Microvillus Inclusion Disease: Successful Treatment of Severe Hyponatremia With High-Dose Fludrocortisone via a Possible Gut-Mediated Mechanism

Microvillus inclusion disease (MVID) is a rare congenital enteropathy marked by severe secretory diarrhea and malabsorption due to microvillous atrophy secondary to accumulation of secretory granules in the apical cytoplasm of enterocytes. Management is supportive with parenteral nutrition and emergent management of acute electrolyte disturbances. We report an 8-month-old male infant with genetically confirmed MVID (<i>MYO5B</i> mutation) who developed life-threatening hyponatremia (serum sodium 118 mmol/L) despite maximal sodium supplementation (>300 mmol/d = 49 mmol/kg/d) and negligible urinary sodium level. High-dose fludrocortisone (100 mcg bid; 28.4 mcg/kg/d) successfully restored serum sodium levels and reduced stool output, with normalization of renin and aldosterone, without adverse effects. This case demonstrates a potential therapeutic benefit of gastrointestinal mineralocorticoid receptor agonism in managing severe hyponatremia in patients with MVID during periods of acute decompensation.