A monozygotic twin design to investigate etiological factors for DCD and ADHD

The high level of comorbidity between Developmental Coordination Disorder (DCD) and Attention Deficit Hyperactivity Disorder (ADHD) suggests that these disorders may have a shared etiology. We used a co-twin control design to study monozygotic (MZ) twins concordant and discordant for DCD and ADHD. In a total of 922 sets of MZ twins, 866 sets were eligible. We found equal numbers of DCD concordant and discordant sets; more ADHD concordant than discordant sets; nine sets in which both twins met criteria for DCD₊ ADHD; 773 sets of twins did not meet criteria for either DCD or ADHD. The only significant sex difference between groups was for ADHD discordant sets, with more males than females. For DCD groups there were no significant sex differences, with slightly more girls than boys affected in both groups. There was a greater number of oxygen perfusion complications in DCD affected than unaffected twins, suggesting that, given equal numbers of DCD concordant and discordant sets and a similar number of DCD girls and boys, the role of pre- and perinatal environmental factors is stronger in the etiology of DCD than in ADHD. Factors such as placental difficulties and hypoxia have also been related to cerebral palsy, which suggests that DCD may fall on the upper end of a continuum of movement disorder that includes cerebral palsy. The results suggest different etiological pathways for DCD and ADHD. Second born twins were at greater risk for oxygen perfusion difficulties in sets concordant for DCD, ADHD, and unaffected for either.