Turner syndrome, also known as 45XO or 45X, is the most common of the sex chromosome abnormalities in females.
The incidence is estimated at 1:2000-5000 of live births, although the in utero rate is much higher (1-2% of conceptions) due to a significant proportion of fetuses with 45X aborting by the 2nd trimester.
In adults, it is one of the most important causes of primary amenorrhea and accounts for approximately one-third of such cases.
Turner syndrome is classically characterized by the absence of one X chromosome copy (45 XO), with the missing chromosome most frequently (two-thirds) being the paternal one. Most cases occur as a sporadic event.
However, the classic genetic change is not present in all cases. Three main subtypes include:
- complete monosomy (45XO): ~60%
- even though it is relatively common, almost all 45 XO fetuses will spontaneously abort, with 70% lost between 16 weeks and term
- partial monosomy (structurally-altered X chromosome): ~15%
- mosaicism (XO and another sex karyotype): ~30%
Unlike the common trisomies, there is no association with maternal age.
- serum alpha-fetoprotein (AFP): decreased
- elevated if hydrops present
- decreased if no hydrops
- serum inhibin
- elevated if hydrops present
- absent if hydrops absent
- glucose intolerance
- inflammatory bowel disease
- hypothyroidism: due to the formation of thyroid antibodies (most commonly Hashimoto thyroiditis)
- gonadal dysgenesis / ovarian dysgenesis
In utero complications include:
- development of hydrops fetalis: usually from fluid overload secondary to lymphatic failure
- cystic hygroma: may appear septated; one of the most typical features of Turner syndrome
- increased nuchal thickness
- increased nuchal translucency
- coarctation of the aorta: 15-20%
- bicuspid aortic valve
- horseshoe kidney / pelvic kidney
- mild IUGR
- features related to complicating hydrops fetalis
- short fetal limbs
- short 4th metacarpal: positive metacarpal sign
- narrowing scapholunate angle: positive carpal sign
- abnormal medial femoral condyle
- decreased carpal angle: Madelung deformity
- short stature
- webbed neck
- valgus deformity of the elbow: increased carrying angle (cubitus valgus)
Treatment and prognosis
Overall prognosis very variable is dependent on associated anomalies. While the vast majority of fetuses abort in the second trimester, some may have long life expectancy. Cases with mosaicism do much better. Mental development is unaffected.
History and etymology
It is named after the American endocrinologist Henry H Turner (1892-1970) 7 who first described the syndrome in 1938.
General differential considerations include:
- Noonan syndrome: can have similar phenotypical features but normal karyotype
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