ARTICLES
Monozygotic Twins Discordant for Attention-Deficit/Hyperactivity Disorder: Ascertainment and Clinical Characteristics

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ABSTRACT

Objective

Nongenetic factors and phenomenology of attention-deficit/hyperactivity disorder (ADHD) were examined in monozygotic (MZ) twin pairs discordant for ADHD.

Method

Recruitment included telephone screening (n = 297 pairs), behavioral ratings obtained from parents and teachers (n = 59 pairs), and, finally, in-person assessment (n = 25 pairs; structured classroom observation, diagnostic interview, psychoeducational evaluation, birth record review, establishment of monozygosity, and anatomic brain imaging). Affected twins were further contrasted with previously studied affected singletons.

Results

Of the 25 MZ twin pairs qualifying for in-person evaluation, only 10 proved discordant for ADHD. Affected twins were mostly comparable with affected singletons on clinical measures, although fathers’ self-ratings of childhood ADHD status were significantly lower in twins than in singletons.

Conclusions

Discordance for ADHD in MZ twins appears to be ascribable to greater environmental discordance and decreased familiality. Despite these differences, affected twins were phenotypically comparable with affected singletons. Thus MZ twins discordant for ADHD, while rare, can inform research on the etiology and pathophysiology of this disorder.

Section snippets

Recruitment

In the absence of a national twin registry, we publicized the study through Web sites, newsletters, and mailings of CHADD (Children and Adults With Attention-Deficit/Hyperactivity Disorder), the National Organization of Mothers of Twins Clubs, Inc., the Virginia Twin Study of Adolescent Behavior & Development at Virginia Commonwealth University, and the American Academy of Child and Adolescent Psychiatry. Paid advertisements were placed through the Canadian group Parents of Multiple Births

RESULTS

Based on the 3-day evaluation, we confirmed full categorical discordance for DSM-IV combined type ADHD in 10 (40%) of 25 twin pairs. Twins were primarily excluded because both cotwins met DSM-IV ADHD criteria (n = 9); three pairs were excluded because neither of the pair met full criteria for ADHD. In a pair discordant for ADHD, the twin who did not meet full ADHD criteria instead met criteria for bipolar II and generalized anxiety disorder. Another twin pair was excluded when we were unable to

DISCUSSION

Recruiting MZ twins discordant for ADHD proved more difficult than originally anticipated. Of 216 eligible pairs, 104 met our inclusion and exclusion criteria, but more than 90% of those proved to be concordant for ADHD status, resulting in a mere 10 pairs in which one twin met DSM-IV criteria for combined type ADHD and the cotwin did not meet criteria for any ADHD subtype. We had expected a higher yield because approximately 120,000 twin pairs are born each year in the United States (Martin et

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  • Cited by (0)

    The study was supported entirely by the Division of Intramural Research Programs, NIMH. The authors thank teachers Ann Davidson, M.Ed., and Susan Job, M.A.; psychologist Barbara Keller, Ph.D.; recreation therapist Debbie Marcus, C.T.R.S.; Jo Procter at NIH Blood Bank; Judy Silberg, Ph.D., of the Virginia Twin Study of Adolescent Behavior & Development at Virginia Commonwealth University; Children and Adults With Attention-Deficit/Hyperactivity Disorder, Inc.; the National Organization of Mothers of Twins Clubs, Inc.; the American Academy of Child and Adolescent Psychiatry; and the families who participated.

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