Restrictive ventricular septal defect resulting in systemic outflow obstruction in adults with Fontan circulation: challenging diagnosis of a serious and potentially fatal complication

: : Fontan operation is a palliative intervention that allows survival in patients with functional univentricular heart by redirecting systemic venous flow to the pulmonary arteries. The result is an entirely passive pulmonary circulation. In this setting, increased afterload may interfere with Fontan's fragile balance and ultimately lead to severe complications including failure of Fontan circulation and sudden death. In patients with Fontan circulation for double inlet left ventricle or tricuspid atresia and discordant ventriculoarterial connections, a restrictive VSD acts haemodynamically as subaortic stenosis. Systemic outflow obstruction can occur late during follow-up and remains undiagnosed in some cases. We report three cases of late onset of subaortic stenosis that clearly illustrate the diagnostic difficulties and the importance of promptly addressing this issue.