Page 13 - Demo

General introduction 111 Millions of people worldwide are affected by one of the nearly 8,000 rare disorders, defined as a condition affecting less than one in 2,000 individuals.1 Around 80 percent of these rare disorders has a genetic origin and many are associated with neurodevelopmental disorders.2 These (rare) genetic neurodevelopmental disorders (RGND) are collectively common, affecting 1-3% of the population directly and another 5% indirectly, i.e., family members coping with care.3,4 There is a marked overlap between individuals with an RGND and with an intellectual disability (ID) (Figure 1). ID is characterized by substantial limitations in both intellectual functioning and adaptive behavior, originating in childhood or adolescence.5–7 Genetic factors play a major role in the etiology of ID, with more than 1700 ID-related genetic disorders already identified,8 apart from exogenous factors such as birth complications, trauma, extreme malnutrition, and infection.9Intellectual disabilityGenetic neurodevelopmental disordersFigure 1. Schematic representation of the populations with intellectual disability and rare genetic neurodevelopmental disorders. Importantly, the figure depicts an indication rather than a precise scaled proportion. RGNDs and ID are often accompanied by complex somatic and neuropsychiatric comorbidities, including epilepsy, sensory deficits, movement disorders, autism spectrum disorders, anxiety disorders, and cognitive and behavioral disturbances, such as irritability, aggression, and Annelieke Muller sHL.indd 11 14-11-2023 09:07