Although intellectual disability is generally nonprogressive, in certain genetic disor- ders (e.g., Rett syndrome) there are periods of worsening, followed by stabilization, and in others (e.g., San Phillippo syndrome) progressive worsening of intellectual function. After early childhood, the disorder is generally lifelong, although severity levels may change over time. The course may be inﬂuenced by underlying medical or genetic conditions and co—occurring conditions (e.g., hearing or visual impairments, epilepsy). Early and ongoing in- terventions may improve adaptive functioning throughout childhood and adulthood. In some cases, these result in significant improvement of intellectual functioning, such that the diagnosis of intellectual disability is no longer appropriate. Thus, it is common practice when assessing infants and young children to delay diagnosis of intellectual disability un- til after an appropriate course of intervention is provided. For older children and adults, the extent of support provided may allow for full participation in all activities of daily liv- ing and improved adaptive function. Diagnostic assessments must determine whether im- proved adaptive skills are the result of a stable, generalized new skill acquisition (in which case the diagnosis of intellectual disability may no longer be appropriate) or whether the improvement is contingent on the presence of supports and ongoing interventions (in which case the diagnosis of intellectual disability may still be appropriate).
Genetic and physiological. Prenatal etiologies include genetic syndromes (e.g., se- disorders), inborn errors of metabolism, brain malformations, maternal disease (including placental disease), and environmental inﬂuences (e.g., alcohol, other drugs, toxins, terato— gens). Perinatal causes include a variety of labor and delivery-related events leading to neonatal encephalopathy. Postnatal causes include hypoxic ischemic injury, traumatic brain injury, infections, demyelinating disorders, seizure disorders (e.g., infantile spasms), severe and chronic social deprivation, and toxic metabolic syndromes and intoxications (e.g., lead, mercury).
Intellectual disability occurs in all races and cultures. Cultural sensitivity and knowledge are needed during assessment, and the individual’s ethnic, cultural, and linguistic back- ground, available experiences, and adaptive functioning within his or her community and cultural setting must be taken into account.
Overall, males are more likely than females to be diagnosed with both mild (average malezfemale ratio 1.6:1) and severe (average malezfemale ratio 12:1) forms of intellectual disability. However, gender ratios vary widely in reported studies. Sex-linked genetic fac- tors and male vulnerability to brain insult may account for some of the gender differences.
A comprehensive evaluation includes an assessment of intellectual capacity and adaptive functioning; identification of genetic and nongenetic etiologies; evaluation for associated medical conditions (e.g., cerebral palsy, seizure disorder); and evaluation for co-occurring mental, emotional, and behavioral disorders. Components of the evaluation may include basic pre- and perinatal medical history, three—generational family pedigree, physical exam- ination, genetic evaluation (e.g., karyotype or chromosomal microarray analysis and testing for specific genetic syndromes), and metabolic screening and neuroimaging assessment.
The diagnosis of intellectual disability should be made whenever Criteria A, B, and C are met. A diagnosis of intellectual disability should not be assumed because of a particular genetic or medical condition. A genetic syndrome linked to intellectual disability should be noted as a concurrent diagnosis with the intellectual disability.
Major and mild neurocognitive disorders. Intellectual disability is categorized as a neu- rodevelopmental disorder and is distinct from the neurocognitive disorders, which are characterized by a loss of cognitive functioning. Major neurocognitive disorder may co- occur with intellectual disability (e.g., an individual with Down syndrome who develops