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. 2012 Aug;53(8):864-73.
doi: 10.1111/j.1469-7610.2012.02527.x. Epub 2012 Feb 10.

Aetiology for the covariation between combined type ADHD and reading difficulties in a family study: the role of IQ

Celeste H M Cheung  1 , Alexis C Wood, Yannis Paloyelis, Alejandro Arias-Vasquez, Jan K Buitelaar, Barbara Franke, Ana Miranda, Fernando Mulas, Nanda Rommelse, Joseph A Sergeant, Edmund J Sonuga-Barke, Stephen V Faraone, Philip Asherson, Jonna Kuntsi
Affiliations

Aetiology for the covariation between combined type ADHD and reading difficulties in a family study: the role of IQ

Celeste H M Cheung et al. J Child Psychol Psychiatry. 2012 Aug.

Abstract

Background: Twin studies using both clinical and population-based samples suggest that the frequent co-occurrence of attention deficit hyperactivity disorder (ADHD) and reading ability/disability (RD) is largely driven by shared genetic influences. While both disorders are associated with lower IQ, recent twin data suggest that the shared genetic variability between reading difficulties and ADHD inattention symptoms is largely independent from genetic influences contributing to general cognitive ability. The current study aimed to extend the previous findings that were based on rating scale measures in a population sample by examining the generalisability of the findings to a clinical population, and by measuring reading difficulties both with a rating scale and with an objective task. This study investigated the familial relationships between ADHD, reading difficulties and IQ in a sample of individuals diagnosed with ADHD combined type, their siblings and control sibling pairs.

Methods: Multivariate familial models were run on data from 1,789 individuals at ages 6-19. Reading difficulties were measured with both rating scale and an objective task. IQ was obtained using the Wechsler Intelligence Scales (WISC-III/WAIS-III).

Results: Significant phenotypic (.2-.4) and familial (.3-.5) correlations were observed among ADHD, reading difficulties and IQ. Yet, 53%-72% of the overlapping familial influences between ADHD and reading difficulties were not shared with IQ.

Conclusions: Our finding that familial influences shared with general cognitive ability, although present, do not account for the majority of the overlapping familial influences on ADHD and reading difficulties extends previous findings from a population-based study to a clinically ascertained sample with combined type ADHD.

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Conflict of interest statement

The authors have disclosed the following competing or potential conflicts of interest: J.K B. has been in the past 3 years a consultant to / member of Advisory Board of / and/or speaker for Janssen Cilag, Eli Lilly, Bristol-Myer Squibb, Organon/Shering Plough, UCB, Shire, Medice, Servier, Bioprojet, Pfizer, and Servier. J.A.S. is a member of an Advisory Board to Eli Lilly and Shire; has received research funding from Eli Lilly; educational grants from Eli Lilly, Janssen-Cilag and Shire; and a speaker’s fee from Eli Lilly, Janssen-Cilag and Shire. E. S.-B. is a member of an Advisory Board to Shire, Flynn Pharma, UCB Pharma and Astra Zeneca; has received research support from Janssen Cilag, Shire and Qbtech and Flynn Pharma; conference support from Shire; is on speaker board for Shire and UCB Pharma; and has been a consultant for UCB Pharma and Shire. S.V.F. received consulting fees and was on Advisory Boards for Shire Development and received research support from Shire and the National Institutes of Health (NIH) in the past year; in previous years, he received consulting fees or was on Advisory Boards or participated in continuing medical education programs sponsored by: Shire, McNeil, Janssen, Novartis, Pfizer and Eli Lilly; he also receives royalties from a book published by Guilford Press: Straight Talk about Your Child’s Mental Health. P.A. has received funding for educational and research activities from Janssen-Cilag and Shire; and has received consultancy and speaker fees from Janssen-Cilag, Shire, Eli Lilly and Flynn Pharma that have been used for educational and research activities. J.K. has received speaker fees from Eli Lilly that have been used for educational and research activities. The other authors report no such associations and declared that they have no competing or potential conflicts of interest arising from the publication of this work..

Figures

Figure 1
Figure 1
Parameters F1–F3 and parameters E1–E3 are estimates from Cholesky models estimating the familial and child-specific environmental factors across IQ, ADHD and Reading Difficulties Questionnaire (RDQ), respectively. Significant paths (p<0.05) are indicated as solid lines and non-significant paths (p≥0.05) are indicated as dotted lines. Note: Parameters F1–F3 represent familial influences on and between the traits. f1,1 represents the sum of the familial influences underlying IQ. f2,1 represents the familial influences from IQ that are also shared with ADHD, while parameter f2,2 represents those familial influences that underlie ADHD, and are not shared with IQ. The sum of familial influences underlying ADHD is therefore the sum of f1,1 (those shared with IQ) and f2,2 (those not shared with IQ). The same inferences apply to RDQ, such that f3,1 represents familial influences underlying RDQ that are shared with ADHD (and to some extent ADHD), f3,2 represents familial influences shared between ADHD and RDQ only, and f3,3 represents familial influences specific to RDQ only. The sum of these three paths represents the sum of all familial influences underlying RDQ. The same inferences can be made for the E parameters, which represent child-specific environmental influences (and subsume any measurement error).
Figure 2
Figure 2
Parameters F1–F3 and parameters E1–E3 are estimates from Cholesky models estimating the familial and child-specific environmental factors across IQ, ADHD and TOWRE, respectively. Significant paths (p<0.05) are indicated as solid lines and non-significant paths (p≥0.05) are indicated as dotted lines.
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