Natriuretic peptide-induced hyponatremia in a patient after surgical resection of a left atrial myxoma

Severe hyponatraemia in a patient with a large left atrial myxoma: a case report

Background

Atrial myxomas (AMs) are the most commonly encountered cardiac tumours. They can be genetically inherited and are commonly found in the left atrium. They usually present with dyspnoea, syncopal episodes, heart failure from mitral valve obstruction, and constitutional symptoms including weight loss, fatigue, and fever. We present a rare case of severe symptomatic hyponatraemia secondary to a large AM and discuss possible aetiology.

Case summary

A 75-year-old Caucasian female presented with acute nausea, vomiting, confusion, and drowsiness. She had a background of palpitations for about 20 years. Her blood test results revealed severe hyponatraemia (serum sodium—103 mmol/L). Further investigations for hyponatraemia including serum cortisol and urine biochemistry suggested Syndrome of Inappropriate Anti-Diuretic Hormone (SIADH) secretion. Computer tomography scan revealed an incidental large left AM. Echocardiography confirmed the AM attached to the left side of the inter-atrial septum and occupying the majority of the left atrium. She was treated medically for hyponatraemia and referred for excision of myxoma. She underwent urgent resection of the myxoma once sodium levels were optimized. Postoperatively, her serum sodium remained low but gradually returned to normal on postoperative Day 11.

Conclusion

This is only the third reported case of significant hyponatraemia associated with a large AM. It has been previously hypothesized that large left AM stretch the atrium causing release of atrial natriuretic peptide and subsequent hyponatraemia. The excision of myxoma and reduction in left atrial size postoperatively with an improvement in sodium levels suggests an association between the two pathologies.