Simple and complex motor skills in children with dyslexia and/or attention deficit/hyperactivity disorder : towards a unifying framework of sequential motor impairments in neurodevelopmental disorders

There is growing evidence that motor abnormalities are present in many neurological illnesses, ranging from neurodevelopmental disorders to neurodegenerative dementia. Theories have emerged suggesting that co-occurring motor impairments across disorders can be indicators of a vulnerable brain state and common aberrant underlying mechanisms. Dyslexia and Attention Deficit disorder with or without hyperactivity (AD) are two prevalent neurodevelopmental disorders and are associated with a collection of cognitive and motor symptoms that often co-occur. The co-occurrence of cognitive symptoms across dyslexia and AD is generally accepted and authors have developed unifying frameworks to better understand accumulating evidence of overlapping symptoms. However, less emphasis has been placed on co-occurring motor impairments, in part due to the inconsistency of findings associated with the many different assessment tools used across studies. In addition, few studies have explored what components of motor abilities are similarly impaired in both disorders. The objective of the current thesis is to clarify the presence of co-occurring motor difficulties in dyslexia and AD by assessing a variety of abilities associated with motor functioning. In addition, the relationship between co-occurring cognitive symptoms and motor difficulties is examined across both disorders to support the putative presence of a common aberrant mechanism that may underlie co-occurring motor weaknesses in dyslexia and AD.  The thesis is comprised of two empirical articles. The first paper assesses fine and gross motor abilities that range from simple to complex sequential coordination, in children with dyslexia only, AD only, and both disorders (Combo). Results suggest that children with dyslexia and/or AD have co-occurring difficulties compared to their typically developing peers on unimanual and bimanual sequential coordination in the presence of preserved simple motor speed. In addition, children with AD have more severe problems in complex bimanual out-of-phase coordination and are impaired on measures of dexterity. These results suggest that children with AD may have weaknesses on a wider range of motor abilities and have more profound bimanual coordination difficulties. The second paper examines the relationship between cognitive abilities and co-occurring sequential motor difficulties in dyslexia and AD. Capabilities in visual working memory and math fluency were found to be significant predictors of motor abilities without differentiation between disorders, with one exception by which math fluency did not contribute to performance on bimanual in-phase coordination in the AD group. Moreover, the distinctive symptoms of reading in dyslexia and inattention in AD did not contribute significantly to sequential motor performance.  The results suggest that the pattern of motor difficulties is similar in dyslexia and AD, and support the presence of common mechanisms that underlie co-occurring motor weaknesses. We suggest that dyslexia and AD often have co-occurring sequential motor difficulties, and to our knowledge these findings are among the first to show a shared relationship between cognitive abilities and sequential motor weaknesses. The findings support the idea that dyslexia and AD are different facets of a common atypical development and that shared sequential motor difficulties are indicators of a vulnerable brain state. The findings provide important information to help guide assessment and early screening of neurodevelopmental disorders, as well as encourage the development and application of motor intervention programs that integrate sequential motor planning.

Keyphrases: Neurodevelopmental disorders, Dyslexia, Attention deficit disorder with or without hyperactivity, Comorbidity, Sequential motor skills, Unimanual and bimanual coordination, Cognitive abilities, Atypical brain development