Re-Emergent Cyanosis Post-Fontan: Unraveling the Differential Diagnosis and Optimizing Targeted Management

Background: The Fontan procedure represents the definitive palliative surgical intervention for patients with single ventricle physiology, establishing a total cavopulmonary connection that redirects systemic venous return directly to the pulmonary arteries. While this procedure aims to normalize systemic arterial oxygen saturation, numerous patients experience persistent or recurrent cyanosis, which significantly impacts long-term morbidity and mortality. The major causes of desaturation include surgically created fenestrations, baffle leaks, pulmonary arteriovenous malformations, and Veno-Venous Collaterals (VVC) draining into the pulmonary veins or left atrium. VVC develop in approximately 20-25% of patients undergoing catheterization following Fontan operations, believed to result from chronically elevated central venous pressure. These collaterals function as right-to-left shunts, leading to systemic desaturation and increasing the risk of paradoxical embolism. Case Illustration: We present a 34-year-old female with tricuspid atresia, pulmonary atresia, ventricular septal defect, atrial septal defect, and hypoplastic right ventricle who underwent a fenestrated Fontan procedure at age three. She presented with a 9-year history of progressive fatigue, dyspnea, palpitations, and chest pain, worsening over 2 years with persistent cyanosis requiring supplemental oxygen. Physical examination revealed oxygen saturation of 81%, digital clubbing, and a grade 3 out of 6 pansystolic murmur. Cardiac magnetic resonance imaging identified a 12-millimeter fistula between the left innominate vein and left upper pulmonary vein. Catheter-based embolization was performed using a 16/18 millimeter patent ductus arteriosus occluder device via a transjugular approach. Post-procedural angiography confirmed successful fistula closure with immediate improvement in oxygen saturation from 81% to 94%. The patient was discharged in stable condition with sustained clinical improvement at follow-up. Conclusions: VVCs represent an important etiology of persistent cyanosis in post-Fontan patients. Advanced cardiac imaging, particularly cardiac magnetic resonance, is critical for identifying these abnormal vascular connections. Transcatheter embolization using appropriately sized occluder devices can provide effective treatment with immediate clinical benefit. However, patient selection remains crucial as closure may negatively impact survival in certain subgroups, particularly those with elevated Fontan pressures. Comprehensive hemodynamic assessment and careful consideration of risks versus benefits are essential before intervention.