45X

45X, also called monosomy X, is a chromosomal configuration in which cells contain only one sex chromosome instead of the usual two. Most individuals with 45X are phenotypically female. The condition is commonly identified clinically as Turner syndrome when chromosomal testing confirms the single X chromosome.

Causes and frequency: 45X results from nondisjunction during egg or sperm formation. In some individuals, the 45X cell line is present only in a subset of cells (mosaic Turner syndrome). Most cases are sporadic and not inherited. A large proportion of conceptions with a 45X karyotype do not result in a live birth.

Clinical features and diagnosis: Key features include short stature, gonadal dysgenesis with streak ovaries, and primary or delayed pubertal development. Lymphedema of hands and feet and a webbed neck are common congenital findings. Many individuals have cardiovascular anomalies, notably coarctation of the aorta and bicuspid aortic valve. Other features can include a shield chest and widely spaced nipples. Diagnosis is made by karyotype analysis from blood or other tissue, and can be determined prenatally through amniocentesis or chorionic villus sampling if indicated.

Management and prognosis: Treatment focuses on addressing growth and puberty, cardiovascular health, and fertility considerations. Growth hormone therapy may improve adult height; estrogen replacement is used to initiate and sustain puberty. Regular monitoring of cardiovascular status is important due to associated risks. Fertility is typically reduced; options include assisted reproduction with donor eggs. With comprehensive care, life expectancy is near that of the general population, though outcomes vary with cardiovascular and other comorbid conditions.

Epidemiology: Turner syndrome, encompassing 45X and related mosaics, occurs in about 1 in 2,000 to 1 in 5,000 live female births.