INTRODUCTION
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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ANTENATAL
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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POSTNATAL
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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WEST
MIDLANDS DATA
To be added.
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