Risk of recurrence after two children with central nervous system malformations in an area of high incidence.

This study was undertaken to provide an estimate of the recurrence risk of central nervous system (CNS) malformation in families with two or more affected children. The hospital records and the records of the Genetic Counseling Clinics were examined for instances where a child born between 1962 to 1973 had spina bifida and anencephalus and was documented as having an affected elder sib; such children were taken as index patients. Parents of 76 families with 89 index patients were interviewed. Fifty-one index patients had younger sibs; of 85 younger sibs, 12 had spina bifida and six anencephalus (1 in 4.7). Little variation was observed in the proportion of subsequent children affected with the type of lesion in the index patient and the affected older sib. Ten index patients with two older affected sibs had 14 subsequent sibs of whom four had a CNS malformation (1 in 3.5). For the purpose of genetic counselling it was suggested that the risk after two and three children with CNS malformations in Northern Ireland was 1 in 5 and 1 in 4, respectively.