Turner syndrome is an important cause of short stature in girls and primary amenorrhea in young women that is usually caused by loss of part or all of an X chromosome. The clinical manifestations and diagnosis of Turner syndrome will be reviewed here. Treatment of this disorder is discussed separately. (See "Management of Turner syndrome (gonadal dysgenesis)".)
Turner syndrome, first described in 1938 , is the most common sex chromosome abnormality in females and occurs in approximately 1/2,500 live births, based on data from Europe, Japan, and the United States . The true prevalence of Turner syndrome is difficult to ascertain, since those with mild phenotypic features may remain undiagnosed .
Molecular studies have shown that the maternal X is retained in two-thirds of patients with Turner syndrome and the paternal X in the remaining one-third . More than one-half of all patients with Turner syndrome have a mosaic chromosomal complement (eg, 45,X/46,XX) detectable in peripheral blood leukocytes [5,6]. Mosaicism with a normal cell line in the fetal membranes may be necessary in all affected cases for adequate placental function and fetal survival .
The identification of mosaicism depends directly upon the method of ascertainment. It varies from 34 percent with conventional cytogenetic techniques to 60 percent with fluorescence in situ hybridization (FISH) techniques to 74 percent in a study in which reverse transcriptase polymerase-chain-reaction (PCR) assays were used .
Some patients with Turner syndrome lack only part of one sex chromosome, and the Turner syndrome phenotype can be seen with a variety of structural abnormalities, such as isochromosomes, rings, or terminal deletions . A rare but very informative class of Turner syndrome includes patients who have deletions of the Y chromosome that remove the testes-determining gene, SRY; these individuals develop as females. Based upon this finding and the fact that males require only one X chromosome for normal development, Ferguson-Smith hypothesized that copies of Turner syndrome genes are also present on the Y chromosome.