Turner Syndrome

Epidemiology and Genetics

Epidemiology

• Incidence: 1 in 2,000–2,500 live births 
• 99% spontaneously abort
• Most frequent gonadal chromosome anomaly in women

Genetics

• Karyotype: Only 1 normal X chromosome is present; the other is missing or impaired. 
  • Complete sex chromosomal monosomy in approximately 45% of live births: 45,X0 (2nd sex chromosome is missing)
  • Sex chromosomal mosaicism in approximately 50% of live births: 
    • Most commonly 46,XX with 45,X0
    • Approximately 10%–12% have mosaicism involving Y chromosome material.
• Etiology: 
  • The underlying process is nondisjunction, which may occur during meiosis or mitosis:
    • In meiosis: Failure of chromosomes (meiosis I) or chromatids (meiosis II) to separate leads to loss of a chromosome (chromosomal monosomy). 
    • In mitosis: Postzygotic nondisjunction leads to different cells within the same individual containing different chromosomal content/karyotypes (chromosomal mosaicism).
  • Nearly 70% of cases are due to meiotic nondisjunction of paternal sex chromosomes; i.e., the paternal X chromosome is lost.
• No known risk factors

Clinical Presentation

Clinical phenotype

• Outer appearance
  • Short stature
  • Pterygium colli (webbed neck)
  • Congenital lymphedema
  • “Shield chest”: broad chest with widely spaced nipples
  • Lymphedema on the back of the hand and foot (sausage-like appearance of fingers and toes)
  • Low hairline
  • Cubitus valgus (outward-turned forearms)
  • Multiple (benign) pigment nevi
  • Ear anomalies such as low-set or rotated ears
  • Nail dysplasias
  • Short 4th metacarpals/metatarsals
  • Kyphosis and/or scoliosis
• Cognition/intelligence
  • Intelligence can be normal or reduced.
  • A deficit in some areas can be present (e.g., mathematics, spatial orientation).
• Behavioral phenotype
  • Increased risk of impaired social skills and/or mood disorders such as anxiety or depression
• Sex development/fertility
  • Hypergonadotropic hypogonadism
    • Primary ovarian failure as development of ovaries typically impaired (streak gonads)
    • Decreased estrogen and progesterone levels with elevated luteinizing hormone (LH) and follicle-stimulating hormone (FSH)
  • Delayed or lack of onset of puberty due to lack of estrogen
    • Lack of breast development
    • Primary amenorrhea
  • Infertility
    • Spontaneous pregnancy very unlikely, but possible in 2%–5% of affected persons

Associated conditions

• Cardiac abnormalities:
  • Found in approximately ⅓ of individuals with Turner syndrome
  • Congenital bicuspid aortic valve → ↑ risk of aortic stenosis and/or aortic insufficiency
  • Congenital coarctation of the aorta (abnormal narrowing in segment of aorta) 
  • Anomalies of the pulmonary veins
  • ↑ risk of aortic dissection (tear occurs in the inner layer of aorta) or rupture; risk goes up during pregnancy
• Malformations of the kidneys and ureters
  • Horseshoe kidneys (normal functioning kidneys congenitally fused, forming a horseshoe shape)
  • Agenesis, rotational anomalies, obstructions
• Vision impairments such as color blindness, cataracts, and strabismus (crossed eyes)
• Hypertension
• Cystic hygroma (abnormal growth/mass, typically on the neck)
• Hearing loss
• Recurrent otitis media
• Celiac disease
• Osteoporosis and pathologic fractures secondary to ↓ estrogen levels
• ↑ risk of gonadoblastoma if Y chromosome material present

Diagnosis and Management

Diagnosis

• If suspected in utero: 
  • Chorionic villi sampling (CVS)
  • Amniocentesis with karyotype 
• Postnatal diagnosis:
  • History and clinical examination
  • Genetic testing: Karyotyping confirms the diagnosis.
  • Labs show: ↓ estrogen and androgen levels, ↑ LH and FSH levels
• Additional screening tests once diagnosis is confirmed:
  • Hearing evaluation
  • Eye exam
  • Neuropsychological, developmental, educational, and behavioral assessments/screenings 
  • Blood pressure measurements in arms and legs
  • Routine labs: thyroid test, lipid screening, glucose screening, liver enzymes
  • Imaging: 
    • Baseline echocardiogram and/or cardiac magnetic resonance imaging (MRI) to check for cardiac defects
    • Renal ultrasound to check for malformations
    • Baseline dual-energy X-ray absorptiometry (DEXA) scan in early adulthood

Management

• Hormonal replacement:
  • Estrogen therapy (life long) initiated in adolescence for sexual maturation/development
  • Cyclic progestin therapy to accompany estrogen within 2 years of beginning estrogen therapy and/or breakthrough bleeding begins
  • Human growth hormone to try to improve short stature; used until bone age reaches 14 years
• Fertility treatment if children are desired
• In the rare instance of Y chromosome material being present, gonadectomy is recommended.

Clinical Relevance

Differential diagnosis

The following conditions are differential diagnoses of Turner syndrome:

• Noonan syndrome: autosomal dominant genetic condition caused by several potential gene mutations. Both men and women can be affected. Clinical presentation includes short stature, webbed neck, and congenital heart defects. Genetic testing confirms the diagnosis. Individuals with Noonan syndrome have a normal karyotype. Developmental delays are more common than in Turner syndrome. 
• Gonadal dysgenesis: congenital condition in which the gonads do not develop. Both men and women are affected. Gonadal dysgenesis is not caused by a chromosomal abnormality. Women have a normal 46,XX karyotype with lack of onset of puberty, primary amenorrhea, and infertility similar to Turner syndrome. The phenotypic features seen in Turner syndrome are not present, however. Therapy with hormone replacement is required.

Related condition

The following condition is related to Turner syndrome as it also causes gonadal insufficiency due to a chromosomal abnormality:

• Klinefelter syndrome: chromosomal abnormality disorder seen in men in which extra X and possibly Y chromosomes are present in karyotype. Causes delayed puberty and primary hypergonadotropic hypogonadism in men. Individuals with this condition tend to present as tall, phenotypic men, with small testes, decreased body hair, gynecomastia, and infertility. Genetic testing confirms the diagnosis. Requires testosterone replacement therapy.