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Neural Tube Defects - NTD
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What are these?
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Neural tube defects are defects in the closure of the spinal column or in the development
of the brain. They occur at a very early stage of embryonic development and are
a result of the failure of the neural canal to close properly.
- When this occurs in the fetus's head, it results in a reduced head size - the head
appears very small and undeveloped. In this condition, which is called anencephalus
and which is always fatal, the brain is only partially developed.
- When the defect occurs in the spinal column, a condition called spina bifida, the
spinal cord and the spinal nerves protrude. The degree of severity is determined
by the part of the spinal column that is affected and the number of defective vertebrae,
but in most cases this is a functional neurological disorder involving paralysis
of the lower limbs and sometimes urinary and fecal incontinence.
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What is the frequency of the defect and how is it caused?
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The frequency of this defect varies according to geographic location, it is estimated
to be about 1:1000 in Israel.
The risk of recurrence in a couple who has already had a pregnancy involving this
defect is 5%, and 10% if there have been two such pregnancies.
The inheritance pattern is multifactorial, meaning both hereditary and environmental
factors are involved.
Some years ago various international research studies found that one of the main
environmental causes of this defect is a maternal deficiency of the vitamin folic
acid.
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Can these defects be prevented?
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Giving the mother supplements of folic acid before and during the first weeks of
pregnancy reduces the risk of recurrence by 70% of the original risk. Hence women
who are at risk because they have had a pregnancy with an affected fetus are advised
to take one 5 mg folic acid tablet daily for at least 8 weeks before becoming pregnant,
and for the first 2 - 3 months of pregnancy. Even women in the general population
without a known increased risk are advised to take folic acid, but a high dosage
is not necessary. Here, 0.4 mg per day is sufficient, and it is best taken with
other multivitamins - see recommendations in "What can be done before pregnancy?"
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How are these defects diagnosed?
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These defects can be effectively diagnosed in a number of ways:
- Measuring the level of the marker
Alpha-Fetoprotein (AFP)
The level of the marker alpha-fetoprotein (AFP) in the mother's blood is measured
as part of the second trimester biochemical screening test. It is used in the evaluation
of the risk for Down syndrome and congenital defects, and a raised level is indicative
of a neural tube defect. The test is performed between the 16th and the 18th week
of gestation. In cases where the level is greater than 2.5 multiples of the median
for a singleton pregnancy or 2.0 multiples of the median for a twin pregnancy, an
open defect in the nervous system should be suspected, and possibly other defects
also. In this situation, the family should be offered genetic counseling in order to discuss what the
various possibilities are, and to decide what further investigations should be undertaken,
such as a directed ultrasound scan. In certain cases, it is advisable to measure
the AFP level in the
amniotic fluid.
It is important to note that in cases where the opening in the spinal column is
covered by skin, there is no increased secretion of alpha-fetoprotein from the fetus
into the amniotic fluid and the maternal blood, and only a thorough system scan
can diagnose this defect. A thorough
ultrasound scan
directed at the spinal column is not performed as a matter of routine in pregnancies
without special problems.
- Ultrasound scan
Anencephalus, which is the most severe type of neural tube defect, can be diagnosed
ultrasonically, usually by the 12th to 14th week of gestation. If the defect is
in the spinal column, the diagnosis is more difficult, but still possible. In most
cases, these defects can be diagnosed during the extended system scan performed
between the 21st and 22nd week.
- Alpha-fetoprotein level in amniotic
fluid
Women with a high level of alpha-fetoprotein in the maternal blood are recommended
to undergo amniocentesis (at the expense of the HMO) in order to determine the level
of this protein in the amniotic fluid. If the level is also elevated in the amniotic
fluid, the risk of a defect is relatively high. This includes neural tube defects
in the spinal column or brain, defects of the kidneys or the closure of the abdominal wall, diseases
affecting the integrity of the skin, etc. In order to differentiate between a defect
of the closure of the neural tube and other conditions, the amniotic fluid is sent
for testing for a substance called acetylcholine esterase. This is secreted into
the amniotic fluid only from the neural tube, so its presence indicates a defect
in the closure of the neural tube. Its absence leaves the other differential diagnoses
for elevated alpha-fetoprotein levels.
Ultrasound examination
has become more accurate and detailed today, so that it can be used effectively
to detect defects in the spinal column or brain. It can detect pathological conditions
in more than 99% of cases. This means that in the case of "precious" pregnancies,
especially when the maternal blood alpha-fetoprotein level is not markedly raised
(up to 3.00 multiples of the median), or when there is another reason for the elevated
level of AFP, such as placental problems or the loss of a fetus in a multi-fetal
pregnancy, it may be possible to avoid amniocentesis. This should be discussed with
the genetic counselor.
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