46,XY
| Swyer syndrome | |
|---|---|
| Classification and external resources | |
| Specialty | medical genetics |
| ICD-10 | Q56.4 |
| ICD-9-CM | 752.7 |
| OMIM | 400044 |
| DiseasesDB | 31464 |
| MeSH | D006061 |
Swyer syndrome, or XY gonadal dysgenesis, is a type of hypogonadism in a person whose karyotype is 46,XY. The person is externally female with streak gonads, and if left untreated, will not experience puberty. Such gonads are typically surgically removed (as they have a significant risk of developing tumors) and a typical medical treatment would include hormone replacement therapy. The syndrome was named by Gerald Swyer, an endocrinologist, based in London, UK.
There are several forms of gonadal dysgenesis. The term “pure gonadal dysgenesis” (PGD) has been used to describe conditions with normal sets of sex chromosomes (e.g., 46,XX or 46,XY), as opposed to those whose gonadal dysgenesis results from missing all or part of the second sex chromosome. The latter group includes those with Turner syndrome (i.e., 45,X) and its variants, as well as those with mixed gonadal dysgenesis and a mixture of cell lines, some containing a Y chromosome (e.g., 46,XY/45,X).
Thus Swyer syndrome is referred to as PGD, 46,XY, and XX gonadal dysgenesis as PGD, 46,XX. Patients with PGD have a normal karyotype but may have defects of a specific gene on a chromosome.
Types include:
The first known step of sexual differentiation of a normal XY fetus is the development of testes. The early stages of testicular formation in the second month of gestation requires the action of several genes, of which one of the earliest and most important is SRY, the sex-determining region of the Y chromosome. Mutations of SRY account for many cases of Swyer syndrome.
When such a gene is defective, the indifferent gonads fail to differentiate into testes in an XY (genetically male) fetus. Without testes, no testosterone or antimüllerian hormone (AMH) is produced. Without testosterone, the wolffian ducts fail to develop, so no internal male organs are formed. Also, the lack of testosterone means that no dihydrotestosterone is formed and consequently the external genitalia fail to virilize, resulting in normal female genitalia. Without AMH, the Müllerian ducts develop into normal internal female organs (uterus, fallopian tubes, cervix, vagina).
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