it was 1·31 (1·12-1·54; p=0·0007). The association between relative age and age at ADHD diagnosis reflected children diagnosed before age 10 years, and the strength of this association increased during recent years (2004-11). Thus, compared with children born between January and April, for those born between May and August, the ADHD incidence ratio was 1·37 (95% CI 1·24-1·53; p<0·0001) and for those born between September and December, the incidence ratio was 1·64 (1·48-1·81; p<0·0001). The relative age effect was not accounted for by comorbid disorders such as CD, ODD, or LD.

INTERPRETATION

In a health service system with low prescribing rates for ADHD, a younger relative age is associated with an increased likelihood of receiving a clinical diagnosis of ADHD. This effect has increased in recent years. Teachers, parents, and clinicians should take relative age into account when considering the possibility of ADHD in a child or encountering a child with a pre-existing diagnosis.

FUNDING

Academy of Finland, Finnish Medical Foundation, Orion Pharma Foundation, Finnish Cultural Foundation.

Uncovering exposures responsible for birth season - disease effects: a global study

Objective

Birth month and climate impact lifetime disease risk, while the underlying exposures remain largely elusive. We seek to uncover distal risk factors underlying these relationships by probing the relationship between global exposure variance and disease risk variance by birth season.

Material and Methods

This study utilizes electronic health record data from 6 sites representing 10.5 million individuals in 3 countries (United States, South Korea, and Taiwan). We obtained birth month–disease risk curves from each site in a case-control manner. Next, we correlated each birth month–disease risk curve with each exposure. A meta-analysis was then performed of correlations across sites. This allowed us to identify the most significant birth month–exposure relationships supported by all 6 sites while adjusting for multiplicity. We also successfully distinguish relative age effects (a cultural effect) from environmental exposures.

Results

Attention deficit hyperactivity disorder was the only identified relative age association. Our methods identified several culprit exposures that correspond well with the literature in the field. These include a link between first-trimester exposure to carbon monoxide and increased risk of depressive disorder (R = 0.725, confidence interval [95% CI], 0.529-0.847), first-trimester exposure to fine air particulates and increased risk of atrial fibrillation (R = 0.564, 95% CI, 0.363-0.715), and decreased exposure to sunlight during the third trimester and increased risk of type 2 diabetes mellitus (R = −0.816, 95% CI, −0.5767, −0.929).

Conclusion

A global study of birth month–disease relationships reveals distal risk factors involved in causal biological pathways that underlie them.

ADHD treatment and diagnosis in relation to children's birth month: Nationwide cohort study from Norway

BACKGROUND

Studies from several countries have reported that children youngest in grade are at higher risk of attention-deficit/hyperactivity disorder (ADHD) diagnosis and treatment. Norwegian children start school the year they turn six, making children born in December youngest in their grade. We used data on medication, specialist healthcare diagnoses, and primary healthcare diagnoses from national registers to investigate associations between birth month and ADHD.

METHODS

All children born in Norway between 1998 and 2006 ( N=509,827) were followed from age six until 31 December 2014. We estimated hazard ratios for ADHD medication and diagnoses by birth month in Cox proportional-hazards models. We compared risk among siblings to control for potentially confounding socioeconomic factors, and assessed risk of receiving ADHD medication by birth month while attending different grades in cross-sectional time-series analyses.

RESULTS

At end of follow-up, 5.3% of boys born in October-December had received ADHD medication, compared with 3.7% of boys born in January-March. Corresponding numbers for girls were 2.2% and 1.3%, respectively. The adjusted hazard ratio for ADHD medication for children born in October-December (reference: January-March) was 1.4 (95% confidence interval: 1.4-1.5) for boys and 1.8 (1.7-2.0) for girls. Analyses with diagnoses as outcome showed consistent results, and analyses restricted to siblings within the study population also supported the findings. Analysis by grade revealed an increased risk for children born late in the year from grade 3 onwards, with most marked differences in higher grades.

CONCLUSIONS

Children youngest in grade had the highest risk of receiving ADHD treatment. Differences were most marked among older children.