Bilateral carcinoid heart disease without intracardiac shunt in a patient with advanced functional small bowel neuroendocrine tumour: a clinical conundrum

Background: Carcinoid heart disease (CHD) is a known complication of advanced functional neuroendocrine tumours (NETs), almost exclusively affecting right-sided cardiac valves. Left-sided involvement is rare and usually attributed to intracardiac shunting or pulmonary sources of serotonin. This case report highlights a rare presentation of CHD involving left- and right-sided valves in the absence of an anatomic shunt or bronchopulmonary NET. Case summary: A 67-year-old woman with a long-standing, functional, small bowel Grade 1 NET and metastatic liver and peritoneal disease presented with worsening dyspnoea and peripheral oedema. She had a 10-year disease history, previously managed with debulking surgery, somatostatin analogues, peptide receptor radionuclide therapy (PRRT), and everolimus. Echocardiography and cardiac magnetic resonance imaging demonstrated extensive left- and right-sided valvular involvement with severe mitral regurgitation, moderate aortic and pulmonic regurgitation, and mild tricuspid regurgitation, yet without intracardiac shunt. An elevated Qp/Qs ratio of 2.6 was attributed to severe left-sided valvular regurgitation. The patient improved on diuretic therapy and was referred for surgical evaluation of valve replacement prior to additional systemic treatment. Discussion: This case illustrates an atypical presentation of bilateral carcinoid valvulopathy in the absence of intracardiac shunting, likely due to overwhelming systemic serotonin from tumour burden. Although serotonin is typically inactivated in the lungs, extensive exposure may surpass this protective mechanism. The potential role of selective serotonin reuptake inhibitors remains inconclusive. Multidisciplinary coordination is essential for optimizing cardiac and oncologic outcomes, especially when systemic therapy such as PRRT is considered. Left- and right-sided CHD may develop in patients with small bowel NETs even without anatomic shunting. High tumour burden and systemic serotonin exposure may override pulmonary inactivation, leading to left-sided involvement. Early recognition and multidisciplinary care are critical for effective management.