Bubbles Don’t Lie: Superior Vena Cava Thrombosis With a Systemic-to-Pulmonary Venous Shunt Mimicking a Patent Foramen Ovale

In patients with ischaemic stroke, right-to-left shunting on a bubble study is often attributed to a patent foramen ovale (PFO) and taken as sufficient grounds for percutaneous closure.We report the case of a 60year-old woman admitted for an ischaemic stroke, in whom transoesophageal echocardiography (TOE) showed microbubbles in the left atrium after right antecubital injection of agitated saline.A PFO was presumed, and the patient was referred for transcatheter closure.During the procedure, repeated attempts to cross the interatrial septum failed.Repeat TOE with agitated saline injected directly adjacent to the interatrial septum showed no septal transit.A subsequent left antecubital injection produced left atrial opacification as abundant as the right-sided injection, with microbubbles consistently entering through the pulmonary veins.This excluded an interatrial shunt and pointed toward a large, fixed, extracardiac right-to-left conduit between the systemic venous return and the pulmonary venous circulation.The closure procedure was therefore abandoned.Contrast-enhanced computed tomography (CT) angiography resolved the discrepancy between the positive bubble study and the absence of an interatrial communication.It showed a superior vena cava thrombosed around a long-standing central venous catheter, with extensive venous collateralisation: the left brachiocephalic vein drained through a network of collaterals into a markedly dilated azygos vein.Largecalibre collaterals anastomosed directly with the pulmonary veins, creating a haemodynamically significant right-to-left pathway into the left atrium that bypassed the pulmonary capillary filter and provided the substrate for paradoxical embolism.After multidisciplinary discussion, the patient was treated with longterm oral anticoagulation with a vitamin K antagonist.This case highlights that a positive bubble study is not synonymous with a PFO.Identifying the site of microbubble entry into the left atrium, and performing bilateral arm injections when findings are atypical or septal crossing fails, can help distinguish an intracardiac from an extracardiac shunt and prevent unnecessary intervention.In patients with a history of central venous catheterisation, superior vena cava thrombosis with collaterals draining into the pulmonary veins is a rare cause of right-to-left shunting and paradoxical embolism, and venous imaging may be considered before closure.