Turner syndrome

Turner syndrome, also known as 45XO or 45X, is the most common of the sex chromosome abnormalities in females.

Epidemiology

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 2trimester.

Clinical presentation

In adults, it is one of the most important causes of primary amenorrhea and accounts for approximately one-third of such cases.

Pathology

Genetics

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.

Markers
  • serum alpha-fetoprotein (AFP): decreased
  • beta HCG
    • elevated if hydrops present
    • decreased if no hydrops
  • serum inhibin
    • elevated if hydrops present
    • absent if hydrops absent
Associations
Complications

In utero complications include:

  • development of hydrops fetalis: usually from fluid overload secondary to lymphatic failure

Radiographic features

Antenatal ultrasound
Postpartum-to-adulthood features
Musculoskeletal
Pelvic ultrasound
Gastrointestinal

Treatment and prognosis

Overall prognosis very variable is dependent on associated anomalies. While the vast majority of fetuses are aborted in the second trimester, some may have a 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) who first described the syndrome in 1938.

Differential diagnosis

General differential considerations include:

  • Noonan syndrome: can have similar phenotypical features but normal karyotype