An epidemiological and aetiological study of children with intellectual disability in Taiwan

A large-scale cytogenetic study of the causes of intellectual disability (ID) in children from special schools and institutions was made in Taiwan between 1991 and 1996. The screening methods and the identification of subjects with ID consisted of both clinical evaluation (i.e. photographs, questionnaires on family, pre-, peri- and postnatal history, and hospital records, including IQ) and further laboratory studies for diagnosis (i.e. standard chromosome analysis, and if indicated, high-resolution banding, cytogenetic fragile-X study or molecular techniques). A total of 11,892 patients were enrolled in this study. After excluding the acquired causes of ID, such as infections and the sequelae of brain insults, or the well-known single-gene disorders and other multifactorial diseases, 4372 (36.8%) cumulative cases were recruited for karyotyping studies according to their phenotypes and medical records. Abnormal karyotypes were noted in 1889 children (43.2% of all selected children). Thus, the overall incidence of chromosomal aberrations in subjects with ID was estimated as 15.9%. Down's syndrome, the most common cause of ID, accounted for 82.4% of all patients with abnormal karyotypes. The causes of ID were considered to be prenatal in 55.2% (n = 6564) of cases, perinatal in 9.5% (n = 1130), postnatal in 3.3% (n = 392) and unknown in 32.0% (n = 3805) of cases. Two large groups were classified: (1) serious ID (37%), including profound, severe and moderate categories; and (2) mild ID (63%). The causes (pre-, peri- and postnatal, and unknown) in these two populations were: 70%, 10.5%, 5.4% and 14.1%, and 46.5%, 8.9%, 2.1% and 42.5%, respectively. Genetic causes accounted for 38.5% (n = 4578) of all cases in this study, including 1557 with Down's syndrome, 233 with fragile-X syndrome, 199 with other various chromosomal abnormalities (i.e. unbalanced translocation, supernumerary markers and structural rearrangements), 238 with a defined or presumed single-gene defect, and 98 with a recognized contiguous gene syndrome (Prader-Willi, 56; Angelman, 34; Williams, 5; and Kallmann, 3); 2120 cases had familial ID. Multiple anomalies of undefined pattern, but without chromosomal aberration, infantile autism, ID of normal phenotype or family history, were of the other categories. Patients with a single-gene disorder or chromosomal aberration, especially those with unbalanced translocated or rearranged chromosomes, had genetic counselling and family studies. Pre-screening with photographs and questionnaires may give a better costbenefit than blind mass cytogenetic studies for each individual with ID.