Paradoxical embolism
Synonyms and keywords: Paradoxical embolization, paradoxical embolus, crossed embolism

# Overview
A paradoxical embolism refers to a phenomenon of dislodging a clot from venous vasculature which traverses through intracardiac or intrapulmonary shunt into systemic circulation . If dislodged into brain, it could cause end-organ ischemia depending on site of blockade e.g brain, kidney, gut, limb and/or heart etc.

# Etiology
It can occur from any condition with any condition with breach in a barrier between right and left sided circulation of heart. The most common pre-existing conditions that results in mixing of arterial and venous blood and eventually lead to paradoxical embolism include:
- Patent Foramen Ovale
- Atrial Septal Defect
- Arteriovenous Shunts
- Ventricular Septal Defects
Factors that enhance clotting mechanism beyond physiological requirements elevate the risks of incidence of paradoxical embolism e.g. genetic disorders of hypercoagulation (factor V Leiden deficiency, anti-thrombin III deficiency, protein C and S deficiency), increased estrogen levels (pregnancies and use of oral contraceptive pills), immobilization (related to surgery or disability) and malignancies .

# Pathophysiology and Presentation
The prerequisites for paradoxical embolism include presence of blood clot on the veins and their eventual bypass passage from venous to arterial blood systems through a breach in integrity of separating right and left sides of heart . When already present clot in form of deep vein thrombosis which is mostly in veins of lower extremities dislodges, it traverses through the right side to the left side of heart and eventually through systemic circulation lodges in end-artery. The manifestation of symptoms depend on size of clot and vessels blocked. The most commonly blocked vessels include:
- Cerebral Arteries (leading to stroke)
- Mesenteric Arteries (leading to acute or chronic mesenteric ischemia)
- Femoral Artery (limb ischemic)
- Renal Artery (renal infarction)
- Coronary Artery (acute myocardial infarction)

# Differential Diagnoses
- Deep vein thrombosis
- Endocarditis
- Atrial fibrillation leading to clot formation

# Work Up
Paradoxical embolism is a diagnosis of exclusion which needs extensive laboratory work up to exclude other causes of possible symptoms manifestation. However, three conditions are required to meet clinical diagnosis :
- Venous source of embolism
- Presence of intracardiac shunt or pulmonary fistula
- Arterial blockage
For detection of hypercoagulability, factor V Leiden assays and levels of anti-thrombin III, protein C and S are required.
Specialized studies for detection of intra-cardiac shunts include:
Echocardiography
Transthoracic and tranesophageal echocardiographies could be used for detection of intra-cardiac shunt anomalies. Due to non-invasive nature of transthoracic     echocardiography, it is mostly used. This method is mostly used for detection of site and size of intra-cardiac defect .
Bubble study using echocardiography could be used for detection of  small intra-cardiac defects. Saline with bubbles is injected through peripheral veins and the detection of bubbles on left side of heart through echocardiogram confirms the presence of intra-cardiac defect .
Transcranial Doppler Sonography
This method is traditionally used for detection of right-to-left cardiac shunts in conjunction with transesophageal echocardiography. Contrast saline with bubbles is injected in venous blood and patient is also to do Valsalva maneuver . Detection of microemboli in middle cerebral vessel through transcranial doppler sonography in conjunction with evidence of intra-cardiac defect via transesophageal echocardiography confirms the presence of defect .
Computed Tomography Resonance
It produced a high resolution imaging of cardiac structure and its vasculature with high sensitivity and specificity . However, it's not commonly used as it could potentially damage heart with exposure to ionizing radiations. Further, it could not evaluate functional aspects.

# Management
Acute management of paradoxical embolism is dependent on specific organ involvement. Generally, the aim is to remove clot to avoid irreversible damage to organs. If cranial and coronary vasculature are involved, patients are treated on basis of stroke and acute myocardial infarction management guidelines, respectively.
Secondary Prevention aims at risk factors that lead to formation of clots and their passage through intra-cardiac shunts.
- Aspirin, an oral anticoagulant, is used to prevent formation of clots .
- Percutaneous closure of intracardiac shunt (closure of an atrial septal defect (ASD), ventricular septal defect (VSD), or patent foramen ovale (PFO) is an outpatient procedure with minimal complications . Surgical Closure of intracardiac shunts is associated with operation related complications.
- Placement of a inferior vena cava (IVC) filter in patients with recurrent lower extremity deep vein thrombosis and contraindicated to anti-coagulants.
- Discontinuation of birth control pills if patient is taking prior to the embolism.
- Smoking cessation for smokers.