Turner syndrome
Turner syndrome, commonly known as 45,X, or 45,X0, is a chromosomal disorder in which cells of females have only one X chromosome instead of two, or are partially missing an X chromosome leading to the complete or partial deletion of the pseudoautosomal regions in the affected X chromosome. Humans typically have two sex chromosomes, XX for females or XY for males. The chromosomal abnormality is often present in just some cells, in which case it is known as Turner syndrome with mosaicism. 45,X0 with mosaicism can occur in males or females, but Turner syndrome without mosaicism only occurs in females. Signs and symptoms vary among those affected but often include additional skin folds on the neck, arched palate, low-set ears, low hairline at the nape of the neck, short stature, and lymphedema of the hands and feet. Those affected do not normally develop menstrual periods or mammary glands without hormone treatment and are unable to reproduce without assistive reproductive technology. Small chin, loose folds of skin on the neck, slanted eyelids and prominent ears are found in Turner syndrome, though not all will show it. Heart defects, Type II diabetes, and hypothyroidism occur in the disorder more frequently than average. Most people with Turner syndrome have normal intelligence; however, some have problems with spatial visualization that can hinder learning mathematics. Ptosis and conductive hearing loss also occur more often than average.
Turner syndrome is caused by one X chromosome, a ring X chromosome, 45,X/46,XX mosaicism, or a small piece of the Y chromosome in what should be an X chromosome. They may have a total of 45 chromosomes or will not develop menstrual periods due to loss of ovarian function genes. Their karyotype often lacks Barr bodies due to lack of a second X or may have Xp deletions. It occurs during formation of the reproductive cells in a parent or in early cell division during development. No environmental risks are known, and the mother's age does play a role. While most people have 46 chromosomes, people with Turner syndrome usually have 45 in some or all cells. In cases of mosaicism, the symptoms are usually fewer, and possibly none occur at all. Diagnosis is based on physical signs and genetic testing.
No cure for Turner syndrome is known. Treatment 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 Turner syndrome is associated.
Turner syndrome occurs in between one in 2,000 and one in 5,000 females at birth. All regions of the world and cultures are affected about equally. Generally people with Turner syndrome have a shorter life expectancy, mostly due to heart problems and diabetes. American endocrinologist Henry Turner first described the condition in 1938. In 1964, it was determined to be due to a chromosomal abnormality.
Presentation
Turner syndrome is associated with a number of physical features, including short stature, heart defects, webbed neck, micrognathia, amenorrhoea, and infertility. The phenotype of Turner syndrome is affected by mosaicism, where cell lines with a single sex chromosome are combined with those with multiple. Individuals with mosaicism of 45,X0/46,XY may be phenotypically male, female, or ambiguous, while those with 45,X0/46,XX will be phenotypically female. Patients with 45,X0/46,XY do not receive the diagnosis of Turner syndrome if phenotypically male. Around 40%–50% of cases of Turner syndrome are true "monosomy X" with a 45,X0 karyotype, while the remainder are mosaic for another cell line, most commonly 46,XX, or have other structural abnormalities of the X chromosome. The classic features of Turner syndrome, while distinctive, may be rarer than previously thought; incidental diagnosis, such as in biobank samples or prenatal testing for older mothers, finds many girls and women with few traditional signs of Turner syndrome.Physiological
Stature
Turner syndrome is associated with short stature. The mean adult height of women with Turner syndrome without growth hormone therapy is around shorter than the mean of women in the general population. Mosaicism affects height in Turner syndrome; a large population sample drawn from the UK Biobank found women with 45,X0 karyotypes to have an average height of, while those with 45,X0/46,XX karyotypes averaged. The strength of the association between Turner syndrome and short stature is such that idiopathic short stature alone is a major diagnostic indication.Growth delay in Turner syndrome does not begin at birth; most neonates with the condition have a birth weight in the lower end of the normal range. Height begins to lag in toddlerhood, with a delayed growth velocity becoming apparent as early as 18 months. Marked short stature becomes obvious in mid-childhood. In undiagnosed preadolescents and adolescents, growth delay may be mistaken for a side effect of delayed puberty and improperly treated.
Short stature in Turner syndrome and its counterpoint, tall stature in sex chromosome polysomy conditions such as Klinefelter syndrome, XYY syndrome, and trisomy X, is caused by the short-stature homeobox gene on the X and Y chromosomes. The absence of a copy of the SHOX gene in Turner's inhibits skeletal growth, resulting both in overall short stature and in a distinctive pattern of skeletal malformations including micrognathia, cubitus valgus, and shorter fingers.
When Turner syndrome is diagnosed in early life, growth hormone therapy can decrease the degree of short stature. Treatment with human growth hormone appears to increase expected adult height by approximately from an otherwise expected norm of –. The effects of growth hormone therapy are at their strongest during the first year of treatment and taper off over time. In some cases oxandrolone, a steroid with a relatively mild masculinizing effect, may be used alongside growth hormone. The addition of oxandrolone to a Turner syndrome treatment regimen adds around to the final height. Oxandrolone is used particularly often in girls diagnosed later in their growth period, due to the reduced impact of growth hormone alone in this population. However, oxandrolone use runs the risk of delayed breast development, voice deepening, increased body hair, or clitoromegaly.
Physical features
In addition to short stature, Turner syndrome is associated with a number of characteristic physical features. These include a short, webbed neck, low hairline, small chin and lower jaw, arched palate, and broad chest with widely-spaced nipples. Lymphedema of the hands and feet is common at birth and sometimes persistent throughout the lifespan.A number of the external manifestations of Turner syndrome are focused on the limbs, hands, and feet. Lymphedema at birth is one of the classic features of the syndrome; though it often resolves during toddlerhood, recurrence in later life is frequent, often without apparent cause. Cases where the retained X chromosome was inherited from the mother more often experience lymphedema than those where it was from the father. As a consequence of lymphedema's effects on nail anatomy, females with Turner syndrome frequently have small, hypoplastic, upturned nails. Their fingers are shorter and the hands are broad. Their feet are puffy, thicker, and swollen. Shortened metacarpal bones, particularly the fourth metacarpal, are a frequent finding. The body shape of individuals with Turner syndrome is frequently quite broad and stocky, as the growth deficiency is more pronounced in the length of bones than in their width. Scoliosis is common in Turner syndrome, and is seen in 40% of girls without growth hormone treatment.
Facial features associated with Turner syndrome include
broad, prominent ears, a low hairline at the nape of the neck, a webbed neck, a small chin with dental malocclusion, and downslanting palpebral fissures. These are thought to be related to lymphedema during the fetal period, specifically to the presence and resorption of excess fluids in the head and neck region. Neck webbing is a particularly distinctive trait of Turner syndrome, leading to many neonatal diagnoses. The underlying etiology of neck webbing is related to prenatal blood flow issues, and even in populations without Turner's has broad health consequences; the rate of congenital heart disease in webbed neck is 150-fold higher than in the general population, while the feature is also associated with reduced height and minor developmental impairments. Some women with Turner syndrome have premature facial wrinkling. Acne is less common in teenage girls and women with Turner syndrome, though the reasons why are unclear.
Other physical features connected to the condition include long eyelashes, sometimes including an additional set of eyelashes, and unusual dermatoglyphics. Some women with Turner's report being unable to create fingerprint passwords due to hypoplastic dermatoglyphics. Unusual dermatoglyphics are common to chromosome anomalies like Down Syndrome. and in the case of Turner's may be a consequence of fetal lymphedema. Keloid scars, or raised hypertrophic scars growing beyond the boundaries of the original wound, are potentially associated with Turner syndrome; however, the association is underresearched. Though traditional medical counselling on the topic urges conservatism about elective procedures such as ear piercing due to the risk of severe scarring, the actual consequences are unclear. Keloids in Turner syndrome are particularly frequent following surgical procedures to reduce neck webbing. Turner syndrome has been associated with unusual patterns of hair growth, such as patches of short and long hair. Armpit and pubic hair is often sparse, while arm and leg hair is often thick. Though armpit hair is reduced in amount and thickness, the pattern in which it is implanted in the skin is as in men, rather than as in women.