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Macrocephaly/Megalencephaly (Increased Head Circumference)
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What is this and what is its significance?
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A larger than average head circumference for the week of pregnancy (more than one
week's difference) is a relatively common finding and has no medical significance
if the fetus's head circumference is in the normal range, especially if one of the
parents has a greater than average head circumference (measured using a standard
tape measure). Even if the fetus's head circumference is above the upper limit of
the normal range, this is not exceptional or rare. However, it must be established
that the rate of growth of the head is constant, that there are no additional defects,
and that there is no enlargement of the lateral ventricles of the brain - in such
situations one of the parents almost always also has a larger head circumference.
An increased head circumference (above the normal range), or macrocephaly, is defined
as a head circumference two standard deviations greater than the mean circumference
for that week of pregnancy (without an increase in the size of the cerebral ventricles).
It occurs in approximately 3% of the normal population and is not usually associated
with specific medical problems, especially if one of the parents also has a large
head circumference and this parent is healthy and normal. It is known that the head
circumference is determined genetically and that there are many families whose members
have large head circumferences that are not associated with any other problems.
Rarely an increased head circumference may be associated with developmental delay and/or other problems such
as epilepsy.
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What should be done when macrocephaly is diagnosed?
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- A 3rd level ultrasound scan should be performed to look for additional defects,
intracranial tumors, etc., and also to determine the fetus's weight and to measure
the width of the cerebral ventricles. In cases where the cerebral ventricles are
also dilated (expanded), the situation is different and the significance of this
and the possible presence of hydrocephalus as well should be evaluated. In cases
where all the body dimensions of the fetus are large, the increased head circumference
is less significant, and the cause of the increased body dimensions should be investigated.
- The head circumferences of the parents and of the other children in the family should
be measured.
- If neither parent has a large head circumference (or one has, and this is accompanied
by neurological problems), then it is worth considering genetic counseling. This will enable the parents
to be given a full explanation about the significance of the condition and the possibility
that the fetus may have a genetic condition, and the advisability of performing
amniocentesis in cases where there is considerable enlargement of the fetal head
circumference can be discussed.
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A combination of two signs: short femur
and slightly increased head circumference
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The anomaly discussed in this section progresses throughout the pregnancy.
If on ultrasound examination during the first or second trimester the head circumference
is found to be greater and the length of the femur less than the number of the weeks
of gestation, and the difference between them is one and a half weeks or more, then
this requires further investigation to ensure that the difference does not increase,
as if it does, this could indicate a problem of bone development.
The vast majority of the cases where a difference has been found are not associated
with significant medical problems in the fetus, although such a combination may
indicate that the fetus has a bone disorder known as achondroplasia.
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Achondroplasia is the most common form of very short stature (the term "dwarfism"
is not used nowadays). In this disorder, the shortening of the femur and the enlargement
of the head circumference become obvious only in the late stages of pregnancy (week
34 and later), which is relatively late to undertake investigations and to act in
accordance with the results. A molecular test can be performed on amniotic fluid
cells in order to identify the mutation in the gene responsible for achondroplasia
as soon as the combination described above is found, immediately after the system
ultrasound scan, while the deviation from the normal is still small and non-diagnostic
(in approximately 99% of cases this test is found to be normal). However, these
cases should be monitored by performing regular ultrasound examinations, and if the difference
increases, amniocentesis can be performed.
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