To examine the clinical and genetic characteristics of intellectual disability.

We conducted a population-based retrospective analysis on the clinical and genetic data of 959 children with diagnosed intellectual disability during a 5-year period (2017–2021) at Oulu University Hospital, Finland.

Pathogenic or likely pathogenic gene variants were detected in 89 of 194 patients (46%) who underwent exome sequencing. Chromosomal abnormalities, including those with low penetrance, were observed in 106 of 530 patients (20%) who underwent chromosomal microarray testing. Chromosomal abnormalities and causative gene variants were more frequently identified in patients with moderate to profound intellectual disability than in those with mild intellectual disability; however, this difference was not significant in the diagnostic yield analysis. Epilepsy, congenital heart disease, hearing loss, ophthalmological abnormalities, and autism spectrum disorder were more common among patients with moderate to profound intellectual disability, whereas attention-deficit/hyperactivity disorder was associated with mild intellectual disability. Chromosomal abnormalities were associated with congenital heart disease and hearing loss, while pathogenic gene variants were associated with epilepsy and ophthalmological abnormalities.

Somatic comorbidities were more common in moderate to profound intellectual disability, whereas attention-deficit/hyperactivity disorder was more frequent in mild intellectual disability.