Congenital adrenal hyperplasia due to 21-hydroxylase deficiency, in all its forms, accounts for over 95% of diagnosed cases of congenital adrenal hyperplasia (CAH), and CAH in most contexts refers to 21-hydroxylase deficiency and different mutations related to enzyme impairment have been mapped on protein structure. Presentation Severe, early onset 21-hydroxylase deficient CAH The two most serious neonatal consequences of 21-hydroxylase deficiency occur: life-threatening salt-wasting crises in the first month of life (for male and female infants alike) and severe virilization of female infants. The subdivision of the early onset CAH into salt-wasting and simple-virilizing forms, which is based on the capacity of the adrenal to produce small amounts of aldosterone in the simple-virilizing form, is often not clinically meaningful, because clinical presentations overlap and all patients lose salt to some degree. Salt-wasting crises in infancy The excessive amounts of adrenal testosterone produce little effect on the genitalia of male infants with severe CAH. If a male infant with CAH is not detected by newborn screening, he will appear healthy and normal and be quickly discharged home to his family. However, the lack of aldosterone results in a high rate of sodium loss in the urine. Urinary sodium concentrations may exceed 50 mEq/L. With this rate of salt loss, the infant cannot maintain blood volume, and hyponatremic dehydration begins to develop by the end of the first week of life. Potassium and acid excretion are also impaired when mineralocorticoid activity is deficient, and hyperkalemia and metabolic acidosis gradually develop. Ability to maintain circulation is further limited by the effect of cortisol deficiency. The early symptoms are spitting and poor weight gain, but most infants with severe CAH develop vomiting, severe dehydration, and circulatory collapse (shock) by the second or third week of life. When brought to a hospital, the 1- to 3-week-old infant will be both underweight and dehydrated by appearance. Blood pressure may be low. Basic chemistries will reveal hyponatremia, with a serum Na+ typically between 105 and 125 mEq/L. Hyperkalemia in these infants can be extreme—levels of K+ above 10 mEq/L are not unusual—as can the degree of metabolic acidosis. Hypoglycemia may be present. This is termed a salt-wasting crisis and rapidly causes death if not treated. As ill as these infants can be, they respond rapidly to treatment with hydrocortisone and intravenous saline and dextrose quickly restores blood volume, blood pressure, and body sodium content, and reverses the