Elsevier

Human Movement Science

Volume 25, Issue 1, February 2006, Pages 110-124
Human Movement Science

DCD and ADHD: A genetic study of their shared aetiology

https://doi.org/10.1016/j.humov.2005.10.006Get rights and content

Abstract

Previous studies have found that rates of attention deficit hyperactivity disorder (ADHD) and developmental coordination disorder (DCD) are very similar, both being approximately 7% in sample populations [Kadesjö, B., & Gillberg, C. (1999). Developmental coordination disorder in Swedish 7-year-old children. Journal of the American Academy of Child and Adolescent Psychiatry, 38, 820–828; Milberger, S., Faraone, S., Biederman, J., Testa, M., & Tsuang, M. (1996). New phenotype definition of attention deficit hyperactivity disorder in relatives for genetic analyses. American Journal of Medical Genetics, 67, 369–377]. The rate of comorbidity between the two has been found to be close to 50% [Barkley, R. (1990). Attention deficit hyperactivity disorder: A handbook for diagnosis and treatment. New York: Guilford Press]. Investigations into the comorbidity of the disorders points to a shared aetiology between them. The aim of the present investigation was to examine the extent to which the shared aetiology is due to common genetic factors to both disorders. We also investigated whether particular subtypes of each disorder were more linked than others. Mailed questionnaires were completed by parents (predominantly mothers) of 1285 twin pairs aged 5 and 16 years from the volunteer Australian Twin Registry (ATR). Included were a DSM-IV-based ADHD form, the alternative SWAN (Strengths and Weaknesses of ADHD Symptoms and Normal Behaviour scale) and the Developmental Coordination Disorder Questionnaire (DCDQ). Statistical analyses including structural equation modelling were carried out to explore the genetic factors of both disorders. The modelling showed a strong shared additive genetic component between most subtypes of ADHD and DCD to the subtypes of the other disorder. Analyses comparing the two ADHD measures showed an overlap of the symptoms captured by each measure but also significant differences. The DCD-fine motor and ADHD-Inattentive were most strongly linked using the DSM-IV based scale. On the SWAN scale the results were similar but the general coordination scale was also very strongly linked. Implications for the use of different assessment tools are discussed.

Introduction

Attention deficit hyperactivity disorder (ADHD) and developmental coordination disorder (DCD) are both childhood disorders identified in the DSM-IV (APA, 1994) which have a population prevalence of approximately 7% (Kadesjö and Gillberg, 1999, Milberger et al., 1996). Studies investigating ADHD have found that around 50% of ADHD cases also have motor problems severe enough to be diagnosed as DCD (Barkley, 1990, Piek et al., 1999, Pitcher et al., 2003). Further, children initially diagnosed with DCD have been found to also meet moderate to severe diagnosis for ADHD (Kadesjö & Gillberg, 1999).

Given the joint evolution of ADHD and DCD from the Minimal Brain Dysfunction classification, it is not surprising that these two disorders may be associated (Piek et al., 1999). Not only are they found together, but many studies report a higher prevalence of boys compared with girls for both ADHD (Eiraldi et al., 1997, Pennington and Ozonoff, 1996) and DCD (e.g., Henderson and Hall, 1982, Kadesjö and Gillberg, 1999). However other studies suggest there are equal numbers of boys and girls affected in DCD (e.g., Hoare & Larkin, 1991). Also, both disorders have been linked to psychosocial problems such as socially inappropriate behaviour, emotional problems, reduced academic performance, and reading and spelling difficulties (Levy et al., 2005, Taylor et al., 2004). Furthermore, when ADHD and DCD are comorbid, the outcome tends to be more severe than when each disorder occurs alone (e.g., Gillberg, 1992, Pitcher et al., 2003, Visser, 2003). This implies that there may be an exclusive shared aetiology of the comorbidity which is distinct from the factors influencing either of the separate disorders.

One of the difficulties in exploring the comorbidity of these two disorders is the difference between the bodies of research existing for each disorder. ADHD has been extensively researched (Voeller, 2004) with many genetic studies confirming the high heritability of ADHD and its subtypes (Levy & Hay, 2001). Several candidate loci, such as DAT1 (Chen et al., 2003), DRD4 (Langley et al., 2004), and MAO (Payton et al., 2001) have been identified and replicated. There are also many well developed measures in questionnaire or interview form and numerous neuropsychological measures, all of which are comprehensively covered in the recently revised European guidelines (Taylor et al., 2004). DCD, however, is much less extensively researched and what literature exists on it and its aetiology is often confusing. This confusion is often due to varying selection criteria, such as different cut-off scores being used in different studies (Piek & Edwards, 1997), and also due to the overlap that many symptoms of DCD have with other disorders such as learning difficulties (Peters, Barnett, & Henderson, 2001). Performance tests such as the Movement Assessment Battery for Children (Henderson & Sugden, 1992) and the McCarron Assessment of Neuromuscular Development (McCarron, 1997) are generally used to identify DCD. Recently, screening tools have been developed, such as the Developmental Coordination Disorder Questionnaire (Wilson, Kaplan, Crawford, Campbell, & Dewey, 2000), which allows the testing of much larger samples of children.

Although the link between ADHD and motor problems has been recognised for many years, there have been few studies that have investigated the motor problems in relation to the three distinct subtypes identified by the DSM-IV (APA, 1994). These subtypes are based on the behavioural symptomatology identified in the child; either predominantly inattentive (ADHD-PI), predominantly hyperactive/impulsive (ADHD-HI), or a combination of both types of symptoms (ADHD-C). One link that appears to have been established is that between inattentive symptomatology and poor motor skills, in particular fine motor control (McGee et al., 1992, Piek et al., 1999, Pitcher et al., 2003). There is also evidence that suggests that gross motor deficits are more likely to occur in ADHD-C compared to the ADHD-PI (Piek et al., 1999). Differences in the prevalence of boys and girls for different subtypes also suggests a greater relationship between DCD and ADHD-PI, as a greater proportion of girls are found with ADHD-PI compared with ADHD-C (Lahey et al., 1994).

The aim of the current study was to take validated questionnaire measures for both ADHD and DCD and look for a shared genetic heritability to both of them using quantitative genetic methodology in a very large twin sample. While this methodology is powerful, it does require extremely large sample sizes (Neale & Cardon, 1992). It can answer the question of why the two disorders co-occur at such a high frequency and with a different outcome than either disorder alone. By studying the heritability in this way we will be able to tease apart the genetic influences from the environmental effects acting upon the disorders to better understand how they come about. From looking at the comorbid disorder we can examine whether it is a general link between ADHD and DCD or whether particular subtypes of each are linked more strongly with each other. Specific comorbidities may point to particular aetiological pathways which can then be explored more fully. By studying the subtypes of the disorders we can also examine whether there are different factors influencing each one as well as common factors which act on all of them.

Different screening measures of ADHD were also tested against each other for equivalence and to explore the effect that different ADHD measures have on finding shared heritability between it and DCD. There has been much concern that different measures of ADHD may yield different genetic results. For example on some questionnaires, particularly short ones such as the Rutter A, parents may exaggerate differences between dizygotic twins and thus inflate the heritability (Thapar, Harrington, Ross, & McGuffin, 2000).

Section snippets

Participants

The participants consisted of 1285 families of twins from the Australian Twin ADHD Project (ATAP) summarised in Levy and Hay (2001). Families were ascertained from the Australian Twin Registry (ATR: http://www.twins.org.au), a nation-wide, volunteer based database of twins and higher order multiple birth families born in Australia. Each family was sent a questionnaire package for returning by pre-paid mail. The project was approved by Curtin University Human Research Ethics Committee and by the

Prevalence

The prevalence of DCD using the published Canadian cut-off (Wilson et al., 2000) was 2% which was rather low compared with previous estimates of between 5% and 19% (APA, 1994, Henderson and Sugden, 1992, Kadesjö and Gillberg, 1999, Wright and Sugden, 1996). Using the mean ± 1.65sd method as used for the SWAN scale it was 8% which more closely approximates the prevalence given in DSM-IV of 6% (APA, 1994) and the recent Australian national survey (Graetz, Sawyer, Hazell, Arney, & Baghurst, 2001).

Discussion

The prevalence of both disorders was found to be comparable to those of previous studies. We found a higher rate of ADHD symptoms in boys than girls with a ratio of approximately 3:1 which is comparable to that found in previous studies.

The univariate analyses demonstrated that all of the subscales of ADHD and DCD have a substantial genetic component. For the ADHD data, both the SWAN and the ATBRS showed similar patterns with the inattentive and hyperactive/impulsive subtypes having substantial

Acknowledgments

This work was funded by a grant from the National Health and Medical Research Council of Australia. We would like to thank Grant Baynam and Kellie Bennett for their assistance with data collection and entry and the co-operation of so many families, as well as the Australian Twin Registry.

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