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CAR: Anomalies - CNS
Neural tube defects

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    Neural tube
      Spina bifida


Introduction Antenatal Postnatal West Midlands Data


Neural tube defects are the second most common congenital malformation after congenital heart defects and include anencephaly, open spina bifida, and encephalocele. Neural tube defects result from disruption in closure of the neural tube and/or vertebral arch development. The most severe defects are likely to be lethal early in fetal life and will end in miscarriage, often without being recognised as fetal anomalies. The spine and brain form early in fetal life, at 16 to 18 days after fertilisation as the cells along the back of the embryo develop into a groove to create the neural tube. The edges of this groove then join to form a hollow tube during the next 7 to 10 days. The edges of the tube join in the middle of the embryo; closure continues towards the bottom of the "back", and up towards the top of the "head".

The aetiology of NTDs is heterogeneous. Several risk factors have been correlated with increased NTD incidence including maternal insulin dependent diabetes, hyperthermia, and obesity at conception. Some NTDs occur in association with autosomal trisomies however, 90% of cases have unknown aetiology. Mothers with a previously affected pregnancy or a positive family history are at high risk. Multiparous women of high maternal age also have an elevated risk of NTDs.

Mothers with low serum folate concentrations have a high-risk of NTDs. A low serum folate concentration may result from either from a poor dietary intake or a disruption in the folate metabolism by the use of anti-epileptic drugs (valproic acid) or a genetic methylenetetrahydrofolate reductase deficiency..

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A marker for open NTDs is raised alphafetoprotein (AFP) in the amniotic fluid and maternal serum. The fetal skin usually prevents blood protein leaking from the fetus into the liquor. The measurement of maternal serum AFP forms the basis of prenatal serum screening performed at 16 to 18 weeks gestation. Depending on the cut-off levels used, serum screening has a detection rate of up to 85% for open spina bifida. It is less effective for encephalocele, which is usually covered by skin. An ultrasound scan is required to confirm a positive serum screening result. In a prenatal ultrasound examination, the fetal head and spine are visualised. This technique, when performed at 18 to 20 weeks gestation, has a high sensitivity for all NTDs.

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Many neural tube defect cases will be correctly identified by prenatal diagnosis and the pregnancy terminated but inevitably some affected children will be born, either because of missed diagnosis or parental choice. Paediatric care of such cases is often complex and difficult, involving careful assessment, attempts at prediction of the likely outcome and intervention in some circumstances.

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To be added.

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© Perinatal Institute 2011