出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2015/05/28 06:02:50」(JST)
| Turner syndrome | |
|---|---|
|
Girl with Turner syndrome before and after an operation for neck-webbing
|
|
| Classification and external resources | |
| ICD-10 | Q96 |
| ICD-9 | 758.6 |
| DiseasesDB | 13461 |
| MedlinePlus | 000379 |
| eMedicine | ped/2330 |
| Patient UK | Turner syndrome |
| MeSH | D014424 |
Turner syndrome (TS) also known as Ullrich–Turner syndrome, gonadal dysgenesis, and 45,X, is a condition in which a female is partly or completely missing an X chromosome.[1] Signs and symptoms vary among those affected. Often there is a short and webbed neck, low-set ears, low hairline at the back of the neck, short stature, and swollen hands and feet at birth. Typically they are without menstrual periods, do not develop breasts, and are unable to have children. Heart defects, diabetes, and low thyroid hormone occur more frequently. Most people with TS have normal intelligence. Many, however, have troubles with spatial visualization such as that needed for mathematics.[2] Vision and hearing problems occur more often.[3]
Turner syndrome is not usually inherited from a person's parents.[4] There are no known environmental risks and the mother's age does not play a role.[4][5] Turner syndrome is due to a chromosomal abnormality in which all or part of one of the X chromosomes is missing or altered. While most people have 46 chromosomes, people with TS usually only have 45.[6] The chromosomal abnormality may be present in just some cells in which case it is known as TS with mosaicism.[3] In these cases the symptoms are usually fewer and possibly there are none at all.[7] Diagnosis is based on physical signs and genetic testing.[8]
There is no cure for Turner syndrome. Treatment, however, may help with symptoms. Human growth hormone injections during childhood may increase adult height. Estrogen replacement therapy can promote development of the breasts and hips. Medical care is often required to manage other health problems with which TS is associated.[9]
Turner syndrome occurs in between 1 in 2000[10] to 1 in 5000 females at birth.[11] All regions of the world and cultures are affected about equally.[4] People with TS have a shorter life expectancy, mostly due to heart problems and diabetes.[3] Henry Turner first described the condition in 1938. In 1964 it was determined to be due to a chromosomal abnormality.[12]
The following is a list of common symptoms of Turner syndrome. It is important to note that an individual may have any combination of symptoms and is unlikely to have all symptoms.
Other features may include a small lower jaw (micrognathia), cubitus valgus,[13] soft upturned nails, palmar crease, and drooping eyelids. Less common are pigmented moles, hearing loss, and a high-arch palate (narrow maxilla). Turner syndrome manifests itself differently in each female affected by the condition; therefore, no two individuals will share the same features.
While most of the physical findings are harmless, there can be significant medical problems associated with the syndrome.
Despite the excellent postnatal prognosis, 99% of Turner-syndrome conceptions are thought to end in spontaneous abortion or stillbirth,[14] and as many as 15% of all spontaneous abortions have the 45,X karyotype.[15] Among cases that are detected by routine amniocentesis or chorionic villus sampling, one study found that the prevalence of Turner syndrome among tested pregnancies was 5.58 and 13.3 times higher respectively than among live neonates in a similar population.[16]
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%[17] and 38%[18] in a group of pure 45,X monosomy. But, considering other karyotype groups, they reported a prevalence of 24.3%[17] and 11%[18] in patients with mosaic X monosomy, and a prevalence of 11% in patients with X chromosomal structural abnormalities.[17]
The higher prevalence in the group of pure 45,X monosomy is primarily due to a significant difference in the prevalence of aortic valve abnormalities and coarctation of the aorta, the two most common cardiovascular malformations.
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 includes bicuspid aortic valve and coarctation (narrowing) of the aorta. Sybert, 1998 found that more than 50% of the cardiovascular malformations observed in her study of individuals with Turner syndrome were bicuspid aortic valves or coarctation of the aorta, alone or in combination.
Other congenital cardiovascular malformations, such as partial anomalous venous drainage and aortic valve stenosis or aortic regurgitation, are also more common in Turner syndrome than in the general population. Hypoplastic left heart syndrome represents the most severe reduction in left-sided structures
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. Since bicuspid valves are capable of regulating blood flow properly, this condition may go undetected without regular screening. However, bicuspid valves are more likely to deteriorate and later fail. Calcification also occurs in the valves,[19] which may lead to a progressive valvular dysfunction as evidenced by aortic stenosis or regurgitation.[20]
With a prevalence from 12.5%[17] to 17.5% (Dawson-Falk et al., 1992), bicuspid aortic valve is the most common congenital malformation affecting the heart in this syndrome. It is usually isolated but it may be seen in combination with other anomalies, particularly coarctation of the aorta.
Between 5% and 10% of those born with Turner syndrome have coarctation of the aorta, a congenital narrowing of the descending aorta, usually just distal to the origin of the left subclavian artery (the artery that branches off the arch of the aorta to the left arm) and opposite to the duct (and so termed "juxtaductal"). Estimates of the prevalence of this malformation in patients with Turner syndrome ranges from 6.9%[17] to 12.5% . A coarctation of the aorta in a female is suggestive of Turner syndrome, and suggests the need for further tests, such as a karyotype.
This abnormality is a relatively rare congenital heart disease in the general population. The prevalence of this abnormality also is low (around 2.9%) in Turner syndrome. However, its relative risk is 320 in comparison with the general population. Strangely, Turner syndrome seems to be associated with unusual forms of partial anomalous venous drainage.[17][21]
In the management of a patient with Turner syndrome it is essential to keep in mind that these left-sided cardiovascular malformations in Turner syndrome result in an increased susceptibility to bacterial endocarditis. Therefore prophylactic antibiotics should be considered when procedures with high risk endocarditis are performed, such as dental cleaning.[20]
Turner syndrome is often associated with persistent hypertension, sometimes in childhood. In the majority of Turner syndrome patients with hypertension, there is no specific cause. In the remainder, it is usually associated with cardiovascular or kidney abnormalities, including coarctation of the aorta.
Two studies have 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.[22]
Sybert, 1998 points out that it remains unproven that aortic root diameters that are relatively large for body surface area but still well within normal limits imply a risk for progressive dilatation.[25]
The prevalence of aortic root dilatation ranges from 8.8%[22] to 42%[20] in patients with Turner syndrome. Even if not every aortic root dilatation necessarily goes on to an aortic dissection (circumferential or transverse tear of the intima), complications such as dissection, aortic rupture resulting in death may occur. The natural history of aortic root dilatation is still unknown, but it is a fact that it is linked to aortic dissection and rupture, which has a high mortality rate.[26]
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 dissection. Routine surveillance is highly recommended.[20]
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 common 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 45,X karyotype.[20]
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.[22] 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). The natural history of aortic root dilatation is unknown, but because of its lethal potential, this aortic abnormality needs to be carefully followed.
Normal skeletal development is inhibited due to a large variety of factors, mostly hormonal. The average height of a woman with Turner syndrome, in the absence of growth hormone treatment, is 4 ft 7 in (140 cm). Patients with Turner's mosaicism can reach normal average height.
The fourth metacarpal bone (fourth toe and ring finger) may be unusually short, as may the fifth.
Due to inadequate production 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.
Approximately one-third of all women with Turner syndrome have one of three kidney abnormalities:
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.
Approximately one-third of all women with Turner syndrome have a thyroid disorder.[citation needed] Usually it is hypothyroidism, specifically Hashimoto's thyroiditis. If detected, it can be easily treated with thyroid hormone supplements.
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. The risk of developing type 2 diabetes can be substantially reduced by maintaining a healthy weight.
Turner syndrome does not typically cause intellectual disability or impair cognition. However, learning difficulties are common among women with Turner syndrome, particularly a specific difficulty in perceiving spatial relationships, such as nonverbal learning disorder. 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. Most Turner Syndrome patients are employed as adults and lead productive lives.
There is also a rare variety of Turner Syndrome, known as "Ring-X Turner Syndrome", which has an approximate 60 percent association with intellectual disability. This variety accounts for approximately 2–4% of all Turner Syndrome cases.[27]
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 45,X.[28][29] Even when such pregnancies do occur, there is a higher than average risk of miscarriage or birth defects, including Turner Syndrome or Down Syndrome.[30] Some women with Turner syndrome who are unable to conceive without medical intervention may be able to use IVF or other fertility treatments.[31]
Usually estrogen replacement therapy is used to spur growth of secondary sexual characteristics at the time when puberty should onset. While very few women with Turner Syndrome menstruate spontaneously, estrogen therapy requires a regular shedding of the uterine lining ("withdrawal bleeding") to prevent its overgrowth. Withdrawal bleeding can be induced monthly, like menstruation, or less often, usually every three months, if the patient desires. Estrogen therapy does not make a woman with nonfunctional ovaries fertile, but it plays an important role in assisted reproduction; the health of the uterus must be maintained with estrogen if an eligible woman with Turner Syndrome wishes to use IVF (using donated oocytes).
Turner syndrome is a cause of primary amenorrhea, premature ovarian failure (hypergonadotropic hypogonadism), streak gonads and infertility. Failure to develop secondary sex characteristics (sexual infantilism) is typical.
Especially in mosaic cases of Turner syndrome that contains Y-chromosome (e.g. 45,X/46,XY) due to the risk of development of ovarian malignancy (most common is gonadoblastoma) gonadectomy is recommended.[citation needed]
Turner syndrome is characterized by primary amenorrhoea, premature ovarian failure, streak gonads and infertility. However, technology (especially oocyte donation) provides the opportunity of pregnancy in these patients.
As more women with Turner syndrome complete pregnancy thanks to modern techniques to treat infertility, it has to be noted that pregnancy may be a risk of cardiovascular complications for the mother. Indeed several studies had suggested an increased risk for aortic dissection in pregnancy.[22] 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 estrogen rate.[20] Of course these findings are important and need to be remembered while following a pregnant patient with Turner syndrome.
Turner syndrome is caused by the absence of two complete copies of the X chromosome in some or all the cells. The abnormal cells may have only one X (monosomy) (45,X) or they may be affected by one of several types of partial monosomy like a deletion of the short p arm of one X chromosome (46,X,del(Xp)) or the presence of an isochromosome with two q arms (46,X,i(Xq))[32] In mosaic individuals, cells with X monosomy (45,X) may occur along with cells that are normal (46,XX), cells that have partial monosomies, or cells that have a Y chromosome (46,XY).[32] The presence of mosaicism is estimated to be relatively common in affected individuals (67-90%).[32]
In the majority of cases where monosomy occurs, the X chromosome comes from the mother.[33] This may be due to a nondisjunction in the father. Meiotic errors that lead to the production of X with p arm deletions or abnormal Y chromosomes are also mostly found in the father.[34] Isochromosome X or ring chromosome X on the other hand are formed equally often by both parents.[34] Overall, the functional X chromosome mostly comes from the mother.
In most cases, Turner syndrome is a sporadic event, and for the parents of an individual with Turner syndrome the risk of recurrence is not increased for subsequent pregnancies. Rare exceptions may include the presence of a balanced translocation of the X chromosome in a parent, or where the mother has 45,X mosaicism restricted to her germ cells.[35]
Turner syndrome may be diagnosed by amniocentesis or chorionic villus sampling during pregnancy.
Usually, fetuses with Turner syndrome can be identified by abnormal ultrasound findings (i.e., heart defect, kidney abnormality, cystic hygroma, ascites). In a study of 19 European registries, 67.2% of prenatally diagnosed cases of Turner Syndrome were detected by abnormalities on ultrasound. 69.1% of cases had one anomaly present, and 30.9% had two or more anomalies.[36]
An increased risk of Turner syndrome may also be indicated by abnormal triple or quadruple maternal serum screen. The fetuses diagnosed through positive maternal serum screening are more often found to have a mosaic karyotype than those diagnosed based on ultrasonographic abnormalities, and conversely those with mosaic karyotypes are less likely to have associated ultrasound abnormalities.[36]
Although the recurrence risk is not increased, genetic counseling is often recommended for families who have had a pregnancy or child with Turner syndrome.
Turner syndrome can be diagnosed postnatally at any age. Often, it is diagnosed at birth due to heart problems, an unusually wide neck or swelling of the hands and feet. However, it is also common for it to go undiagnosed for several years, typically until the girl reaches the age of puberty/adolescence and she fails to develop properly (the changes associated with puberty do not occur). In childhood, a short stature can be indicative of Turner syndrome.[37]
A test, called a karyotype or a chromosome analysis, analyzes the chromosomal composition of the individual. This is the test of choice to diagnose Turner syndrome.
As a chromosomal condition, there is no cure for Turner syndrome. However, much can be done to minimize the symptoms. For example:[38]
Approximately 99 percent of all fetuses with Turner syndrome result in spontaneous termination during the first trimester.[42] Turner syndrome accounts for about 10 percent of the total number of spontaneous abortions in the United States.[citation needed] The incidence of Turner syndrome in live female births is believed to be around 1 in 2000.[10]
The syndrome is named after Henry Turner, an endocrinologist from Illinois, who described it in 1938.[43] In Europe, it is often called Ullrich–Turner syndrome or even Bonnevie–Ullrich–Turner syndrome to acknowledge that earlier cases had also been described by European doctors.
The first published report of a female with a 45,X karyotype was in 1959 by Dr. Charles Ford and colleagues in Harwell, Oxfordshire and Guy's Hospital in London.[44] It was found in a 14-year-old girl with signs of Turner syndrome.
|
||||||||||||||||||||||||||||||||||||||||||||||||||||||||
|
全文を閲覧するには購読必要です。 To read the full text you will need to subscribe.
| リンク元 | 「ターナー症候群」 |
| 関連記事 | 「X」「syndrome」 |