Turner Syndrome

Genetics and Karyotypes

• The missing or defective X is most often paternal in origin (in ~75-80% of cases, the retained X is maternal), indicating the error usually occurs in paternal gametogenesis.
• 45,X is the most common cytogenetic abnormality in spontaneous abortions, accounting for ~18% of chromosomally abnormal abortions. Only ~1% of 45,X embryos survive to birth.
• Y chromosome material is present in 5-10% of mosaic Turner patients (e.g., 45,X/46,XY). These individuals carry a significantly higher risk of gonadoblastoma and require gonadectomy.
• Mosaic patients have milder phenotypes; some with 45,X/46,XX mosaicism may present only with primary amenorrhea and can rarely even conceive.

Clinical Features

Neonatal/Infantile

• Lymphedema of the dorsum of hands and feet (due to lymphatic obstruction)
• Cystic hygroma (markedly distended lymphatics at the nape of the neck) - resolves into neck webbing

Somatic Features

• Short stature - below the 3rd percentile; the hallmark feature
• Webbing of the neck (pterygium colli)
• Low posterior hairline
• Shield chest with widely spaced nipples
• Cubitus valgus (increased carrying angle of the arms)
• Pigmented nevi
• High-arched palate

Cardiovascular (25-50% of patients)

• Predominantly left-sided defects: coarctation of the aorta and bicuspid aortic valve are most frequent
• Aortic root dilation in ~30%; 100-fold increased risk of aortic dissection
• Cardiovascular abnormalities are the leading cause of death in childhood
• Notably, ~5% of young females initially diagnosed with coarctation of the aorta have Turner syndrome

Renal

• Horseshoe kidney and other structural anomalies

Gonadal/Reproductive

• Failure to develop secondary sex characteristics at puberty
• Infantile genitalia, minimal breast development, sparse pubic hair
• Primary amenorrhea (most patients)
• Streak ovaries - fibrous strands devoid of follicles, the result of accelerated oocyte loss (complete by age 2 years)
• Infertility in the vast majority

Endocrine/Metabolic

• Autoimmune hypothyroidism - occurs in up to 50% of patients (especially those with isochromosome Xq)
• Glucose intolerance, insulin resistance, obesity, NAFLD - subset of patients can develop full metabolic syndrome
• Note: growth hormone therapy (used for short stature) worsens insulin resistance

Neurological/Cognitive

• Intellect is generally normal
• Subtle deficits in visual-spatial information processing are described

Pathogenesis

1. Streak ovaries: Both X chromosomes are needed for normal oocyte maintenance. In Turner syndrome, fetal ovaries develop normally until ~18 weeks gestation, after which the absence of a second X drives accelerated oocyte loss, complete by age 2 years. This has been described as "menopause before menarche."
2. SHOX gene haploinsufficiency: The short stature homeobox gene (SHOX) at Xp22.33 lies in the pseudoautosomal region and escapes X-inactivation - meaning both normal males and females have two active copies. Loss of one copy in Turner syndrome causes short stature. SHOX deletions also occur in 2-5% of otherwise normal children with short stature. (In contrast, the extra copy in Klinefelter syndrome contributes to tall stature.)
3. Other X-linked loci: Cardiac malformations and endocrine abnormalities cannot be explained by SHOX alone; additional genes on the short arm contribute to somatic features, while genes on the long arm affect fertility and menstruation.

Diagnosis

• Karyotyping is the definitive test. Conventional cytogenetics detects mosaicism in ~30% of cases; more sensitive molecular techniques (FISH, SNP arrays) raise this to ~75%.
• Clinical clue in adults: short stature + primary amenorrhea = strong suspicion for Turner syndrome.
• Prenatal diagnosis: can be identified on ultrasound (cystic hygroma, fetal hydrops) and confirmed by amniocentesis/CVS.

Management Highlights

• Growth hormone therapy to improve final adult height (started in childhood)
• Estrogen replacement therapy at puberty to induce secondary sex characteristics and protect bone density
• Cardiac surveillance - regular echocardiography and MRI for aortic root dilation/coarctation
• Thyroid function monitoring (TSH annually)
• Fertility: assisted reproduction (oocyte donation + IVF) is possible for those desiring pregnancy, though pregnancy carries cardiovascular risks (particularly aortic dissection)
• Gonadectomy in patients with Y chromosome material to prevent gonadoblastoma

Recent Evidence (2024-2026)

• A 2025 meta-analysis (PMID 41243107) examined the global prevalence of autoimmune diseases in Turner syndrome, reinforcing the well-known autoimmune predisposition.
• A 2025 systematic review (PMID 39743222) explored karyotype-phenotype correlations in non-mosaic X chromosome structural rearrangements.
• A 2025 meta-analysis (PMID 40557696) assessed the impact of karyotype on congenital heart disease risk in Turner syndrome, confirming karyotype significantly influences cardiovascular phenotype.