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Familial risks of anencephaly and spina bifida in British Columbia McBride, Mary Louise


The purpose of this study has been to assess the risk of central nervous system trial formations to sibs of individuals born in British Columbia with either anencephaly or spina bifida cystica, two related central nervous system defects. Risks of anencephaly, spina bifida cystica, and other central nervous system anomalies were derived, both for all sibs and for subsequent sibs of the index case. The sex, type of malformation, parity, and type of birth (livebirth or stillbirth) of the index case were all considered in the estimation of sibling risk. In addition, various hypotheses regarding the aetiology of these malformations were discussed. An attempt was made to fit a model for polygenic inheritance to the data and to estimate the degree of genetic determination in the causation of these defects in the province. Index cases of anencephaly and spina bifida cystica were ascertained from routinely-collected records on morbidity and mortality, obtained from the Division of Vital Statistics of the British Columbia Department of Health. All affected cases born in the province from 1952 to 1970 were considered probands. The family information was acquired using linked groupings of British Columbia marriage, birth and stillbirth records for the period from 1946 to 1970. The use of these records was intended to avoid biases in ascertainment of indes cases and to provide complete family information. The empiric risk of anencephaly or spina bifida cystica to all sibs of individuals born in British Columbia with either of these defects was 2.4%, about fifteen times the population incidence. The risk to subse-quent-sibs of the first affected individual in a family (2.1%) was not significantly different from the risk to all sibs. There was no difference in risk when the sex, type of malformation, parity, or type of birth of the proband were taken into account. Brothers and sisters of index cases had the same risk of either anencephaly or spina bifida cystica, and there were equal proportions of each defect among sibs. The risk of recurrence of either of these anomalies after two previously affected sibs was 4.8%, or approximately double the risk after one affected sib. No increased risk of any other central nervous system defect was observed in the families of the index cases. The sibling risk of anencephaly and spina bifida cystica in British Columbia is much lower than that reported elsewhere. Comparison of the results of this study with other family studies of anencephaly and spina bifida cystica suggest that-geographical differences in risk can be attributed largely to environmental factors in causation. The risk is, however, large enough to justify the continuation of amniocentesis service to mothers of children with anencephaly or spina bifida cystica. The linked family records available in British Columbia can be utilized further in order to study the sibling risks of recurrence of other congenital malformations in the province, in particular those with a higher frequency or those that present greater medical problems in the community.

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