CAH, the endocrine emergency the screen prevents

Before newborn screening for congenital adrenal hyperplasia, an unknown but substantial fraction of affected newborns died in the second or third week of life of an apparent sudden infant death whose cause was actually salt-wasting adrenal crisis. Other affected children were misassigned at birth because the most common form of CAH masculinizes the external genitalia of XX infants enough to produce ambiguous or apparently male anatomy. The combination of an early lethal endocrine crisis and a developmental misclassification at the moment of birth made CAH a condition the medical system was actively missing in two different ways.

CAH is on the newborn screening panel of every US state and most high-income economies. The screening marker is 17-hydroxyprogesterone, 17-OHP, elevated in the most common form of CAH because of the upstream block in cortisol synthesis. Detection is on the dried blood spot. Treatment is hormone replacement, started before the salt-wasting crisis. The condition is now manageable from the first week of life. The before-and-after picture is one of the cleanest among RUSP additions.

What CAH is

Congenital adrenal hyperplasia is a family of autosomal recessive disorders of cortisol biosynthesis. The most common form, accounting for roughly 95 percent of cases, is 21-hydroxylase deficiency, caused by variants in CYP21A2 on chromosome 6p21. 21-hydroxylase converts 17-hydroxyprogesterone to 11-deoxycortisol on the path to cortisol. Without the enzyme, cortisol production fails, ACTH drives the adrenal cortex to grow (the hyperplasia), and the upstream substrate 17-OHP accumulates. The accumulated substrate is shunted into androgen biosynthesis. The clinical picture follows the biochemistry: cortisol deficiency, salt-wasting from concomitant aldosterone deficiency in severe forms, and androgen excess.

The classic salt-wasting form presents in the first two weeks of life with vomiting, dehydration, weight loss, hyponatremia, hyperkalemia, hypoglycemia, and cardiovascular collapse. Without treatment, mortality is high. The simple virilizing form has the cortisol and androgen excess phenotype without the severe salt-wasting and presents later in childhood with virilization. The non-classic form, milder and partial, presents in adolescence or adulthood with hyperandrogenism that can mimic polycystic ovary syndrome. Less common variants of CAH involve other steroidogenic enzymes and produce different mixtures of cortisol deficiency, salt-wasting or salt retention, hypertension, and atypical genital development.

In XX newborns with classic CAH, prenatal androgen exposure produces virilized external genitalia ranging from clitoromegaly to a phenotype that can be misread as male at first inspection. Internal Mullerian structures (uterus, Fallopian tubes, ovaries) are typically normal. In XY newborns, the genitalia appear typical and the disorder is not visible at birth.

Detection

Newborn screening for CAH measures 17-OHP on the dried blood spot, typically by immunoassay with mass spectrometry confirmation in many states for borderline samples. False positives are non-trivial in preterm infants because 17-OHP is physiologically elevated in prematurity. Most state programs use birth-weight or gestational-age stratified cutoffs to manage that. Confirmation uses serum 17-OHP, electrolytes, plasma renin activity, and CYP21A2 sequencing.

Reported live-birth incidence of classic CAH in unscreened populations runs roughly 1 in 14,000 to 1 in 18,000 globally. Higher rates are documented in some founder populations including Ashkenazi Jews, Yupik Eskimos in Alaska, and several Mediterranean populations.

What management looks like

Standard of care is lifelong hormone replacement. Glucocorticoid replacement, typically hydrocortisone in childhood and a longer-acting glucocorticoid in adulthood, replaces the missing cortisol and suppresses the ACTH drive that fuels androgen excess. Mineralocorticoid replacement, fludrocortisone, replaces aldosterone in salt-wasting forms. Sodium chloride supplementation is part of infant management until salt intake from food becomes adequate. Stress dosing of glucocorticoid during illness, surgery, or trauma is essential for life because the adrenal cortex cannot respond to stress without exogenous cortisol.

Crinecerfont, a corticotropin-releasing factor type 1 receptor antagonist marketed as Crenessity, was approved by the FDA in December 2024 for adults and pediatric patients aged 4 and older with classic CAH. The drug reduces the supraphysiological glucocorticoid doses that have historically been required to suppress androgen excess, which over time produces the iatrogenic Cushing-syndrome-like adverse effects of long-term high-dose steroid therapy. Crinecerfont allows lower glucocorticoid dosing while maintaining androgen control.

Surgical decision-making for virilized external genitalia in 46,XX infants with classic CAH has changed substantially in the past two decades. The historical practice of early infant feminizing surgery has given way to deferred surgical decisions, multidisciplinary teams, and approaches that involve the affected individual in decisions about surgery when they are old enough to participate. The shift reflects long-term outcome data and advocacy from people who underwent infant surgery and developed concerns about consent, sexual function, and gender identity later in life. Practice still varies by center and country.

Outcomes after screening

Universal newborn screening shifted CAH outcomes substantially in the populations where it was implemented. Salt-wasting crisis in the second or third week of life, the historical lethal event, is now uncommon because hormone replacement starts before the crisis develops. Time to diagnosis in 46,XY infants, who have no genital marker, dropped from a mean of weeks (after the crisis presented) to days (from screening). The clinical question shifted from how to rescue the crashing newborn to how to manage hormone replacement across decades.

The cost-effectiveness case for CAH screening is well established and similar to that for biotinidase and congenital hypothyroidism: a treatable condition with a catastrophic untreated course, a reliable dried blood spot assay, and a treatment that costs little once the diagnosis is made.

What this looks like for a family

A baby is born and the heel-prick is sent. On day 4, the state lab calls the pediatrician with an elevated 17-OHP. On day 5, the family is in the pediatric endocrinology clinic. Hydrocortisone is started. Fludrocortisone is started if electrolytes confirm salt-wasting. The baby is fed, monitored, weighed, and goes home. The parents learn how to dose the hydrocortisone, how to recognize when to give a stress dose, when to go to the emergency department, what to put on the medical alert bracelet that will travel with the child for life.

That is what CAH care looks like in practice when the screen catches it. The endocrine emergency that historically killed an unknown number of newborns has become an outpatient appointment.