Turner syndrome

Turner syndrome encompasses a number of chromosomal abnormalities, of which monosomy X, is the most common. It occurs in 1 out of every 2,500 female births. Instead of the normal XX sex chromosomes for a female, only one X chromosome is present and fully functional. This is called 45,X or 45,X0, although other genetic variants occur. In Turner syndrome, female sexual characteristics are present but underdeveloped.

Symptoms

Common symptoms of Turner syndrome include:

Short stature
Lymphoedema (swelling) of the hands and feet
Broad chest (shield chest) and widely-spaced nipples
Low hairline
Low-set ears
Reproductive sterility
Amenorrhea

Other symptoms include a small lower jaw, cubitus valgus (turned-out elbows), a webbed neck, and soft, upturned nails. Less common are pigmented moles, schizophrenia, hearing loss, and a high-arch palate. Turner syndrome manifests itself differently in each female affected by the condition, and no two individuals will share the same symptoms.

Genetics

Turner syndrome is characterized by the presence of a single complete functioning sex-determining chromosome, the X chromosome, instead of two, either XX or XY. This chromosomal arrangement normally occurs in four ways.

During meiosis in either parent, a nondisjunction event can occur that leaves the gamete, either oocyte or spermatocyte with neither X nor Y chromosome. When this gamete combines with a gamete from the other parent (with a normal X chromosome), the embryo lacks the normal two chromosomes. Normally, humans have 46 chromosomes, so this leaves the embryo with 45 chromosomes and a single X chromosome, denoted 45,X (or, sometimes 45,XO, where the "O" is used as a placeholder). This is found in 50% of individuals with Turner syndrome.

During development leading up to gametogenesis, an X chromosome loses a portion of the smaller arm, with the end of the long arm wrapping around — creating a ring-like structure. The chromosome is missing the short arm of the X chromosome. When combined with a normal X chromosome in an embryo, the individual has just a single copy of the information on the short arm of the X chromosome. This is denoted as 46, XrXp-, where rXp- means a ring chromosome missing the small (p) arm of the chromosome. Other chromosomal variants have two long arms of the X chromosomes attached, and are called isochromosomes. Variants of chromosomal structure occur in 30% of individuals with Turner syndrome.

During an early cell division of the zygote, a nondisjunction event occurs and creates a cell line that is missing the second X chromosome. In this case, individuals most likely have a mixture of 46,XX cells and 45,X cells. It is classified this as a mosaic. The exact mixture of those two cell types depends on when the nondisjunction occurred. It is possible for a very early nondisjunction from a 46,XY zygote to be completely 45,X (and are indistinguishable from the first classification). If the nondisjunction occurs later, a 45,X/46,XY individual will develop as a male, without Turner syndrome. Mosaicism is found in about 20% of individuals with Turner syndrome.

Very rarely, the embryo has a normal X chromosome and a portion of the Y chromosome. In these cases, the Y chromosome does not have a functional SRY (and so develops as a female), the diagnosis is gonadal dysgenesis (see OMIM entry for SRY). It is possible that some Turner syndrome diagnosis is due to gonadal dysgenesis, particularly when it is caused by a large deletion of the Y chromosome.

There is no equivalent syndrome which results in a Y chromosome with no X, as such a condition is incompatible with life. Because an embryo with Turner syndrome doesn't have a Y chromosome (or, doesn't have a functional SRY on the Y chromosome), it will move along the path to female development.

Risk factors for Turner syndrome are not well known. Nondisjunctions increase with maternal age, such as for Down syndrome, but that effect is not clear for Turner syndrome.

Incidence

Approximately 98% of all fetuses with Turner syndrome spontaneously abort. Fetuses with Turner syndrome make up about 10% of the total number of spontaneously aborted fetuses in the United States. The incidence of Turner syndrome in live births is now believed to be 1 in 2000.

History

The syndrome is named after Henry Turner, an Oklahoma endocrinologist, who described it in the 1940s. In Europe, it is often called Ullrich-Turner syndrome or even Bonnevie-Ulrich-Turner syndrome to acknowledge that earlier cases had also been described by European doctors.

Diagnosis

Turner syndrome may be diagnosed by an amniocentesis during pregnancy. Sometimes, fetuses with Turner syndrome are identified by abnormal ultrasound findings (i.e. heart defect, kidney abnormality, cystic hygroma, ascities). Although the recurrence risk is not increased, genetic counseling is often recommended for families who have had a pregnancy or child with Turner syndrome.

A blood test, called a karyotype, analyzes the chromosomal composition of the individual. This is the most commonly used blood test to diagnose Turner syndrome.

Medical consequences of Turner syndrome

While most of the symptoms of Turner syndrome are harmless, some can lead to significant medical problems.

Cardiovascular

Price et al (1986 study of 156 female patients with Turner syndrome) showed a significantly greater number of deaths from diseases of the circulatory system than expected, half of them due to congenital heart disease. When patients with congenital heart disease were omitted from the sample of the study, the mortality from circulatory disorders was not significantly increased.

Cardiovascular malformations are a serious concern as it is the most common cause of death in adults with Turner syndrome. It takes an important part in the 3-fold increase in overall mortality and the reduced life expectancy (up to 13 years) associated with Turner syndrome.

Often associated with webbing of the neck and coarctation of the aorta.

Prevalence of cardiovascular malformations

The prevalence of cardiovascular malformations among patients with Turner syndrome ranges from 17% (Landin-Wilhelmsen et al, 2001) to 45% (Dawson-Falk et al, 1992).

The variations found in the different studies are mainly attributable to variations in non-invasive methods used for screening and the types of lesions that they can characterize (Ho et al, 2004). However Sybert, 1998 suggests that it could be simply attributable to the small number of subjects in most studies.

Different karyotypes may have differing prevalence of cardiovascular malformations. Two studies found a prevalence of cardiovascular malformations of 30% (Mazanti et al, 1998 …594 patients with Turner syndrome) and 38% (Gotzsche et al, 1994 …393 patients with Turner syndrome) in a group of pure 45X monosomy. But considering other karyotype groups, they reported a prevalence of 24.3% (Mazanti et al, 1998) and 11% (Gotzsche et al, 1994) in patients with mosaic X monosomy , and Mazanti et al, 1998 found a prevalence of 11% in patients with X chromosomal structural abnormalities.

The higher prevalence in the group of pure 45X monosomy is primarily due to a significant difference in the prevalence of aortic valve abnormalities and aortic coarctation, the two most common cardiovascular malformations.

Though according to Sybert, 1998 there is inadequate data to allow conclusions for regarding phenotype-karyotype correlations in regard to cardiovascular malformations in Turner syndrome because the number of individuals with the less common karyotype groups that has been studied is too small. Other studies also suggest the presence of hidden mosaicisms that are not diagnosed on usual karyotypic analyses in some patients with 45X karyotype.

In conclusion the associations between karyotype and phenotypic characteristics, including cardiovascular malformations, remain questionable.

Congenital heart disease

The most commonly observed are congenital obstructive lesions of the left side of the heart, leading to reduced flow on this side of the heart. This mainly includes bicuspid aortic valve and coarctation of the aorta. Sybert, 1998 found that more than 50% of the cardiovascular malformations observed in her study were bicuspid aortic valve or coarctation of the aorta alone or in combination.

Other congenital cardiovascular malformations such partial anomalous venous drainage and aortic stenosis or regurgitation are also more common in Turner syndrome compared with the general population. Hypoplastic left heart syndrome represents the most severe reduction in left-sided structures. Though rare in the population at large, this feature is quite common in those with Turner syndrome.

Bicuspid aortic valve. Up to 15% of adults with Turner syndrome have bicuspid aortic valves, meaning that there are only two, instead of three, parts to the valves in the main blood vessel leading from the heart. Because bicuspid valves are capable of regulating blood flow properly, this condition may go undetected without regular screening. Because bicuspid valves must work harder than ordinary heart valves, they are more likely to deteriorate and later fail.

With a prevalence from 12,5% (Mazanti et al, 1998) to 17,5% (Dawson-Falk et al, 1992), it is the most common congenital malformation affecting the heart. Hypotheses have been proposed to explain this abnormal aortic valve but the pathogenesis remains unknown. It is usually isolated but it may be seen in combination with other anomalies, particularly coarctation of the aorta (a quite common association). As this malformation calcifies it may lead to a progressive valvular dysfunction as evidenced by aortic stenosis or regurgitation (Elsheikh and al, 2002).

Coarctation of the aorta. Between 5% and 10% of those born with Turner syndrome have coarctation of the aorta, a condition which if untreated causes blood to flow in a thin, fast stream against the side of the heart. This can lead to heart rupture (see below). Coarctation of the aorta can be corrected surgically as soon as it is detected.

Coarctation of the aorta consists in a congenital narrowing of the descending aorta, usually just distal to the origin of the left subclavian artery and opposite to the duct (and so termed “juxtaductal”). While it's more common in boys, estimates of the prevalence of this malformation in patients with Turner syndrome ranges from 6,9% (Mazanti et al, 1998) to 12,5% (Dawson-Falk et al, 1992). A coarctation of the aorta in a female might be quite suggestive of Turner syndrome. It has to be noted the important relative risk of 160 found by the Mazanti et al, 1998 study.

Partial anomalous venous drainage. This abnormality is quite a rare congenital heart disease in the general population. The prevalence of this abnormality is quite low (around 2,9%) in Turner syndrome. But it is interesting to note that with a relative risk of 320 it has the highest one, in comparison with the general population. Strangely, Turner syndrome seems to be associated with quite unusual forms of partial anomalous venous drainage. (Mazanti et al, 1998 and Prandstraller et al, 1999)

For the management of a patient with Turner syndrome it is essential to keep in mind that all these left-sided cardiovascular malformations often found in Turner syndrome result in an increased susceptibility to infective endocarditis. Therefore prophylactic antibiotics should be considered when procedures with high risk of infection need to be performed (Elsheikh et al, 2002).

Turner syndrome is often associated with persistent hypertension, sometimes even in childhood. In the majority of Turner syndrome patients with hypertension, there is no specific cause. In the remainder, it is usually associated with heart or kidney abnormalities.

Aortic dilation, dissection and rupture

Two studies had suggested aortic dilatation in Turner syndrome, typically involving the root of the ascending aorta and occasionally extending through the aortic arch to the descending aorta, or at the site of previous coarctation of the aorta repair (Lin et al, 1998).

Firstly Allen et al, 1986 who evaluated 28 girls with Turner syndrome found a significantly greater mean aortic root diameter in patients with Turner syndrome than that of the control group (matched for body surface area). Nonetheless it is important to add that the aortic root diameter found in Turner syndrome patients were still well within the limits.
This has been confirmed by the study of Dawson-Falk et al, 1992 who evaluated 40 patients with Turner syndrome. They presented basically the same findings, that means a greater mean aortic root diameter which however remains in the normal range for body surface area.

Sybert, 1998 points out the fact that it however remains unproven that aortic root diameters, relatively large for body surface area but still well within normal limits, imply a risk for progressive dilatation.

Prevalence: The prevalence of aortic root dilatation ranges from 8,8% (Lin et al, 1986) to 42% (Elsheikh et al, 2001) in patients with Turner syndrome.
Even if not every aortic root dilatation necessarily goes on to dissect, complications such as dissection (circumferential or transverse tear of the intima), rupture and possibly death often occur. Indeed even if the natural history of aortic root dilatation is still unknown, the risk of aortic dissection or rupture could be as high as 60% according to some studies.
Aortic dissection affects 1% to 2% of patients with Turner syndrome.
As a result any aortic root dilatation should be seriously taken into account as it could become a fatal aortic abnormality. Routine surveillance is highly recommended (Elsheikh et al, 2002).

Risk factors: It is well established that cardiovascular malformations (typically bicuspid aortic valve, coarctation of the aorta and some other left-sided cardiac malformations) and hypertension predispose to aortic dilatation and dissection in the general population.
At the same time it has been shown that these risk factors are highly present in Turner syndrome. Indeed these same risk factors are found in more than 90% of patients with Turner syndrome who develop aortic dilatation. Only a small number of patients (around 10%) have no apparent predisposing risk factors.
It is important to note that the risk of hypertension is increased 3-fold in patients with Turner syndrome. Because of its relation to aortic dissection blood pressure needs to be regularly monitored and hypertension should be treated aggressively with an aim to keep blood pressure below 140/80 mmHg.
It has to be noted that as with the other cardiovascular malformations, complications of aortic dilatation is commonly associated with 45X karyotype (Elsheikh et al, 2002).

Pathogenesis: The exact role that all these risk factors play in the process leading to such fatal complications is still quite unclear.
Aortic root dilatation is thought to be due to a mesenchymal defect as pathological evidence of cystic medial necrosis has been found by several studies. The association between a similar defect and aortic dilatation is well established in such conditions such as Marfan Syndrome. Also, abnormalities in other mesenchymal tissues (bone matrix and lymphatic vessels) suggests a similar primary mesenchymal defect in patients with Turner syndrome (Lin et al, 1986).
However there is no evidence to suggest that patients with Turner syndrome have a significantly higher risk of aortic dilatation and dissection in absence of predisposing factors. So the risk of aortic dissection in Turner syndrome appears to be a consequence of structural cardiovascular malformations and hemodynamic risk factors rather than a reflection of an inherent abnormality in connective tissue (Sybert, 1998).
As no studies have been able to clearly expose the natural history of aortic root dilatation and because of its lethal potential, this aortic abnormality needs to be carefully followed-up.

Pregnancy: As more women with Turner syndrome complete pregnancy thanks to the new modern techniques to treat infertility, it has to be noted that pregnancy may be at risk for cardiovascular complications.
Indeed several studies had suggested an increased risk for aortic dissection in pregnancy (Lin et al, 1998). Three deaths have even been reported. The influence of estrogen has been examined but remains unclear. It seems that the high risk of aortic dissection during pregnancy in women with Turner syndrome may be due to the increased hemodynamic load rather than the high oestrogen rate (Elsheikh et al, 2002).
Of course these findings are important and need to be remembered while following a pregnant patient with Turner syndrome.

Cardiovascular malformations in Turner syndrome are also very serious, not only because of their high prevalence in that particular population but mainly because of their high lethal potential and their great implication in the increased mortality found in patients with Turner syndrome. Congenital heart disease needs to be explored in every female newly diagnosed with Turner syndrome. As adults are concerned closed surveillance of blood pressure is highly needed to avoid a higher risk of fatal complications in aortic root dilatation.

A greater knowledge of the natural history and pathogenesis is needed in order to propose a better management and prevention of these malformations. It would therefore be interesting to examine the different causes mentioned by some studies in order to explain why patients with Turner syndrome are more susceptible to cardiovascular malformations that the general population.

Skeletal

Normal skeletal development is inhibited due to a large variety of factors, mostly hormonal. The head, neck, and chest of women with Turner syndrome are usually of normal size, but the arms and legs are unusually short. The average height of a woman with Turner syndrome is 4'7", or about 140cm.

The fourth metacarpal bone (fourth toe and ring finger) may be unusually short.

Due to inadequate circulation of estrogen, many of those with Turner syndrome develop osteoporosis. This can decrease height further, as well as exacerbate the curvature of the spine, possibly leading to scoliosis. It is also associated with an increased risk of bone fractures.

Kidney

Approximately one-third of all women with Turner syndrome have one of three kidney abnormalities:

A single, horseshoe-shaped kidney on one side of the body, and no kidney on the other side.
An abnormal urine-collecting system.
Poor blood flow to the kidneys.

Some of these conditions can be corrected surgically. Even with these abnormalities, the kidneys of most women with Turner syndrome function normally. However, as noted above, kidney problems may be associated with hypertension.

Thyroid

Approximately one-third of all women with Turner syndrome have a thyroid disorder. Usually it is hypothyroidism, specifically Hashimoto's thyroiditis. If detected, it can be easily treated with thyroid hormone supplements.

Diabetes

Women with Turner syndrome are at a moderately increased risk of developing type 1 diabetes in childhood and a substantially increased risk of developing type 2 diabetes by adult years.

Cognitive

Turner syndrome does not cause mental retardation or impair cognition. However, women with Turner syndrome may have a specific difficulty perceiving spatial relationships. This may also manifest itself as a difficulty with motor control or with mathematics. While it is non-correctable, in most cases it does not cause difficulty in daily living.

Reproductive

Women with Turner syndrome are almost universally infertile. While some women with Turner syndrome have successfully become pregnant and carried their pregnancies to term, this is very rare and is generally limited to those women whose karyotypes are not 45X0. Even when such pregnancies do occur, there is a higher than average risk of miscarriage or birth defects. Some women with Turner syndrome who are unable to conceive without medical intervention may be able to benefit from the use of IVF or other fertility treatments.

Usually hormone replacement is used to spur growth of secondary sexual characteristics at the time when puberty should onset. This generally induces menstruation, though the vast majority of women with Turner syndrome will still be infertile.

Treatment

As a chromosomal condition, there is no "cure" for Turner syndrome. However much can be done to minimize the symptoms. For example:

Growth hormone, either alone or with a low dose of androgen, will improve growth and probably final adult height. Growth hormone is approved by the Food and Drug Administration for treatment of Turner syndrome and is covered by many insurance plans.

Estrogen replacement therapy has been used since the condition was described in 1938 to promote development of secondary sexual characteristics. Estrogens are also important for maintaining good tissue and bone integrity.

Modern reproductive technologies have also been used to help women with Turner syndrome become pregnant if they desire. For example, a donor egg can be used to create an embryo, which is carried by the Turner syndrome woman.

Sources

Genetics Fact Sheet (from genetics.com.au)

References

External links

Turner Syndrome at the National Institute of Child Health and Human Development
Noonan Syndrome (Turner syndrome with normal Karyotype) at OMIM
Genetic and Rare Disease Information Center (NIH) Has several US government links to Turner syndrome.

Syndromes

Gourt Home::Gourt :: Turner syndrome