Children and adolescents with ID/IDD are highly heterogeneous across and within disease groups, with a broad range of cognitive abilities and behaviors that can vary across their life course, making measurements difficult. A major obstacle is the lack of agreed upon endpoints to assess function. The measures need to be sensitive, reliable, valid, able to detect change over time, and appropriate for use in specific populations [52]. There are limitations across patient assessments (self or interview), caregiver (observations), clinician (direct examination or interview) and combined assessments. There is a particular need to develop clinical rating scales for use in youth who are nonverbal. The targets need to include: Cognition (e.g., executive function, visuospatial skills, memory, learning, inhibition, attention); Behavior and emotion (e.g., anxiety, arousal, stress response, irritability, sociability); Physical/medical (e.g., motor symptoms, sleep disorders, sensory alterations such as ocular or auditory function, seizures, endocrine or immune regulation, respiratory or autonomic dysregulation); and Adaptive behavior (e.g., functional abilities in a variety of living situations, such as home, community or school settings).
ID/IDD services should be designed with the knowledge that a very high proportion of persons with ID/IDD will have substantial and persistent co-occurring mental disorders across the lifespan [53▪▪]. Primary public health and secondary clinical interventions should aim at reducing the prevalence and severity of co-occurring mental disorder in ID/IDD populations. Mental health service development needs to identify and leverage supports and to enhance capacity in education and skills training among parents and families of persons with ID/IDD, with special relevance to the burden of care of co-occurring mental disorders.