Spina bifida and anencephaly: fewer patients, more problems.

Spina bifida and anencephaly: fewer patients, more problems Anencephaly and spina bifida aperta occur when the neural tube fails to develop normally and are the most striking of the common malformations. Ever since the pioneering studies of Record and McKeown' 2 it has been evident that the birth prevalence of these defects-the number of affected infants born, dead or alive, expressed as a proportion of all live births and stillbirths-varies widely and that these variations are broadly similar for the two defects. 4 Some of the variations for example, those observed with sex, family relationship , and colour-may be due to genetic factors. Others-notably differences in liability between first born and other children of the same families, and trends over space and time in populations of apparently similar genetic composition-are probably related to the prenatal environment. These genetic and environmental influences may produce their effects by affecting either the proportion of embryos which become malformed (incidence) or the proportion of malformed embryos which result in spontaneous abortion. Most of the effects that have been studied appear to reflect differences in incidence, though the available evidence is limited.4 Accordingly it is widely believed that many genes and environmental factors affect development of the neural tube. In each individual embryo it is suggested that these effects add up to a score, and that neural tube defects arise in those whose score exceeds a critical threshold.5 Three questions have recently emerged from the continued surveillance of the prevalence of defects of the neural tube and the search for specific environmental factors which might influence this prevalence. Firstly, is the recent decline in birth prevalence the result of improved antenatal screening and termination of pregnancy? Secondly, can sodium valproate cause spina bifida? Thirdly, what further tests are needed of the hypothesis that the intake of vitamins affects prevalence ? On the first point, prevalence began to decline in England and Wales after 1972. The national notification rate fell between 1972 and 1981 from 147 per thousand live births and stillbirths to 0 39 for anencephaly, and from 1-88 to 1 04 for spina bifida.6 7 Defects of the neural tube also seem to have become less common in Northern Ireland,8 the United States,9 10 and Australia,"1 12 though in these countries the reduction has been less striking than in England and Wales. The reduction has been greater for anencephaly than for spina bifida except …