Ventriculomegaly (mild dilation of the ventricles of the brain)
What is this and what is its significance?
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
Ventriculomegaly (unilateral or bilateral) of up to 9 - 10 mm is not rare, and if
it is an isolated finding, it is not associated with a significant increase in the
risk for mental retardation. However, it is necessary
to establish that:
Ventriculomegaly of up to 9 - 10 mm is associated with an increased statistical
risk of chromosome disorders,
and based on the results of a number of studies, with albeit only a small number
of pregnancies, this risk has been estimated to be approximately 1% - 5%. Because
of this the genetic counselor may consider that it is worth recommending amniocentesis, taking into account the week of gestation,
other test results, etc.
- There are no other defects in the brain or elsewhere.
- No increase in the width of the ventricles beyond 9 - 10 mm is found on follow-up
within 2 - 3 weeks.
- The fetuss head circumference is normal and corresponds with that of each of the
parents, since a
small head circumference in the presence of ventriculomegaly can indicate
a small brain volume.
- There is normal fetal motility. The width of the ventricles must be monitored by
ultrasound in order to rule out further, pathological dilation. It is, of course,
also necessary to rule out intrauterine viral infection in the fetus as a possible
cause of ventriculomegaly.
Ventriculomegaly is defined as mild when the width of the ventricles (unilateral
or bilateral) is between 10 and 14 mm. The implications for the fetuss mental functioning
are determined by the cause of the ventriculomegaly and by its severity and rate
of progress. The most common causes of this type of ventriculomegaly are:
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.
- Blockage 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 mothers 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.
Risk of neurological problems in cases of mild ventriculomegaly (over 10 mm)
Even when ventriculomegaly is an isolated finding, there is nevertheless a risk
of mental retardation and neurological problems. There are reports in the literature
that the risk of neurological disorders, including mild ones such as learning problems,
fine motor problems, etc., is approximately 20% - 30% in these children; however
most of these disorders are not severe.
Obviously, there is a correlation with the extent of dilation. A study that divided
patients with ventriculomegaly into two groups - those whose ventricles were between
11 and 12 mm in width and those whose ventricles were between 12.5 and 15 mm in
width - showed that there was a significant difference between the two groups. The
risk of retardation in the group with dilation up to 12 mm was only 3%, whereas
the risk in the second group was significantly higher.
What should be done if mild ventriculomegaly is diagnosed?
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.
- 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 monitor the
width of the ventricles every two to three weeks in order to ascertain that it is
- Investigation for
intrauterine infection should be carried out by testing the mothers 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
- 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.
These decisions should be based on the severity of the ventriculomegaly and the
presence of additional findings or a family history.
- 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.
This is an extreme state of ventriculomegaly with increased pressure on the brain
tissue itself. Here there is a progressive dilation of the ventricles from week
to week. It is caused by an obstruction in the canals or apertures of the brain,
which results in increased accumulation of fluid in the ventricles. Usually this
appears in the third trimester of pregnancy.
In some cases it is caused by bleeding and the formation of a clot that obstructs
the outlets of the ventricles, and in other cases by a genetic defect that is more
common in males. If other defects are also present, the cause may be genetic and
more complex, such as a chromosome disorder.
This condition can have major implications for the mental functioning of the fetus,
depending on the severity, the presence of additional defects, the cause of the
hydrocephalus, gestational age, and the rate of progression of the dilation of the
In cases of hydrocephalus, 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, etc., as
well as whether any further tests, such as MRI and special
DNA tests, should be carried out.