What are the causes of a mural thrombus in the distal abdominal aorta?

Causes of Mural Thrombus in the Distal Abdominal Aorta

Mural thrombus in the distal abdominal aorta without aneurysm or significant atherosclerosis is most commonly caused by systemic hypercoagulable states, inflammatory conditions, or underlying prothrombotic disorders, particularly in younger patients. 1, 2

Primary Etiologic Categories

Hypercoagulable and Prothrombotic States

• Systemic hypercoagulability is the predominant mechanism when mural thrombus occurs in a normal or minimally atherosclerotic aorta, particularly in patients under 50 years of age. 1, 3
• Specific prothrombotic disorders include protein C deficiency, protein S deficiency, antithrombin III deficiency, factor V Leiden mutation, prothrombin gene mutations, and hyperhomocysteinemia. 4
• Presence of lupus anticoagulant or anticardiolipin antibodies represents another important prothrombotic mechanism. 4
• Inflammatory bowel disease, particularly active ulcerative colitis, creates a hypercoagulable state that can directly cause abdominal aortic mural thrombus formation. 5

Atherosclerotic and Degenerative Disease

• Atherosclerotic plaque rupture with overlying thrombus formation is the most common mechanism in older patients with cardiovascular risk factors. 4
• Penetrating atherosclerotic ulcers in the abdominal aorta allow blood to enter the aortic media, creating localized thrombus formation. 4
• Mild atherosclerosis of the aortic wall increases the risk of mural thrombus formation by 2.5-fold compared to completely normal aortic walls. 2

Infectious and Inflammatory Causes

• Mycotic aneurysms and aortic infections can present with mural thrombus, though this is less common in the infrarenal abdominal aorta (15-25% of mycotic aneurysms) compared to other locations. 4
• Vasculitic diseases including Takayasu arteritis, Behçet's syndrome, and giant cell arteritis can affect the abdominal aorta and promote thrombus formation. 4
• Syphilitic aortitis and other infectious aortitides represent rare but important inflammatory causes. 4

Iatrogenic and Traumatic Causes

• Cardiac catheterization, angioplasty procedures, and aortic surgery can cause iatrogenic aortic dissection or endothelial injury leading to mural thrombus. 4
• Blunt abdominal trauma can cause aortic wall injury with subsequent thrombus formation. 4

Drug-Associated Causes

• Cocaine and amphetamine use are associated with aortic aneurysm formation and dissection, which can subsequently develop mural thrombus. 4
• Paradoxically, heparin therapy itself has been historically implicated in promoting white thrombus formation in the aorta, though this represents a more generalized prothrombotic problem. 6

Critical Risk Factor Clustering

Patients with mural thrombus typically present with at least two of five key risk factors: heart disease, recent thrombophlebitis, heparin therapy, abdominal atherosclerosis, and postoperative status. 6

Location-Specific Patterns

• Mural thrombi in the distal abdominal aorta (infrarenal region) are more commonly associated with atherosclerotic disease compared to thoracic locations. 4, 6
• Thrombi can occur anywhere from T-10 to the aortic bifurcation, with suprarenal location in approximately 50% of cases. 6

Imaging Characteristics That Suggest Etiology

Features Suggesting Infectious Cause

• Absence or minimal calcification in the aortic wall suggests mycotic aneurysm rather than atherosclerotic disease. 4
• Periaortic soft tissue stranding, fluid, or gas strongly suggests infectious etiology. 4
• Rapid enlargement indicates active infection or impending rupture. 4

Features Suggesting Atherosclerotic Cause

• Common intimal calcifications and fusiform shape suggest atherosclerotic disease. 4
• Mural thrombus is more common with atherosclerotic aneurysms than with mycotic aneurysms. 4

Important Clinical Pitfalls

• Smoking is significantly more prevalent in patients who ultimately require surgical intervention, suggesting more aggressive disease. 2
• The presence of mild atherosclerosis (rather than completely normal aorta) increases recurrence risk and should lower the threshold for surgical intervention. 2
• Thrombus location in the ascending aorta or arch carries 12-18 times higher odds of recurrence compared to abdominal locations, though this is less relevant for distal abdominal thrombi. 2
• Malignancy-associated hypercoagulability represents an often-overlooked cause of aortic mural thrombus. 4