Estimated rates of concordancy for the Sudden Infant Death Syndrome in twins.

Abstract

Spiers, P. S. (Epidemiology Branch, Office of Epidemiology and Biometry, National Institute of Child Health and Human Development, National Institutes of Health, Building 31. Room 2A10, 9000 Rockville Pike, Bethesda, Md. 20014). Estimated rates of concordancy for the sudden infant death syndrome in twins. Am J Epidemiol 100: 1–7, 1974.—A review of the literature concerning the occurrence of the Sudden Infant Death Syndrome (SIDS) in both members of twin pairs suggested that genetic factors were not of great importance in the etiology of the syndrome. Using 8967 collated twin pairs born in North Carolina over the period 1959–1967 as a data base, the concordancy rates and the dependency ratios (death rates for surviving twins vs. death rates for all members of particular twin subsets) were calculated for members of like-sexed and unlike-sexed twin pairs who died aged 7–364 days either at home or in the hospital. SIDS accounts for the majority of home deaths but cases occurring in hospitals are rare. It was predicted that among home deaths (excluding those of clearly traumatic origin) the concordancy rates for like- and unlike-sexed twin pairs would not be significantly different from each other. Concordancy rates of 8.6% and 7.6% for like- and unlike-sexed pairs, respectively, confirmed this prediction. However, this measure of concordancy is biased according to the degree of difference between the incidence of SIDS in males and females. Available data indicate that the incidence of home deaths in males is greater than that in females, and the bias can be overcome by comparing the concordancy rates in MM pairs, 7.8%, and MF pairs, 7.6%. The dependency ratios for MM pairs and MF pairs, 4.7 and 4.5, respectively, were also nearly equal, but the ratio for FF pairs was 8.8. In all there were 17 pairs of twins with both members dying at home, and in 14 instances the co-twin died within 30 days of the death of the first twin. An average dependency ratio of 5.5 suggests that the excessive risk of death for the surviving twin is limited to a period of about 1 month, and that the excessive dependency ratio is explained by a transient common experience. Due to the inability of this study to isolate precisely instances of SIDS and the relatively small numbers of deaths analyzed, these conclusions should be viewed as suggestive rather than definitive.