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Bilateral Ventriculomegaly Between 10 and 14 mm
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What is this and what is its significance?
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On a cross-sectional scan using the ultrasound transducer it can be seen that the
brain is divided into two hemispheres. Within each hemisphere there is a ventricle
(cavity or chamber) that contains fluid that lines and protects the surrounding
soft brain tissue. The fluid in the ventricles is produced by spongy tissue, known
as the choroid plexus,
that appears to float in the ventricular cavity. Mild ventriculomegaly is defined
as dilation of the lateral ventricles of the brain that does not raise the intracranial
pressure and does not increase from one scan to the next. In the second and third
trimester, the normal width of the ventricle (including mild ventriculomegaly without
significant mental handicap) is up to 10 mm. Dilation up to 14 mm is considered
mild.
Dilation in excess of 14 mm is significant dilation, especially if there is an increase
in the ventricular width from week to week. See information sheet: " Ventriculomegaly
in excess of 14 mm and/or progressive ventriculomegaly - hydrocephalus".
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The common causes for this type of ventriculomegaly are:
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- Obstruction of the fluid passages out of the ventricles because of bleeding and
the formation of a clot in the drainage outlets. The fluid formed in the ventricles
then accumulates and results in ventriculomegaly with an increased head circumference.
- Defects in the structure of the brain arising from genetic causes that may or may
not be hereditary that disrupt the normal, complex development of the brain. In
cases in which further defects are detected in the brain in particular and in other
organs in general, there is an increased risk that the fetus may have a syndrome,
with a significantly increased risk of mental retardation.
- Environmental effects such as disrupted blood flow to the brain or infections (viral
or other pathogen) during pregnancy. The mother's blood can be examined to test
for infection by some of the common pathogens. If the cause is found to be a viral
infection that is known to cause ventriculomegaly, there is, of course, an increased
risk of fetal developmental retardation.
- Chromosome disorders. If the ventriculomegaly is an isolated finding, the risk of
an associated
chromosome disorder is between 2% and 5%. However, the presence of other
abnormal findings on ultrasound examination significantly raises the risk- according
to some reports to as high as 25%. In these cases it is worth recommending that
the mother undergo amniocentesis
for chromosome
analysis.
It is worth noting that apart from obstruction of the fluid passages, which usually
leads to a larger than average head circumference, the other causes of ventriculomegaly
tend to be associated with a decreased brain volume, and therefore a relatively
small head circumference.
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What are the risks associated with mild ventriculomegaly when no other defects are
found on ultrasound examination and no further expansion of the ventriculomegaly
is detected on monitoring?
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There is a risk for defects that affect the health of the fetus even when the ventriculomegaly
is the sole finding on ultrasound examination. The risk is estimated to be approximately
23%. The defects include:
- Defects that are undetectable by ultrasound examination in pregnancy but which may
be detected at birth: 8.5%.
- Chromosomal
problems: 4%. These can be detected by amniocentesis. In most cases in which
a chromosomal problem is detected, there are additional reasons for performing amniocentesis,
such as advanced maternal age
or increased risk for Down syndrome based on screening tests (biochemical screening
or increased nuchal translucency). It is open to question whether a 35-year-old
woman with normal screening test results that indicate a low risk for Down syndrome
should undergo amniocentesis based solely on the presence of mild ventriculomegaly.
It must be assumed that the risk for a chromosomal problem in this case is not very
high, but because there is no definitive answer to this question, there is apparently
an indication for amniocentesis that is not based solely on the presence of ventriculomegaly.
- Developmental delay 12%. The risk is increased by
the following criteria:
A. It is more likely in girls (23%) than in boys (4%).
B. It is more likely in ventriculomegaly in excess of 12 mm (14%) than of 10 - 12
mm (4%).
C. It is more likely when the ventriculomegaly is first detected in the second trimester
rather than late in the third.
It should be noted that follow-up studies of development in children born with mild
ventriculomegaly have not been undertaken for long enough to know what the long-term
consequences are. Therefore it is not known whether these children have learning
difficulties or milder developmental disorders. The estimate is that the frequency
of such disorders is slightly increased.
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What should be done if mild ventriculomegaly is diagnosed?
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- A thorough
ultrasound system scan should be carried out to look for additional defects
in the structure of the brain or other organs. It is also important to look for
defects in the genitalia, eyes, heart and growth rate. Normal fetal movement should
be ascertained. It is important to monitor the width of the ventricles, because
sometimes the ventriculomegaly is unilateral, in which case the prognosis is better.
It is also important to monitor the diameter of the head once every two to three
weeks to ascertain that it is not expanding more than the normal amount.
- Investigation for
intrauterine infection should be carried out by testing the mother's blood in
order to look specifically for the known teratogenic pathogens (agents that are
known to infect and harm the fetus) such as TORCH, varicella-zoster virus, Epstein-Barr virus, etc.
- Fetal echocardiography.
- Genetic counseling
should be offered in order to give the parents a thorough explanation about the
implications of the finding and to plan further follow-up such as whether it is
advisable to undergo amniocentesis to rule out a chromosome disorder in the fetus.
As noted above, when there is mild ventriculomegaly, there is a definite indication
to perform amniocentesis. Each case, however, must be considered individually, based
on how important the pregnancy is, the week of gestation, the couple's wishes, the
presence of other defects, the results of the screening tests for Down syndrome
(biochemical screening and nuchal translucency), etc. These are discussed within
genetic counseling. Based on the severity of the ventriculomegaly and the presence
or absence of additional findings or a family history, further follow-up and molecular
or other tests should be considered.
- In cases of mild or more severe ventriculomegaly, it is advisable to refer the parents
to a multidisciplinary clinic in a hospital for professional counseling by a pediatric
neurologist, a geneticist, an expert in ultrasound examinations and a neonatologist,
in order to collate all the data and discuss the various possibilities and courses
of treatment that are available, etc. The decisions taken will depend on the severity
of the problem, the presence of additional findings, etc.
- In the last few years, high-speed MRI machines have been developed that enable examination
of the fetal brain (this could not be done before because the fetus often moves
and the older MRI machines were not quick enough). Cumulative experience shows that
this test has a great advantage in identifying defects that cannot be detected by
ultrasound, and it is recommended in cases of mild or more severe ventriculomegaly,
especially when other defects are also present. The question of whether the result
of an MRI scan can be normal and there still be a high risk for a developmental
problem has been asked. It can be assumed that the risk in this case is lower than
that described above, but there are no definitive answers to this question.
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