Turner syndrome

Turner syndrome (or 45X) is the most common ofsex chromosome abnormalities in females. 

Epidemiology

The incidence is estimated at 1:2000-5000 of live birth, although the the in utero rate is much higher (1-2% of conceptions) due to a significant proportion of foetusesfetuses with 45X aborting by the 2^nd trimester. 

Clinical presentation

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

Pathology

Genetics

Turner syndrome is classically characterised with 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 commontrisomies, there is no association with maternal age.

Markers

• serum alpha feto protein (AFP): decreased
• beta HCG
  • elevated if hydrops present
  • decreased if no hydrops
• serum inhibin
  • elevated if hydrops present
  • absent if hydrops absent

Associations

• hypertension
• glucose intolerance
• inflammatory bowel disease
• hypothyroidism: due to formation of thyroid antibodies (most commonlyHashimoto thyroiditis)
• gonadal dysgenesis/ovarian dysgenesis

Complications

In utero complications complications include

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

Radiographic features

Intra-uterine ultrasound findings

• cystic hygroma: may appear septated; one of the most typical features of Turner syndrome
• increasednuchal 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

Post partum to adulthood findings

Musculoskeletal

• scoliosis
• short 4^th 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 elbow: increased carrying angle (cubitus valgus)

Pelvic ultrasound

• streaky uterus
• streak ovary ovary

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 mosiacism do much better. Mental development is unaffected.

History and etymology

It is named after Henry Turner who first described the syndrome in 1938.

Differential diagnosis

General differential considerations include

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