Concordance Rates of Adolescent Idiopathic Scoliosis in a Danish Twin Population

Study Design. Clinical, radiological, and genetic determination of zygosity of twin pairs from the Danish Twin Registry who self-reported having adolescent idiopathic scoliosis (AIS). Objective. To establish concordance rates of AIS. Summary of Background Data. The etiology of and the true mode of inheritance of AIS remain unclear. Studies on concordance in twin pairs provide a basis for analyzing the influence of genetic versus environmental factors. In 2007, using self-report of AIS from the Danish Twin Registry, concordance rates of 0.13 in monozygotic twins and 0.00 in dizygotic twins were reported. Methods. All 46,418 twins registered in the Danish Twin Registry born from 1931 to 1982 were sent a survey, which included questions about scoliosis. The survey was returned by 34,944 individuals (75.3%) representing 23,204 pairs. From the present study, 548 individuals representing 274 complete twin pairs in which at least one twin selfreported having scoliosis were invited to a clinical and radiological examination. Zygosity was established by genetic testing. Results. A total of 182 individuals (33.2%) of the original cohort agreed to participate, 128 of whom had scoliosis by selfreport. There were 91 twin pairs, in which one or both twins had reported scoliosis and 36 individuals whose twin did not want to participate. Only 35 (27%) of the 128 participants with self-reported scoliosis had a clinically and radiologically confirmed curve. Calculating concordance in twins with Cobb angles greater than 10°, we found that the pairwise concordance rate was 0.4 (0.10-0.70) for monozygotic and 0.05 (-0.05-0.15) for dizygotic twins, P = 0.05, probandwise concordance was 0.45 (0.16-0.74) for monozygotic and 0.1 (-0.03-0.23) for dizygotic pairs. Conclusion. Concordance rates in a Danish twin population using genetic testing for zygosity are higher than previously reported using self-report. Although not statistically significant, the differences in monozygotic and dizygotic pairs indicate that genetic factors may be of importance for scoliosis.