Apparent mineralocorticoid excess mimicking Bartter syndrome in infancy

Objectives Apparent mineralocorticoid excess (AME) syndrome is a rare monogenic disorder caused by loss-of-function variants in the HSD11B2 gene encoding 11β-hydroxysteroid dehydrogenase type 2 (11β-HSD2). Deficient enzyme activity results in cortisol-mediated activation of the mineralocorticoid receptor, leading to low-renin hypertension. Because affected infants frequently present with polyuria and hypokalemic metabolic alkalosis, AME syndrome is often misdiagnosed as Bartter syndrome, resulting in significant diagnostic delay. Case presentation A 13-month-old girl presented with polyuria, hypokalemic metabolic alkalosis, hypercalciuria, and failure to thrive and was initially managed as Bartter syndrome. During follow-up, persistent hypertension accompanied by markedly suppressed plasma renin activity and aldosterone levels prompted evaluation for monogenic hypertension. Urinary steroid analysis demonstrated a markedly reduced cortisone-to-cortisol ratio (0.02), consistent with 11β-HSD2 deficiency. Next-generation sequencing of the HSD11B2 gene identified a homozygous likely pathogenic splice-site variant, c.665-1G>A, confirming the diagnosis of AME syndrome. Treatment with spironolactone, amlodipine, potassium supplementation, and dietary salt restriction achieved blood pressure control and improved electrolyte abnormalities. Conclusions AME syndrome should be included in the differential diagnosis of infants presenting with hypokalemic metabolic alkalosis and polyuria, particularly when hypertension is detected. Systematic blood pressure assessment is essential in this clinical context, as early recognition of this monogenic disorder may prevent long-term cardiovascular and renal complications.