Hyponatremia, Metabolic Acidosis, and Abnormal Newborn Screen in a Preterm Neonate.

A 2,250-g male neonate is born at 34 weeks of gestation via spontaneous vaginal delivery to a 30-year-old gravida 1, para 0 woman after presentation to a community hospital with uterine contractions and spontaneous rupture of membranes. The mother receives 1 dose of ampicillin, 1 dose of betamethasone, and magnesium before delivery. At delivery, the neonate is vigorous and crying. His Apgar scores are 8 and 9 at 1 and 5 minutes after birth, respectively, with no abnormalities noted on physical examination. In the first couple of hours after birth, he develops persistent oxygen desaturations and is started on continuous positive airway pressure.He is admitted to the NICU for prematurity and low birthweight. Admission examination demonstrates nondysmorphic features, no respiratory distress with good air exchange, regular heart rate and rhythm without murmur, soft and flat abdomen, patent anus, normal external male genitalia, unremarkable neurologic findings, and pink skin color.Chest radiography shows mild diffuse ground-glass reticular densities. He is placed on a stock solution of parenteral nutrition via a peripheral intravenous catheter. Antibiotics are started because of the mother’s unknown group B Streptococcus status and preterm labor. Initial surveillance of electrolytes demonstrates sodium, potassium, calcium, phosphorus, and glucose levels within normal limits. On day 4 after birth, the neonate continues on nasal canula for tachypnea, receiving parenteral nutrition, and advancing gavage feeds of maternal breast milk or formula. A capillary blood gas measurement shows mild metabolic acidosis, with a pH of 7.28, Pco2 32 mm Hg (4.26 kPa), and bicarbonate 19 mEq/L (19 mmol/L).On day 6 after birth, repeat surveillance of electrolytes shows a sodium concentration of 126 mEq/L (126 mmol/L) and potassium of 6.7 mEq/L (6.7 mmol/L). Glucose concentration is 53 mg/dL (2.94 mmol/L). Blood gas continues to show mild metabolic acidosis with a pH of 7.27, Pco2 31 mm Hg (4.12 kPa), and bicarbonate 19 mEq/L (19 mmol/L). Results of the state newborn screening demonstrate an elevated 17-hydroxyprogesterone (17-OHP) of 221 nmol/L (normal <80 nmol/L) on dried blood spot testing, or 14,586 ng/dL (normal <5,280 ng/dL) as serum equivalent. A confirmatory serum 17-OHP level is sent that same day. The pediatric endocrinology service is consulted for the concerning laboratory results.The neonate is transferred to a tertiary care pediatric hospital for further management. He is started on hydrocortisone and fludrocortisone replacements along with sodium chloride supplementation. Repeat serum 17-OHP before treatment is greater than 50,000 ng/dL, and the actual diagnosis of classic congenital adrenal hyperplasia (CAH) due to 21-hydroxylase deficiency (21-OHD) is made.The neonate continues to appear well with normal vital signs, including blood pressure. He is breathing room air, receiving full enteral feeds, and working on oral feeds with the occupational therapist. Parenteral nutrition has been discontinued.The differential diagnosis for hyponatremia and hyperkalemia in a neonate includes CAH, pseudohypoaldosteronism, inappropriate parenteral nutrition composition, and exogenous diuretic use.CAH is a group of autosomal recessive disorders encompassing enzyme deficiencies in the adrenal steroidogenesis pathway and is the most common cause of primary adrenal insufficiency in children. The classic form of CAH affects 1 in 15,000 neonates and is commonly identified through newborn screening, (1) with screening mandated across the United States since 2008. (2) Most cases of CAH are caused by 21-OHD, which is marked by an elevated serum 17-OHP. In the steroidogenic pathway, 17-OHP is the precursor in the enzymatic reaction that is catalyzed by 21-OHD, and is the primary analyte tested in newborn screening for CAH. If treatment is not initiated early, neonates may present in acute salt-wasting adrenal crisis with vomiting, hypotension, and severe dehydration, electrolyte abnormalities, and hypoglycemia. (1)(3)(4)(5) Female neonates with classic CAH also present with virilization of external genitalia at birth.False-positive newborn screening results for CAH are common in preterm neonates, related to the fact that healthy preterm neonates have higher overall levels of 17-OHP compared with term neonates. (6) Thus, many screening programs have established reference ranges that are based on weight and gestational age.For neonates with classic CAH, therapy should be initiated soon after birth to reduce the potentially life-threatening sequelae of CAH. Classic CAH is treated with steroid hormone replacements including hydrocortisone (to replace cortisol) and fludrocortisone (to replace aldosterone). Supplemental sodium chloride (salt) is also recommended for neonates with classic CAH who could have salt-wasting.