What is Turner syndrome?

What is Turner Syndrome?

Turner syndrome is a chromosomal disorder affecting phenotypic females characterized by complete or partial absence of one sex chromosome, most commonly presenting as 45,X karyotype, with short stature and ovarian failure as the most prevalent clinical findings, but critically associated with significantly increased cardiovascular mortality, particularly from aortic dissection. 1, 2

Genetic Basis

Turner syndrome results from loss of the second X chromosome, occurring in approximately 1 in 2,000 to 2,500 live female births 3, 4. The karyotype variations include:

• Classic monosomy X (45,X): 40-50% of cases
• Monosomy X mosaicism: 3-25%
• Isochromosome X: 10-18%
• Ring X chromosome: 10-16%
• Other structural abnormalities: Including Y chromosome mosaicism (6-12%) and various deletions 3

Importantly, the paternal X chromosome is absent in three-quarters of patients, though this has no current clinical utility 3. Pregnancy loss occurs in approximately 99% of Turner syndrome conceptions, making live births relatively rare 3.

Cardinal Clinical Features

Primary Manifestations

Short stature is the most common presenting feature (54% of cases) and occurs in nearly all untreated patients 1, 5. This results from haploinsufficiency of the SHOX gene on the X chromosome.

Ovarian dysgenesis and hypergonadotropic hypogonadism affect approximately 66% of patients, leading to:

• Primary amenorrhea or early ovarian failure
• Infertility in the vast majority
• Only 5% may have abbreviated menstrual function before developing amenorrhea
• Spontaneous pregnancy occurs in only 1-2% of all patients 6, 4

Cardiovascular Complications: The Critical Mortality Risk

Cardiovascular disease represents the leading cause of death in Turner syndrome, with standardized mortality increased threefold compared to the general population 7. The cardiovascular burden includes:

Structural heart defects:

• Bicuspid aortic valve: 10-25% of patients 1, 2
• Aortic coarctation: 7-18% of patients 1, 8
• Ascending aortic dilation: 33% when defined as ascending-to-descending aortic diameter ratio >1.5 1
• Other anomalies: Hypoplastic left heart, partial anomalous pulmonary venous return 7

Aortic dissection risk:

• Population-based incidence: 36 per 100,000 Turner syndrome-years (1.4% lifetime risk) versus 6 per 100,000 in general population 1
• Average age at dissection: 31 years, with less than half of patients surviving the event 1
• Critical distinction: Dissection occurs at smaller absolute aortic diameters than in the general population due to patients' short stature 1, 2
• 85% of dissections occur in the ascending aorta, 15% in the descending aorta 2

Risk factors for dissection include:

• Aortic dilation (indexed to body surface area)
• Bicuspid aortic valve
• Aortic coarctation
• Systemic hypertension 1

However, the absence of these risk factors does not preclude dissection risk 6.

Additional Comorbidities

• Endocrine disorders: Hypothyroidism, glucose intolerance, metabolic syndrome
• Autoimmune conditions: Increased prevalence throughout lifespan 4
• Skeletal abnormalities: Osteoporosis risk, cubitus valgus, short fourth metacarpals
• Renal anomalies: Horseshoe kidney, collecting system abnormalities
• Neurocognitive profile: Specific learning difficulties, particularly with visuospatial processing and executive function 9, 10
• Lymphatic abnormalities: Lymphedema in infancy, webbed neck 3

Diagnostic Approach

Diagnosis requires karyotype analysis demonstrating X chromosome abnormality in a phenotypic female. Turner syndrome may be recognized:

• Prenatally: Via ultrasound findings (cystic hygroma, hydrops, cardiac anomalies)
• In infancy/childhood: Due to lymphedema, cardiac defects, or growth failure
• In adolescence/adulthood: Due to short stature (most common), delayed puberty, or primary amenorrhea 3, 5

Essential Screening and Surveillance

At diagnosis, all patients require 2:

• Transthoracic echocardiography (TTE) to evaluate for bicuspid aortic valve, aortic root/ascending aortic dilation, coarctation, and other congenital defects
• Cardiac MRI for comprehensive aortic assessment
• Renal ultrasound
• Thyroid function testing
• Hearing assessment

For patients ≥15 years old, the Aortic Size Index (ASI) must be calculated to properly assess dissection risk 2:

• ASI = maximal aortic diameter (cm) / body surface area (m²)
• ASI >2.0 cm/m² = abnormal
• ASI ≥2.5 cm/m² = significantly increased dissection risk

Ongoing surveillance imaging intervals 2:

• Without risk factors: Every 5 years in children, every 10 years in adults, and before pregnancy
• With ASI >2.3 cm/m²: At least annually
• With risk factors: Interval depends on aortic diameter, ASI, and growth rate

Management Principles

Growth hormone therapy during childhood and adolescence is standard treatment for short stature, used in approximately 72% of patients 5. Studies show no evidence of left ventricular hypertrophy with growth hormone treatment 1.

Estrogen replacement therapy is essential for:

• Pubertal development
• Bone health
• Cardiovascular protection
• Psychosocial well-being

Surgical intervention for aortic disease 2:

• Reasonable in patients ≥15 years with ASI ≥2.5 cm/m² plus risk factors for dissection
• May be considered in those with ASI ≥2.5 cm/m² without risk factors

Critical Pregnancy Considerations

Pregnancy dramatically increases mortality risk in Turner syndrome, estimated at ≥100-fold higher than baseline 6. This applies to:

• Spontaneous pregnancy (extremely rare)
• Donor oocyte pregnancy (more common route)

All Turner syndrome women are at risk of aortic rupture during pregnancy, regardless of:

• Prior spontaneous menses
• Age >30 years
• Absence of known cardiovascular abnormalities 6

The physiological changes of pregnancy may increase rupture risk in future years even for those who survive delivery 6. Thorough cardiovascular assessment and counseling about maternal mortality risk is mandatory before any pregnancy attempt 11.

Prognosis and Lifespan Management

Turner syndrome requires lifelong multidisciplinary care involving endocrinology, cardiology, gynecology, genetics, and other specialties as needed 4, 7. The condition is associated with increased morbidity and mortality throughout the lifespan, with cardiovascular complications being the predominant cause of premature death 1, 7.

Transition to adult care is frequently inadequate, resulting in suboptimal health outcomes and decreased quality of life 10. Approximately 80% of patients complete middle to high-level education, though 22% require educational support 5.

The recurrence risk for phenotypically normal parents with a child diagnosed with Turner syndrome is low 3.