Early assessment of ambiguous genitalia.

T o discover that there is uncertainty about the sex of one’s newborn baby is devastating and often incomprehensible for most parents. It is paramount that clear explanations and investigations are commenced promptly, and that no attempt is made to guess the sex of the baby. Extreme sensitivity is required, and ideally the baby should be managed in a tertiary centre by a multidisciplinary team including a paediatric endocrinologist and a paediatric urologist. Early involvement of a clinical psychologist with experience in this field should be mandatory. Other professionals including geneticists and gynaecologists may also become involved. There must be access to specialist laboratory facilities and experienced radiologists. The incidence of genital ambiguity that results in the child’s sex being uncertain is 1 per 4500, although some degree of male undervirilisation, or female virilisation may be present in as many as 2% of live births. Parents require reassurance that either a male or female gender will definitely be assigned. However the outcome of some of the investigations may take some weeks, and registration of the child’s birth should be deferred until gender has been assigned. This may require communication with the Registrar of Births, and a skilled clinical psychologist will help the parents in deciding what to tell family and friends in the interim. It is also helpful (if appropriate) to reassure the parents that their child is otherwise healthy. While not all intersex conditions are apparent at birth (for example, complete androgen insensitivity may only become apparent in a child with a testis within an inguinal hernia, or at puberty with primary amenorrhoea and lack of androgen hair), only those presenting with genital ambiguity at birth will be considered in this article. An understanding of sex determination and differentiation is essential to direct appropriate investigations and to establish a diagnosis. Genetic sex is determined from the moment of conception and determines the differentiation of the gonad. The differentiation of the gonad in turn determines the development of both the internal genital tracts and the external genitalia and thus phenotypic sex, which occurs later in development (from about 5–6 weeks of gestation). Both male and female genitalia differentiate from the same structures along the urogenital ridge. At about 4 weeks after fertilisation, primordial germ cells migrate from the yolk sac wall to the urogenital ridge that develops from the mesonephros. The urogenital ridge also contains the cells that are the precursors for follicular or Sertoli cells and steroid producing theca and Leydig cells. The ‘‘indifferent’’ gonads form on the genital ridges. The development of the fetal adrenals and gonads occur in parallel, as before migration, the potential steroidogenic cells of both originate from the mesonephros. There are many genes and transcription factors that are expressed in both tissues (for example, SF1 and DAX1), and hence mutations in these genes may affect both adrenal and gonadal development (fig 1). In addition, WT1 is expressed in the kidney and gonad, hence the association of Wilms’ tumour and gonadal dysgenesis in Denys-Drash syndrome, for example. The undifferentiated gonad is capable of developing into either an ovary or a testis. The theory that the ‘‘default’’ programme generates an ovary is probably not correct, although the exact role of ‘‘ovarian determining’’ genes in humans is unclear at present. In contrast, testicular development is an active process, requiring expression of the primary testis determining gene SRY, and other testis forming genes such as SOX9. Transcription factors such as SF1 and WT1 are also required for development of the undifferentiated gonad, as well as for the activation of the other male pathway genes required for testis development and the consequent development of male internal and external genitalia. DAX1 and Wnt 4 are two genes that may act to ‘‘antagonise’’ testis development. Over-expression of DAX1 (through duplication of Xp21) and Wnt4 (through duplication of 1p35), have been associated with impaired gonadal development and undervirilisation in a small number of karyotypic 46 XY males. Mutations or duplications in the various genes responsible for gonadal differentiation and the subsequent development of the internal and external genital phenotype genes may be responsible for gonadal dysgenesis and in some cases complete sex reversal (table 1). Wnt4 is also expressed in the Müllerian ducts and in the absence of anti-Müllerian hormone (AMH) (also known as Müllerian inhibiting substance) and testosterone, Müllerian structures develop, while the Wolffian ducts involute. AMH promotes regression of Müllerian structures and as the only source of AMH in the fetus is the testes, the absence of a uterus in a baby with ambiguous genitalia is evidence that there has been functional testicular tissue (Sertoli cell) present. Testosterone produced from Leydig cells promotes differentiation of the Wolffian ducts and hence the internal male genitalia (vas deferens, epididymis, and seminal vesicles). Testosterone is converted to dihydrotestosterone [DHT] by the enzyme 5areductase. DHT masculinises the external genitalia from about 6 weeks gestation, and the degree of masculinisation is determined by the amount of fetal androgen present (irrespective of source) and the ability of the tissues to respond to the androgens. Defects in any part of this pathway (including gene mutations and chromosomal abnormalities (for example, 46XY/46XX, 45X/46XY), inappropriate hormone levels, or end-organ unresponsiveness) may result in genital ambiguity, with undervirilisation of an XY individual, virilisation of an XX individual, or the very rare true hermaphrodite (an individual with both ovarian tissue with primary follicles and testicular tissue with seminiferous tubules which may be in separate gonads or ovotestes).