Motor functioning in young people with Attention-Deficit/Hyperactivity Disorder, combined type: A clinical and ocular motor investigation.

Background
Attention-Deficit/Hyperactivity Disorder Combined Type (ADHD-CT) is one of the most common neurodevelopmental disorders, estimated as prevalent in 5.3% of young people worldwide. In addition to its core symptoms of inattention and hyperactivity/impulsivity, up to 50% of young people with ADHD-CT exhibit clinically significant gross and fine motor problems. Emerging research suggests that these motor problems may be associated with co-occurring Autism Spectrum Disorder (ASD) symptoms. However, little is currently known about the underlying neurobiology of these problems or their functional impact. The overarching aim of this thesis was to advance current understanding of the motor problems experienced by young people with ADHD-CT, from both a neurobehavioural perspective, using ocular motor measures, and from a clinical, functional perspective. This approach was conceptualised in the context of the International Classification of Functioning, Disability and Health (ICF, 2001) framework.
Method
Participants were young males aged 7 to 15 years old either diagnosed with ADHD-CT or without ADHD-CT (i.e. Typically Developing, TD). Sample sizes varied for each study (from N = 24 to N = 58) and were convenience samples. Neurobehavioural studies (1 and 2) utilised saccade adaptation and volitional saccade ocular motor paradigms. Functional studies (3 and 4) utilised the Children’s Assessment of Participation and Enjoyment (CAPE) and a customised parent questionnaire on the occurrence of accidental injuries (e.g., bruises, cuts) respectively. Motor proficiency was assessed using the Movement ABC–2 (MABC-2). Dimensional measures of ADHD-CT symptoms (Conners Rating Scale-Revised), ASD symptoms (Social Responsiveness Scale), anxiety and emotional-behavioural disturbance (Developmental Behavioural Checklist) were also quantified. The two groups were compared on outcome variables, and associations between dimensional variables were measured.
Results
Motor problems and neurobehavioural (ocular motor) findings:
Study 1 and 2 revealed anomalous saccade adaptation in young people with ADHD-CT, and significantly larger saccade amplitudes with altered main sequence (peak velocity: amplitude) relationships for volitional saccades. These findings reflected impairments in processes that monitor the mismatch between eye and target position, and adjust saccade amplitude, velocity and duration accordingly; processes that rely on the functional integrity of the cerebellum.
Motor problems and participation:
Study 3 revealed young people with ADHD-CT participated in activities that were located closer to home and reported greater enjoyment of their activities compared to TD young people. The results indicated that young people with ADHD-CT who participated in more solitary and home-based activities have greater emotional-behavioural disturbance.
Motor problems and accidental injuries:
Study 4 revealed a significantly greater number of parent-reported accidental injuries for young people with ADHD-CT, compared with TD young people, which was correlated with increased hyperactivity/impulsivity, ASD, and anxiety symptoms, but not motor proficiency.
Conclusion
This thesis advances current understanding of motor problems in young people with ADHD-CT from neurobehavioural, clinical and functional perspectives. The pattern of ocular motor deficits in Studies 1 and 2 suggest that motor problems in ADHD-CT may be underpinned by disruption to cerebellar neural circuitry, potentially representing neurobiological overlap with ASD. Differences in participation context and increased accidental injuries highlight the importance of assessing these areas of function clinically, to develop appropriate interventions for young people with ADHD-CT. This may be especially pertinent for those young people with ADHD-CT who have additional ASD and emotional-behavioural symptomology.