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INTRODUCTION

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The American Association of Intellectual and Developmental Disabilities defines intellectual disability as “a disability characterized by significant limitations in both intellectual functioning and in adaptive behavior, which covers many everyday social and practical skills.” Intellectual disability originates before the age of 18.

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Autism spectrum disorder is a group of developmental disabilities that are characterized by significant impairments in social communication and social interaction, and affected individuals manifest restricted and repetitive behaviors, interests, or activities.

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Intellectual disability and autism spectrum disorder frequently co-occur. Approximately 10% to 15% of all children with intellectual disability have autism spectrum disorder or autistic traits. On the other hand, up to 40% of all children with autism spectrum disorder have intellectual disability.

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Both intellectual disability and autism spectrum disorder are extensively heterogeneous from a genetic viewpoint. Several hundred genes and chromosomal loci have been identified, mutations of which cause or predispose to the respective disorders. Some genetic syndromes manifesting intellectual disability and/or autism spectrum disorder are clinically so distinct and easily recognizable that a given set of symptoms and features will suggest the diagnosis and determine a very targeted diagnostic approach. However, most individuals with intellectual disability or autism spectrum disorder are nondysmorphic, and the overall clinical presentation is nonspecific, rendering them “idiopathic.”

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The genetic diagnostic approach to intellectual disability and autism spectrum disorder has changed dramatically during recent years because of the availability of genome-wide methodologies, such as chromosome microarray analysis and whole-exome sequencing.

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PATHOGENESIS AND EPIDEMIOLOGY FOR INTELLECTUAL DISABILITY

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There are many possible causes of intellectual disability, including prenatal infections, teratogenic exposures, perinatal complications, as well as postnatal brain infections, traumatic brain injury, or toxic exposure. However, especially in developed countries, the majority of cases of intellectual disability (approximately 80%) have an underlying genetic cause. Using the currently available technology, that cause can be identified in up to 60% of all patients who present for diagnostic evaluation.

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The prevalence of intellectual disability in the United States ranges from 0.7% to 1.4%. While males and females are equally represented among individuals with mild intellectual disability, there is an overrepresentation of males among individuals with moderate and severe intellectual disability (male-to-female ratio 1.4:1). This has mostly been attributed to genetic factors, especially X-linked forms of intellectual disability.

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PATHOGENESIS AND EPIDEMIOLOGY FOR AUTISM SPECTRUM DISORDER

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The etiologic causes of autism spectrum disorder are not as well understood as those of intellectual disability. Based on twin studies, the heritability of autism spectrum disorder has been estimated to be between 60% and 90%. Nongenetic risk factors for autism spectrum disorder include prematurity and low birth weight, gestational diabetes, maternal infections during pregnancy, and prenatal drug exposure (eg, valproate, selective serotonin reuptake inhibitors). Genetic causes of autism spectrum disorder can be identified in 20% to 40% of cases referred for evaluation. The likelihood ...

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