The term monosomy is used to describe the absence of one member of a pair of chromosomes. Therefore, there is a total of 45 chromosomes in each cell of the body, rather than 46. 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 be said to have "monosomy X." Monosomy X is also known as Turner syndrome.
Turner syndrome is a genetic disorder that occurs in girls. It causes them to be shorter than others and to not undergo normal puberty as they grow into adulthood. The severity of these problems varies among affected individuals. Other health problems may also be present involving the heart or renal system (for example, kidneys). Many of the health problems affecting girls with Turner syndrome can be managed or corrected with appropriate medical treatment. Turner syndrome occurs in approximately 1 in 2,000 to 2,500 females born.
The name "Turner syndrome" comes from the doctor, Dr. Henry Turner, who first described the collection of findings in 1938. It was not until 1959 that the cause of Turner syndrome (having only a single X chromosome) was identified.
Normally in reproduction, 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 undergo a cell division where the 46 chromosomes are divided in half and the egg and the sperm cells end up with 23 chromosomes each. When a sperm with 23 chromosomes fertilizes an egg with 23 chromosomes, the baby ends up with a complete matched set of 46 chromosomes, half from the biological father and half from the biological mother.
Sometimes, an error occurs when an egg or sperm cell is forming, causing it to have a missing sex chromosome. When this cell fails to contribute a sex chromosome to the embryo, so that there is only one X sex chromosome in the fetus, then Turner syndrome results. Having a single copy of a particular chromosome, rather than the usual pair, is called "monosomy." Turner syndrome is also known as "monosomy X." The missing sex chromosome error can occur in either the mother's egg cell or the father's sperm cell; however, it is usually an error that occurred when the father's sperm cell was forming. There is nothing known that a father could have done or not done that would have caused or prevented the sperm from having a missing sex chromosome. (The chance for Turner syndrome to occur is, therefore, not associated with the increasing age of the mother). The features of Turner syndrome result from having a missing X chromosome in each of the body's cells.
About half of all instances of Turner syndrome result from full monosomy X. Others have a mosaic pattern (two or more chromosome patterns in the cells). A small percentage of instances of Turner syndrome result from having the normal number of chromosomes (46 total), but missing a portion of the X chromosome. When only part of an X chromosome is missing (called a deletion), not all of it, girls with Turner syndrome usually have milder features of the syndrome. The features of Turner syndrome that are present depend on which part of the X chromosome they are missing.
About half of girls born with Turner syndrome will have puffy hands and feet at birth, in addition to wideness and webbing of the neck. 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 and they often go away prior to birth; but, in some cases, are still present in the newborn period.
Girls with Turner syndrome usually have a low hairline at the back of the neck, minor differences in the shape and position of the ears, a broad chest with widely spaced nipples, an increased number of small brown moles (nevi) on the skin, and deep-set nails. Short stature 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 absent ovaries. Ovaries produce estrogen and without it, incomplete sexual development occurs. The usual signs of puberty—breast development, menstruation, and growth of pubic and axillary hair—do not occur without hormone therapy. The resulting infertility is not correctable, in most cases. Heart problems, kidney problems, and thyroid problems are also common in Turner syndrome and should be evaluated early on. About one in ten girls with Turner syndrome is born with coarctation of the aorta (constriction of the main artery leaving the heart), which sometimes needs to be corrected surgically. Other features which have been seen in Turner syndrome include feeding problems during infancy, middle ear infections, skeletal problems, and "cubitus valgus." Cubitus valgus basically describes a situation where a person standing with their arms at their sides, will have their elbows slightly bent. They are unable to keep their arms perfectly straight at their sides. Other reported medical findings in Turner syndrome include diabetes, dry skin, high blood pressure, a small jaw, and a narrow high-arched palate (the 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, and may have problems in the areas of spatial perception and an increased incidence of specific learning disabilities.
When a girl is born with features suggestive of Turner syndrome, a small blood sample is usually taken and the chromosomes are analyzed to determine the absence of a sex chromosome. Sometimes, girls with Turner syndrome do not exhibit any problems as infants or children, and it is only when they fail to go through puberty that a doctor begins to suspect that Turner syndrome may be present. Again, not all girls with Turner syndrome exhibit all features described here. There is great variability. Some girls with Turner syndrome are diagnosed during the newborn period, while others are diagnosed during childhood and in their late teens.
Chromosomal abnormalities, such as Turner syndrome, can often be diagnosed before birth by analyzing cells in the amniotic fluid or from the placenta. Fetal ultrasound during pregnancy can also give information about the possibility of Turner syndrome, but ultrasound is not 100 percent accurate, and many babies with Turner syndrome may look the same on ultrasound as those without Turner syndrome. A chromosome analysis, whether performed on a blood sample, cells from the amniotic fluid, or placenta, is very accurate.
In general, in each subsequent pregnancy, the chance of having another baby with Turner syndrome would not be increased over the general population risk that affects every woman.
After birth, the doctor usually takes a blood sample from a baby suspected of having Turner syndrome to perform a chromosomal analysis (called a karyotype). This confirms the physical findings of Turner syndrome and determines the underlying chromosomal abnormality. Your doctor may explain the results of the test to you or refer you to a genetic doctor or genetic counselor who can explain the results of chromosomal tests, as well as what tests are available to diagnose chromosome problems before a baby is born.
There is no cure for Turner syndrome; however, many of the more serious problems can be treated. For example, growth hormone and androgen therapy can be given to increase final adult height; hormone replacement therapy can be given so that girls will develop secondary sexual characteristics; coarctation (narrowing to an abnormal width) of the aorta can be surgically corrected if needed; and medications are available to treat high blood pressure, diabetes, and thyroid problems. Women with Turner syndrome can give birth to children, if desired, through the use of egg donation.