Turner Syndrome

Turner syndrome is a chromosomal condition of females caused by complete or partial loss of one X chromosome, classically the 45,X karyotype. Its hallmarks are short stature and ovarian insufficiency, often with characteristic physical features and an increased risk of cardiovascular and other anomalies. It is the principal sex chromosome monosomy compatible with live birth.

Definition

Turner syndrome is a sex chromosome disorder of phenotypic females defined by the complete or partial absence of one X chromosome, in all cells or in a mosaic pattern, classically the 45,X karyotype, leading to gonadal dysgenesis and short stature.

Scope

The entry covers the genetic basis, mechanisms, characteristic features, and epidemiology of Turner syndrome, together with the role of mosaicism and the structure of multidisciplinary care as reflected in international guidelines. It is reference-educational and does not provide growth-hormone or estrogen dosing or individualised clinical recommendations.

Key concepts

• 45,X (monosomy X) karyotype
• Mosaicism (e.g. 45,X/46,XX) and structural X abnormalities
• SHOX haploinsufficiency and short stature
• Gonadal dysgenesis and primary ovarian insufficiency
• Congenital cardiovascular anomalies (e.g. bicuspid aortic valve, coarctation)
• Genes escaping X-inactivation
• Multidisciplinary lifelong surveillance

Mechanisms

Turner syndrome results from loss of all or part of one X chromosome, often through nondisjunction or anaphase lag, with mosaicism common because much of the loss occurs after fertilisation. Two X chromosomes are needed for normal ovarian development and maintenance, so their effective reduction to one leads to accelerated loss of oocytes and gonadal dysgenesis. Haploinsufficiency of SHOX, a gene in the pseudoautosomal region that escapes X-inactivation, contributes to short stature and skeletal features, and reduced dosage of other escape genes underlies further aspects of the phenotype, including the predisposition to cardiovascular anomalies.

Clinical relevance

Turner syndrome is associated at the group level with short stature, ovarian insufficiency, and an elevated risk of cardiovascular, renal, endocrine, and hearing problems, which is why international guidelines frame care as lifelong multidisciplinary surveillance. This entry describes the condition for orientation and literature appraisal and is not a basis for individual diagnosis, growth or hormone therapy, or aortic-risk management, which require specialist care.

Epidemiology

Turner syndrome occurs in approximately 1 in 2,000 to 1 in 2,500 liveborn females, although the great majority of 45,X conceptions miscarry, so it is far more frequent at conception than at birth. Newborn cytogenetic surveys helped define its live-birth frequency, and cohort studies have documented diagnostic delay and increased mortality relative to the general female population.

History

Henry Turner described the clinical triad of short stature, webbed neck, and cubitus valgus in 1938, and Otto Ullrich had reported related cases earlier, giving rise to the alternative name Ullrich-Turner syndrome. The chromosomal basis was established in 1959 with identification of the 45,X karyotype, making Turner syndrome one of the first human conditions linked to a specific chromosomal abnormality.

Debates

How should cardiovascular and aortic risk be monitored across the lifespan?

Because aortic dilatation and dissection are major causes of excess mortality, the optimal imaging modality, frequency, and thresholds for surveillance are an active focus of the international guidelines and ongoing discussion.

Seminal works

• turner-1938
• gravholt-2017
• gravholt-2008

Frequently asked questions

What causes Turner syndrome?

It is caused by the complete or partial loss of one X chromosome in females, classically the 45,X karyotype, usually arising sporadically; mosaic and structural variants of the X chromosome are also common.

Why is care for Turner syndrome described as lifelong and multidisciplinary?

Because the condition is associated with a range of issues across organ systems, including cardiovascular, growth, ovarian, renal, and hearing concerns, international guidelines frame care as coordinated surveillance over the lifespan rather than a single intervention.