Congenital Nephrogenic Diabetes Insipidus Secondary to Antenatal Type IVa Bartter Syndrome.

Bartter syndrome (BS) is a rare autosomal recessive salt-wasting tubulopathy caused by genetic defects in the thick ascending limb of the loop of Henle. Type IVa BS, presents with severe salt wasting, sensorineural hearing loss, and impaired urinary concentrating ability. We report a 6-month-old female infant with antenatal type IVa BS due to a homozygous BSND variant, who developed secondary congenital nephrogenic diabetes insipidus (NDI), complicating her early clinical course. She presented with failure to thrive, recurrent dehydration, severe electrolyte imbalances, and profound polyuria. Diagnostic evaluation, including genetic testing and a water deprivation test with vasopressin challenge, confirmed impaired urinary concentration of renal origin. Management involved correction of electrolyte disturbances, Indomethacin therapy, and supportive care. This case highlights the importance of recognizing secondary congenital NDI in patients with type IVa BS when polyuria and hypernatremia are more pronounced than typically observed, allowing for earlier intervention and optimized clinical outcomes.