21-Hydroxylase Deficiency screening
Conditions justifying newborn screening for any disorder include (1) a simple test with an acceptable sensitivity and specificity, (2) a dire consequence if not diagnosed early, (3) an effective treatment if diagnosed, and (4) a frequency in the population high enough to justify the expense. In the last decade more states and countries are adopting newborn screening for salt-wasting CAH due to 21-hydroxylase deficiency, which leads to death in the first month of life if not recognized.
The salt-wasting form of CAH has an incidence of 1 in 15,000 births, is potentially fatal within a month if untreated, and steroid replacement is a simple, effective treatment. However, the screening test itself is less than perfect. While the 17OHP level is easy to measure and sensitive (rarely missing real cases), the test has a poorer specificity. Screening programs in the United States have reported that 99% of positive screens turn out to be false positives upon investigation of the infant. This is a higher rate of false positives than the screening tests for many other congenital metabolic diseases. While each screen costs less than US$2, the program costs well over US$100,000 for each case detected.
When a positive result is detected, the infant's family and doctor must be notified, and the infant must be referred to a pediatric endocrinologist to confirm or disprove the diagnosis. Since most infants with salt-wasting CAH become critically ill by 2 weeks of age, the evaluation must be done rapidly despite the high false positive rate.
Prenatal diagnosis and treatment
Since CAH is an autosomal recessive disease, most children with CAH are born to parents unaware of the risk and with no family history. However, once a first child has CAH, it can be predicted that each successive child will have a 25% chance of being born with the disease (12.5% chance of an affected girl, 12.5% chance of an affected boy). Few families would choose not to continue with a pregnancy of a second child with CAH but all would wish to minimize the degree of virilization of a girl. There is no known prenatal harm to a male fetus from CAH, so treatment can begin at birth.
Adrenal glands of female fetuses with CAH begin producing excess testosterone by the 9th week of gestation. The most important aspects of virilization (urogenital closure and phallic urethra) occur between 8 and 12 weeks. Theoretically, if enough glucocorticoid could be supplied to the fetus to reduce adrenal testosterone production by the 9th week, virilization could be prevented and the difficult decision about timing of surgery avoided.
The challenge of preventing severe virilization of girls is twofold: detection of CAH at the beginning of the pregnancy, and delivery of an effective amount of glucocorticoid to the fetus without causing harm to the mother.
The first problem has not yet been entirely solved, but it has been shown that if dexamethasone is taken by a pregnant woman enough can cross the placenta to suppress fetal adrenal function.
At present no program screens for risk in families who have not yet had a child with CAH. For families desiring to avoid virilization of a second child, the current strategy is to start dexamethasone as soon as a pregnancy has been confirmed even though at that point the chance that the pregnancy is a girl with CAH is only 12.5%. Dexamethasone is taken by the mother each day until it can be safely determined whether she is carrying an affected girl.
Whether the fetus is an affected girl can be determined by chorionic villus sampling at 9-11 weeks of gestation, or by amniocentesis at 15-18 weeks gestation. In each case the fetal sex can be determined quickly, and if the fetus is a male the dexamethasone can be discontinued. If female, fetal DNA is analyzed to see if she carries one of the known abnormal alleles of the CYP21 gene. If so, dexamethasone is continued for the remainder of the pregnancy at a dose of about 1 mg daily.
Most mothers who have followed this treatment plan have experienced at least mild cushingoid effects from the glucocorticoid, but have borne daughters whose genitalia are much less virilized.