Trimethoprim-induced hyponatremia mimicking SIADH in a patient with pulmonary nocardiosis: use of point-of-care ultrasound in apparent euvolemic hypotonic hyponatremia.

A 72-year-old man with chronic obstructive pulmonary disease and depression presented to the emergency department (ED) with progressive worsening of shortness of breath. He required intubation and mechanical ventilation. The patient improved with therapy, but his endotracheal aspirate culture was positive for Nocardia cyriacigeorgica The patient was started on high dose Bactrim and discharged. He presented to the office 5 days later with confusion, and his serum sodium was 113 mmol/L. Based on a euvolemic physical examination, consistent serum and urine studies, he was diagnosed with syndrome of inappropriate antidiuretic hormone secretion (SIADH) likely from citalopram. However, treatment for SIADH failed to improve his serum sodium level. A bedside ultrasound revealed an inferior vena cava diameter of 0.7 cm with a complete inspiratory collapse that was inconsistent with SIADH. The patient was correctly diagnosed with salt-losing nephropathy from trimethoprim, and the medication was discontinued. He received therapy initially with intravenous normal saline and then salt tablets. His sodium improved within 2 weeks.