AN EPIDEMIOLOGIC AND ETIOLOGIC STUDY OF CHILDREN WITH INTELLECTUAL DISABILITY IN TAIWAN

A large-scale cytogenetic study of the causes of intellectual disabili ty (ID) in children from special schools and institutions was made in Taiwan between 1991 and 1996. The screening methods and the identifica tion of subjects with ID consisted of both clinical evaluation (i.e. p hotographs, questionnaires on family, pre-, peri- and postnatal histor y, and hospital records, including IQ) and further laboratory studies for diagnosis (i.e. standard chromosome analysis, and if indicated, hi gh-resolution banding, cytogenetic fragile-X study or molecular techni ques). A total of 11892 patients were enrolled in this study. After ex cluding the acquired causes of ID, such as infections and the sequelae of brain insults, or the well-known single-gene disorders and other m ultifactorial diseases, 4372 (36.8%) cumulative cases were recruited f or karyotyping studies according to their phenotypes and medical recor ds. Abnormal karyotypes were noted in 1889 children (43.2% of all sele cted 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 we re classified: (I) serious ID (37%), including profound, severe and mo derate 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 ca uses accounted for 38.5% (n = 4578) of all cases in this study, includ ing 1557 with Down's syndrome, 233 with fragile-X syndrome, 199 with o ther various chromosomal abnormalities (i.e. unbalanced translocation, supernumerary markers and structural rearrangements), 238 with a defi ned or presumed single-gene defect, and 98 with a recognized contiguou s gene syndrome (Prader-Willi, 56; Angelman, 34; Williams, 5; and Kall mann, 3); 2120 cases had familial ID. Multiple anomalies of undefined pattern, but without chromosomal aberration, infantile autism, ID of n ormal phenotype or family history, were of the other categories. Patie nts with a single-gene disorder or chromosomal aberration, especially those with unbalanced translocated or rearranged chromosomes, had gene tic counselling and family studies. Pre-screening with photographs and questionnaires may give a better costbenefit than blind mass cytogene tic studies for each individual with ID.