Note: This copy is for your personal non-commercial use only. To order presentation-ready copies for distribution to your colleagues or clients, contact us at www.rsna.org/rsnarights. PARADOXICAL EMBOLISM PARADOXICAL 1571 Paradoxical Embolism: Role of Imaging in Diagnosis and Treat- ment Planning1 Farhood Saremi, MD Neelmini Emmanuel, MD Paradoxical embolism (PDE) is an uncommon cause of acute Philip F. Wu, BS arterial occlusion that may have catastrophic sequelae. The pos- Lauren Ihde, MD sibility of its presence should be considered in all patients with an David Shavelle, MD arterial embolus in the absence of a cardiac or proximal arterial John L. Go, MD source. Despite advancements in radiologic imaging technology, Damián Sánchez-Quintana, MD, PhD the use of various complementary modalities is usually necessary to exclude other possibilities from the differential diagnosis and Abbreviations: DVT = deep venous thrombo- achieve an accurate imaging-based diagnosis of PDE. In current sis, IVC = inferior vena cava, PDE = paradoxical embolism, PFO = patent foramen ovale practice, the imaging workup of a patient with symptoms of PDE usually starts with computed tomography (CT) and magnetic RadioGraphics 2014; 34:1571–1592 resonance (MR) imaging to identify the cause of the symptoms Published online 10.1148/rg.346135008 and any thromboembolic complications in target organs (eg, Content Codes: stroke, peripheral arterial occlusion, or visceral organ ischemia). 1From the Departments of Radiology (F.S., Additional imaging studies with modalities such as peripheral N.E., P.F.W., L.I., J.L.G.) and Cardiovascular venous Doppler ultrasonography (US), transcranial Doppler US, Medicine (D.S.), University of Southern Califor- echocardiography, and CT or MR imaging are required to detect nia, USC University Hospital, 1500 San Pablo St, Los Angeles, CA 90033; and Department of peripheral and central sources of embolism, identify cardiac and/ Human Anatomy, University of Extremadura, or extracardiac shunts, and determine whether arterial disease is Badajoz, Spain (D.S.Q.). Recipient of a Cer- tificate of Merit award for an education exhibit present. To guide radiologists in selecting the optimal modalities at the 2012 RSNA Annual Meeting. Received for use in various diagnostic settings, the article provides detailed January 3, 2013; revision requested April 4 and received July 13; accepted July 19. For this information about the imaging of PDE, with numerous radiologic journal-based SA-CME activity, the authors, and pathologic images illustrating the wide variety of features that editor, and reviewers have disclosed no relevant may accompany and contribute to the pathologic process. The relationships. Address correspondence to F.S. (e-mail: [email protected]). roles of CT and MR imaging in the diagnosis and exclusion of PDE are described, and the use of imaging for planning surgical SA-CME LEARNING OBJECTIVES treatment and interventional procedures is discussed. After completing this journal-based SA- ©RSNA, 2014 • radiographics.rsna.org CME activity, participants will be able to: ■■Describe the causes of PDE and se- quelae in target organs. ■■Discuss the specific uses of various im- Introduction aging modalities in the diagnosis of PDE. Paradoxical embolism (PDE) is usually definitively diagnosed at ■■Recognize CT and MR imaging fea- autopsy or at radiologic imaging when a thrombus that crosses an tures that are pertinent for the diagnosis of PDE and for posttreatment evaluation. intracardiac defect is seen in the setting of arterial embolic damage in end organs (eg, stroke). Imaging evaluation of patients in whom See www.rsna.org/education/search/RG. the presence of PDE is suspected usually necessitates the use of more than one modality. Peripheral Doppler ultrasonography (US) and echocardiography are well-established methods for assessing thromboembolic processes. Although echocardiography is the prime modality for depicting a shunt across a patent foramen ovale (PFO), no single modality can cover the whole spectrum of findings in the imaging workup of PDE. 1572 October Special Issue 2014 radiographics.rsna.org The article outlines the optimal imaging ap- proach in various clinical settings and the value Table 1: Essential Elements of PDE contributed by each imaging modality for accu- Systemic embolism confirmed by clinical, an- rate diagnosis of PDE. The current roles of com- giographic, or pathologic findings without an puted tomography (CT) and magnetic resonance apparent source on the left side of the heart or in the proximal arterial tree (ascending aorta) (MR) imaging in identifying cardiac and extra- Embolic source within the venous system cardiac abnormalities known to contribute to the development of PDE and detecting sequelae in Abnormal intracardiac or intrapulmonary commu- nication between the right and left circulations target organs are emphasized, and the utility of Pressure gradient that promotes a right-to-left shunt supplemental US studies is reviewed. Strategies at some point during the cardiac cycle for treating PDE, including interventional tech- niques, also are described. Historical target organs. Additional imaging studies, includ- Background and Definitions ing peripheral venous Doppler US, transcranial In 1877, Cohnheim (1) reported the first case Doppler US, echocardiography, and CT, are used of PDE by describing the path of an embolus to detect peripheral and central sources of embo- through a septal defect in the heart. In 1881, lism, arterial disease, and cardiac or extracardiac Zahn (2) reported an autopsy study in which shunts. Further diagnostic testing often includes thrombosis of the uterine vein, multiple systemic continuous long-term electrocardiographic re- emboli, and a branched thrombus within a PFO cordings, blood chemistry panels, and coagula- were seen in the same cadaver. Later, in 1885, he tion tests. used the term paradoxical embolism to describe a condition in which emboli derived from the ve- Types of Embolism nous system reached the systemic arterial system Thrombi from tributaries of the IVC are the through an abnormal communication between major sources of embolism, but emboli of fat, the heart chambers (3). air, amniotic fluid, and tumor tissue have also Four essential elements contribute to the devel- been described (10–15). Fat embolism syndrome opment of PDE: systemic embolism, an embolic is primarily a pulmonary disease (10). Shunt- source, a right-to-left shunt, and a pressure gradi- ing of fat or other material across a PFO can be ent across the shunt (Table 1) (3–6). The diagnosis precipitated by increased right atrial pressure for of PDE is considered definitive when it is based on a variety of reasons, including changes in body a finding at autopsy or at imaging of a thrombus position, breathing patterns, and intrathoracic that crosses an intracardiac defect in the setting of pressure. Paradoxical air embolism can lead to an arterial embolus (4). A diagnosis of PDE in the cerebral lesions in scuba divers (11). Cerebral absence of these findings is considered presump- air embolism can occur through central venous tive (4,6). The triad of systemic embolism, venous catheters (12). Patients undergoing neurosurgery thrombosis, and intracardiac communication in a sitting position have a risk for paradoxical defines the clinical diagnosis of PDE and allows air embolism (13). In these cases, preoperative treatment with a high level of confidence (7,8). detection of PFO and additional monitoring and The diagnosis of PDE is termed “possible” if an special care during surgery are advised. Amni- arterial embolus and PFO are detected; many phy- otic fluid embolism can rarely be complicated sicians treat patients on the basis of a diagnosis of by PDE resulting from increased pressure in the “possible PDE” (9). right side of the heart due to the release of vaso- Most early case reports of paradoxical embolus active substances when amniotic fluid enters the were based on autopsy findings (4). Later, an in- pulmonary circulation (14). tracardiac right-to-left shunt was demonstrated in Imaging findings of PDE complications in the a living patient when dye injected into the inferior brain are probably similar for different types of vena cava (IVC) appeared earlier than expected embolism, and the clinical history is important at the left brachial artery (6). Limited catheter- for final diagnosis. Air emboli absorb quickly and ization of the right side of the heart was proposed are best depicted in an early stage at CT. as a method for excluding an intracardiac shunt in patients with coexistent venous thrombosis or Peripheral Sources pulmonary embolism and arterial embolism. of Venous Thromboembolism In current practice, the imaging workup of a Venous thrombosis in the legs may be the most patient for PDE usually starts with CT and MR common source of embolus. Approximately 90% imaging. These modalities are used to diagnose of symptomatic pulmonary emboli arise from thromboembolic sequelae of arterial embolism in thrombi located in the leg veins (8,16). In most RG • Volume 34 Number 6 Saremi et al 1573 studies, the prevalence of deep venous thrombo- venography for evaluation after a stroke, DVT sis (DVT) in patients with acute pulmonary em- was found within 3.25 days after the occurrence bolism appears to be higher than that in patients of a stroke in 27% of those with cryptogenic with a cryptogenic stroke and PFO (16,17). In brain ischemia and an interatrial communica- many cases of PDE, the source