Genetic syndromes associated with Intellectual Disability (ID) often evidence increased prevalence of co-occurring conditions, such as attention deficit hyperactivity disorder (ADHD). However, these co-occurring conditions are often overlooked in genetic syndrome populations and characteristics are attributed to the genetic syndrome, without considering other possible explanations, known as diagnostic overshadowing. Research has shown high ADHD prevalence rates and different symptom presentations in genetic syndrome populations, compared to individuals with ADHD in non-syndromic populations. However, research in this area is limited, variable, and warrants updating. This thesis explores the prevalence and profile of ADHD in genetic syndromes, associated with ID.  

Part A presents a systematic review and meta-analysis of the ADHD prevalence in six genetic syndromes (Down Syndrome, Williams Syndrome, Fragile X Syndrome, Angelman Syndrome, 15q13.3 Microdeletion Syndrome, Tuberous Sclerosis Complex), while also exploring ADHD presentation, neurodevelopmental profiles associated with increased ADHD risk and the measures used to assess ADHD in this population. 

 Part B investigates the ADHD prevalence and profile in a large clinic sample of children and young people with Sturge-Weber syndrome (SWS), a neurocutaneous genetic syndrome, affecting approximately 1 in 50,000 individuals. It also explores cognitive and behavioural characteristics associated with ADHD in SWS and evaluates the clinical utility of two ADHD assessment tools; Conners-3 and CPT-3, in this population.  

Findings from both studies evidence high ADHD prevalence across genetic syndromes, including SWS. Inattention symptoms were more common across syndromes reported in the systematic review, while combined ADHD symptoms were more frequent in the SWS sample. Significant variability was found in ADHD assessment tools, while no additional diagnostic value from the CPT-3, over and above the Conners-3, was observed. Cognitive and behavioural characteristics associated with ADHD in these populations are discussed. This thesis highlights the need for timely ADHD assessment and treatment in genetic syndrome populations associated with ID.