A 5-Week-Old Male Infant With Severe Vomiting: Osmosis USMLE Study Question of the Week

February 17, 2017

Answer: C. Loss of function mutation in the Na-K-Cl cotransporter

This patient's metabolic alkalosis, history of polyhydramnios, polyuria (constant diaper change), and dysmorphic facies are suggestive of neonatal Bartter syndrome. Neonatal Bartter syndrome is a rare renal tubular defect that results in impaired salt reabsorption on the thick ascending limb of the loop of Henle. The development of Bartter syndrome is the result of a loss of function mutation in the Na-K-Cl cotransporter (NKCC2).

Characteristically patients with this syndrome have low concentrations of potassium (hypokalemia), increased blood pH (alkalosis), low or normal blood pressure, hypercalciuria/nephrocalcinosis, and increased levels of plasma renin and aldosterone.

In 90% of the cases, polyhydramnios is seen between 24-30 weeks' gestation. Common clinical manifestations include increased urinary frequency (polyuria), excessive thirst (polydipsia), and sensorineural deafness. If left untreated, patients may progress to develop severe dehydration and renal failure.

Major Takeaway: A loss of function mutation in NKCC2 produces Bartter syndrome, an autosomal recessive disorder characterized by hypokalemic metabolic alkalosis, with normal to low blood pressure.

For more on Batter syndrome, read here.

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