What is the difference between a thrombus and an embolus in a patient presenting with absent pulses?

Thrombus vs. Embolus in Acute Limb Ischemia

A thrombus is a blood clot that forms locally at the site of arterial occlusion (typically on pre-existing atherosclerotic plaque), while an embolus is a clot or other material that travels from a distant source (usually the heart or proximal arteries) and lodges in a downstream vessel—this distinction is critical because embolic occlusions typically occur in previously healthy arteries and present more acutely, whereas thrombotic occlusions develop on diseased vessels with pre-existing symptoms.

Key Pathophysiologic Differences

Thrombus Formation

• A thrombus develops in situ at the site of vascular occlusion, typically forming on top of a ruptured or ulcerated atherosclerotic plaque in patients with chronic arterial disease 1
• Thrombotic occlusions occur in vessels with pre-existing atherosclerotic narrowing, meaning patients usually have a history of claudication or other symptoms of chronic limb ischemia before the acute event 1
• The underlying vessel is diseased with significant atherosclerotic burden, which CT angiography will reveal as diffuse plaque throughout the arterial tree 1

Embolus Characteristics

• An embolus originates from a distant source and travels through the bloodstream to lodge in a previously normal or minimally diseased artery 2
• The most common sources are cardiac (atrial fibrillation, ventricular thrombus, valvular disease, or endocarditis) or proximal arterial sources (aortic or iliac atherosclerotic plaque) 2
• Embolic occlusions typically occur at arterial bifurcations where the vessel caliber suddenly narrows, such as the common femoral artery bifurcation or popliteal trifurcation 1
• The vessel distal and proximal to the embolus is relatively normal on imaging, without significant atherosclerotic disease 1

Clinical Presentation Differences

Acute-on-Chronic (Thrombotic)

• Patients present with acute worsening of chronic symptoms rather than sudden onset 1
• History reveals prior claudication, rest pain, or tissue loss indicating pre-existing peripheral arterial disease 1
• Collateral circulation is often well-developed, which may provide some protection and result in less severe ischemia initially 1
• The limb may be more salvageable because chronic disease allows time for collateral development 1

Acute Embolic

• Presentation is sudden and dramatic with the classic "6 Ps": pain, pallor, pulselessness, paresthesias, paralysis, and poikilothermia 1
• No prior history of claudication or limb symptoms in most cases 1
• Absent collateral circulation means the ischemia is typically more severe and progresses more rapidly 1
• Higher risk of irreversible ischemia if not treated within 4-6 hours 1

Diagnostic Imaging Distinctions

CT angiography is the preferred imaging modality and will clearly differentiate between thrombotic and embolic causes 1:

Thrombotic Pattern on CTA

• Diffuse atherosclerotic disease throughout the arterial tree with calcified plaque 1
• Gradual tapering or occlusion at the site of chronic stenosis 1
• Well-developed collateral vessels visible around the occlusion 1
• Multiple levels of disease commonly present 1

Embolic Pattern on CTA

• Abrupt cutoff of contrast at the site of lodgment, typically at a bifurcation 1
• Normal-appearing proximal and distal vessels without significant atherosclerotic disease 1
• Absence of collateral circulation 1
• Meniscus sign may be visible where the embolus lodges 1

Management Implications

Thrombotic Occlusion Management

• Initial anticoagulation with unfractionated heparin to prevent propagation 1, 3
• Catheter-directed thrombolysis may be effective if symptoms present <14 days, with success rates of 85-95% 3
• Endovascular intervention or bypass surgery addresses both the acute occlusion and underlying atherosclerotic disease 1
• Long-term antiplatelet therapy and risk factor modification are essential 3

Embolic Occlusion Management

• Immediate anticoagulation with unfractionated heparin (80 U/kg bolus, then 18 U/kg/h infusion) 1, 3
• Surgical thromboembolectomy is often preferred for embolic occlusions because the vessel is otherwise healthy and the embolus can be extracted with a Fogarty catheter 1
• Mandatory cardiac workup to identify and treat the embolic source (echocardiography, rhythm monitoring) 2
• Long-term anticoagulation is typically required to prevent recurrent embolism, especially if atrial fibrillation or other cardiac source is identified 3

Critical Pitfalls to Avoid

• Do not assume chronic symptoms mean this is "just worsening PAD"—acute-on-chronic presentations still represent vascular emergencies requiring urgent revascularization 1
• Do not delay imaging with screening tests like ABI—proceed directly to CT angiography when acute limb ischemia is suspected 1
• Do not miss the cardiac source in embolic cases—failure to anticoagulate for atrial fibrillation or treat valvular disease leads to recurrent emboli 2
• Do not forget that right heart thrombi can be a source of paradoxical embolism if a patent foramen ovale is present, causing arterial emboli despite venous origin 4, 5
• Mobile thrombi in the right heart or aorta carry particularly high embolic risk and may require urgent intervention beyond anticoagulation alone 2, 6