Developmental Dyslexia and/or co-occurring Attention Deficit: Investigation of prevalence, underlying cognitive deficits, and family risk in a self-selected sample of parents and children

Reading acquisition is a major developmental achievement and plays a critical role in determining academic achievement. Developmental dyslexia is a heritable disorder with up to 66% of at-risk children later diagnosed with the disorder. Children with dyslexia represent a vulnerable group at high risk of underachievement at school, as well as for poorer socio-emotional outcomes. Dyslexia and Attention Deficit Hyperactivity Disorder (ADHD) commonly co-occur, and recent theories hypothesise that it is the inattentive (AD) symptom domain that is most closely associated with literacy problems. Despite an extensive overlap in symptoms, and a close genetic association, they have primarily been investigated as distinct disorders. However, recent theoretical models propose a multiple deficit framework to examine shared cognitive deficits as an explanation for their co-occurrence. This research had seven aims: (1) investigate the prevalence of single and multiple developmental disorders, in a sample of children with a previous diagnosis of dyslexia or attention deficit, with or without hyperactivity disorder (ADHD, AD), or who had been identified by parents as underachieving in school; (2) determine whether the severity of the deficits found would be greater in children with more than one disorder compared to a single disorder; (3) validate an adult self-report protocol used to screen for dyslexia and ADHD, and use this measure to estimate family risk of each disorder; (4) determine the prevalence of symptoms of dyslexia, ADHD or AD in children with and without a family risk of dyslexia, ADHD or AD; (5) determine whether a single, double, multiple, or an intergenerational multiple deficit model is the best predictor of reading fluency; (6) examine the extent that difficulties with phonological awareness and rapid automatic naming differentiate dyslexia, and difficulties with interference control differentiate ADHD and AD; and (7) examine the Multiple Deficit Model as an explanation of the co-occurrence between dyslexia + ADHD, and dyslexia + AD using cognitive variables identified in the literature as common to both disorders. Seventy-two children aged 9 to 11 years of age, were assessed on a broad range of cognitive, reading, language, motor and attention measures. In Study 1 (Chapter 3), prevalence rates were estimated for the single and multiple developmental disorders using the clinical and subclinical scores from the reading, attention, language and motor measures. The hypothesis that most children would show evidence of more than one developmental disorder was not met using clinical criteria, as the overall prevalence rates for single and multiple disorders were similar. However, when subclinical symptoms were included, most children diagnosed with one disorder had subclinical symptoms of at least one other disorder. Results from one-way ANOVAs showed the groups with co-occurring dyslexia + AD, or dyslexia + ADHD were not more impaired than the single disorder groups, indicating that the severity of each disorder was not influenced by the number of disorders experienced by each child. The Adult Reading Questionnaire (ARQ) and Adult ADHD Self-Report Scale (ASRS) are self-report protocols designed to evaluate reading, writing and spelling proficiency, expressive language difficulties (word finding), as well as problems of organization, attention and hyperactivity in adults. One hundred and seventeen parents (64 mothers and 53 fathers) completed the self-report questionnaire and the data was used to replicate the factor structures of the ARQ and ASRS separately (Study 2, Chapter 4). The parent data was then used to estimate family risk of dyslexia and ADHD or AD in the child sample, and the results supported the hypothesis that children from a family with parent-reported reading difficulties were significantly more likely to have clinical symptoms of dyslexia than children without family risk. Similarly, children whose parents reported symptoms of ADHD were more likely to have clinical and subclinical symptoms of ADHD, than children coming from a family without parental report of these disorders. Correlation, regression, logistic regression analyses, and odds ratios indicated a strong influence of parent-reported reading difficulties on child reading difficulties, as 70.4% of children at family risk of dyslexia had scores in the clinical range on the reading measure. The association between parent-reported symptoms and a child diagnosis was weaker for ADHD, as less than half (42.9%) the children at family risk met criteria for a clinical diagnosis, however, when subclinical symptoms were included, the children of parents with self-rated symptoms of ADHD were more likely to have clinical or subclinical symptoms than children from no-risk families. No significant association was found between parent-reported symptoms and a child diagnosis of AD. In Study 3 (Chapter 5), correlation and multiple regression analyses were used to evaluate a number of cognitive variables (phonological awareness, rapid naming, and expressive language) identified in the research literature to be impaired in individuals with dyslexia. Single, double, and multiple deficit frameworks were used to determine which model best predicted reading fluency, with the results showing that children presented with a combination of deficits, indicating various pathways to dyslexia. Using the intergenerational multiple deficit model to examine the association between parent-reported reading difficulties and children’s reading fluency, it was shown that parent self-reported difficulties accounted for a substantial proportion (18.6%) of the variance in children’s reading fluency scores. After controlling for parent self-reported reading skills, phonological awareness and rapid letter naming each made additional contributions to the explanation of children’s reading fluency. Finally, a multiple deficit model was used to achieve the final research aim (Study 4, Chapter 6). Mixed factorial ANOVAs, and moderated regression analyses were conducted to determine if impairments in phonological awareness and rapid automatic naming were unique to dyslexia, and if impairments in interference control were unique to ADHD and AD. The contribution of working memory and reading comprehension were examined as explanations of the co-occurrence between dyslexia and ADHD or AD. The ability to inhibit distractors (interference control) was found to be impaired in each of the three groups, however some differences were noted. The group with dyslexia responded more slowly on the incongruent Flanker condition, and the ADHD group was less accurate on both the congruent and incongruent conditions, while the AD group were found to be both slower and less accurate on each of the conditions. Phonological awareness and reading comprehension were found to be unique predictors of dyslexia. The relationship between reading fluency and working memory was moderated by AD and ADHD. When symptoms of AD and ADHD were low, working memory improved as reading fluency improved, however this was not the case when symptoms of AD and ADHD were high, where an increase in reading fluency was not accompanied by increases in working memory capacity. Although weaker, reading fluency and rapid naming was moderated by AD, however no association was found for ADHD. From a theoretical perspective, this research advances our understanding of the co-occurrence between dyslexia, ADHD and AD in school age children, as well as the associations between some of the cognitive variables associated with the disorders. This research highlights the high prevalence of dyslexia among children at family risk, and the important contribution of parent-reported difficulties to the explanation of their children’s reading fluency. At a practical level this research is expected to contribute to the better identification of children with reading and attention difficulties. The findings demonstrate the substantial overlap between the symptoms of dyslexia and inattention, and the importance of assessing for both disorders. The success of reading interventions has been found to be limited, and this may be due to the high prevalence of attention deficits which co-occur with dyslexia and that may interfere with remediation. When underlying attentional difficulties are identified, appropriate cognitive training programs can be implemented in conjunction with reading remediation to improve academic performance in school.