What are monosomies?
Most people have 46 chromosomes in their cells. Monosomy means that a person is missing
one member of a pair of chromosomes. Instead of 46, the person has only 45 chromosomes.
For example, if a baby is born with only one X sex chromosome, rather than the usual
pair (either two X's or one X and one Y sex chromosome), the baby would have monosomy
X. Monosomy X is also known as Turner syndrome.
What is Turner syndrome?
Turner syndrome is a genetic disorder that occurs in girls. It causes them to be shorter
than others. They also don’t go through normal puberty as they grow into adulthood.
The seriousness of these problems varies from girl to girl. A child with Turner syndrome
may also have other health problems. These include problems of the heart or kidneys.
Many of the health problems affecting girls with Turner syndrome can be managed or
fixed with treatment. Turner syndrome occurs in about 1 in 2,000 to 2,500 girl babies.
What causes Turner syndrome?
When a baby is conceived, the egg cell of the mother and the sperm cell of the father
start out with the usual number of 46 chromosomes. The egg and sperm cells both divide,
so that the chromosome number is half of 46. The egg and the sperm cells then have
23 chromosomes each. When a sperm with 23 chromosomes fertilizes an egg with 23 chromosomes,
the baby will have a complete set of 46 chromosomes. Half are from the father and
half are from the mother.
Sometimes an error occurs when an egg or sperm cell is forming, causing it to have
a missing sex chromosome. When this cell can’t give a sex chromosome to the embryo,
the embryo ends up with only one X sex chromosome. This causes Turner syndrome.
The missing sex chromosome error can occur in either the mother's egg cell or the
father's sperm cell. But it is usually an error that happened randomly when the father's
sperm cell was forming.
About half of all cases of Turner syndrome happen because of full monosomy X. Others
have a mosaic pattern. This means the body’s cells have 2 or more chromosome patterns.
A small portion of cases result from having the normal number of 46 chromosomes, but
with part of the X chromosome missing. When only part of an X chromosome is missing
(deletion), a girl with the syndrome will usually have milder features. The features
of Turner syndrome that are present depend on which part of the X chromosome is missing.
What are the signs and symptoms of Turner syndrome?
About half of girls born with Turner syndrome will have puffy hands and feet at birth.
They will also have a wide neck with webbing. During a pregnancy, the doctor may have
seen a structure called a cystic hygroma during fetal ultrasound. Cystic hygromas
are fluid-filled sacs at the base of the neck. They often go away before birth. But
sometimes the sac is there when the baby is born.
Girls with Turner syndrome usually have:
A low hairline at the back of the neck
Small differences in the shape and position of the ears
Broad chest with widely spaced nipples
More small brown moles (nevi) on the skin than normal
Being shorter than normal is the most visible feature of Turner syndrome. The average
adult height of a woman with Turner syndrome is 4 feet, 8 inches.
Most women with Turner syndrome are born with poorly formed or missing ovaries. Ovaries
make estrogen, and without it, a girl with Turner syndrome will not develop sexually.
The usual signs of puberty—breast development, menstruation, and growth of pubic hair
and hair elsewhere on the body—don’t happen unless the girl is given hormone therapy.
A girl with the syndrome will usually be infertile as an adult.
Other common problems with Turner syndrome affect the heart, kidney, and thyroid.
About 1 in 10 girls with Turner syndrome is born with coarctation of the aorta. This
means the main artery that leaves the heart is narrowed. This problem sometimes needs
to be corrected surgically.
Other problems seen in Turner syndrome include:
Feeding problems during infancy
Middle ear infections
Cubitus valgus. This means that when a girl stands with her arms at her side, her
elbows will be slightly bent. She can’t keep her arms perfectly straight at her side.
High blood pressure
Narrow top of the inside of the mouth
Girls with Turner syndrome generally have normal intelligence. They tend to score
higher on their verbal IQ than their nonverbal IQ. They may have problems in the areas
of spatial perception. They may also have certain learning disabilities.
How is Turner syndrome diagnosed?
If the healthcare provider thinks that your newborn may have Turner syndrome, he or
she will usually take a small sample of your baby’s blood. The lab will look at the
sample to see if the sex chromosome is missing.
Sometimes a girl with Turner syndrome doesn’t have any problems as an infant or child.
It’s only when she doesn’t go through puberty or is shorter than her peers that her
doctor may suspect Turner syndrome.
Chromosome problems such as Turner syndrome can often be diagnosed before birth. This
is done by looking at cells in the amniotic fluid or from the placenta. This can also
be done by looking at the amount of fetal DNA in the mother's blood (noninvasive prenatal
Fetal ultrasound during pregnancy can also show the possibility of Turner syndrome.
But ultrasound is not 100% accurate. Many babies with Turner syndrome may look the
same on ultrasound as those without Turner syndrome.
A chromosome analysis done on a blood sample or on cells from the amniotic fluid or
placenta is very accurate.
What is the risk of having another child with Turner syndrome?
In general, a woman who has a child with Turner syndrome is not at increased risk
of having another baby with the condition.
After birth, the doctor usually takes a blood sample from a baby suspected of having
Turner syndrome. The lab does a chromosome analysis (karyotype). This confirms the
physical findings of Turner syndrome and shows the chromosome problem. Your doctor
may explain the results of the test to you. Or your doctor may refer you to a genetic doctor
or genetic counselor. This expert can explain the results of chromosome tests, as
well as tests available to diagnose chromosome problems before a baby is born.
Can Turner syndrome be cured or prevented?
There is no cure for Turner syndrome. But many of the more serious problems can be
treated. For example, growth hormone and androgen therapy can increase the final adult
height of a girl with the syndrome. A girl can also take hormone therapy to develop
secondary sex traits. Surgery can fix coarctation of the aorta, if needed. And medicines
are available to treat high blood pressure, diabetes, and thyroid problems. A woman
with Turner syndrome can have children by using donor eggs.