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These disorders relate to patients with abnormalities of testicular differentiation or androgen synthesis or activity. The most common disorder of androgen synthesis is 5∝‐reductase deficiency secondary to mutations in the SRD5A2 gene. This enzyme is involved in the virilisation of the external genitalia during embryogenesis. The genitalia are ambiguous at birth, but at puberty can develop further male characteristics.

Androgen insensitivity syndromes, complete or partial, are due to a mutation in the androgen receptor genes leading to hormone resistance in the presence of a 46,XY karyotype. In the complete form, there are no functional androgen receptors. The external genitalia are female, but the Müllerian structures are absent and patients most frequently present with failure of menstruation. There is a blind‐ending vagina and bilateral testes, usually in the abdomen or inguinal canal. As systemic oestrogen is present, secondary sexual characteristics do appear, but pubic hair is sparse or absent. There is an increased risk of testicular neoplasia, and the testes are usually removed after puberty.

In partial forms of androgen insensitivity, there is some androgen receptor function, and although the external genitalia are female, there is a degree of virilisation at puberty which may include phallic enlargement and labioscrotal fusion.