STEMI complicated with serum sodium of 113 mmol/L from syndrome of inappropriate antidiuretic hormone secretion: How worse could it be?

Hyponatremia commonly occurs in acute coronary syndrome and has been recognized as a worse prognostic indicator in patients with ST-segment elevation myocardial infarction (STEMI). However STEMI with preexisting hyponatremia from syndrome of inappropriate antidiuretic hormone secretion (SIADH) has never been described in the literature. We describe a case of 59-year-old woman who presented with STEMI and received emergent percutaneous coronary intervention who also had SIADH with the lowest serum sodium measurement of 113 mmol/L. Initially, she was treated with hypertonic saline to reduce central nervous system complications. Then, vasopressin receptor 2 antagonist and demeclocycline were started as well as fluid restriction and salt tablet. Her sodium level and clinical symptoms improved. Subsequently, we found cavitary right upper lung mass and a biopsy report revealed small cell lung cancer as a cause of SIADH. Severe hyponatremia from SIADH complicated with STEMI could potentially have reduced adverse outcomes by normalizing sodium level through vasopressin receptor 2 antagonist or demeclocycline. <Learning objective: Hyponatremia in STEMI from SIADH, prognosis, and treatment options.>.