What is an aneurysm?

What is an Aneurysm?

An aneurysm is an abnormal focal dilation of an artery where the vessel wall becomes weakened and expands to a diameter more than 1.5 times (>50%) larger than the normal predicted diameter for that vessel. 1

Core Definition and Pathophysiology

An aneurysm represents a permanent, localized expansion of an arterial wall with attenuation (thinning) of the vessel structure. 1 This occurs when the mechanical stress from blood pressure exceeds the structural strength of the weakened arterial wall. 2

The fundamental pathology involves:

• Thinning of the media and adventitia due to loss of vascular smooth muscle cells 2
• Degradation of the extracellular matrix that normally maintains vessel integrity 2
• Weakening of the vessel wall that allows progressive expansion under arterial pressure 1

Types of Aneurysms

Saccular Aneurysms

Saccular aneurysms are the most common type, characterized by a distinct neck and dome structure where axial and lateral dimensions are nearly equal. 1, 3 These typically occur at major arterial bifurcations, particularly at the Circle of Willis in cerebral circulation. 3, 4

Fusiform Aneurysms

These involve circumferential dilation of the entire arterial segment, creating an elongated, spindle-shaped expansion. 1

Other Pathological Types

Additional variants include dolichoectatic, dissecting, serpentine, traumatic, and mycotic (infectious) aneurysms, each with distinct pathophysiological mechanisms. 1

Anatomical Distribution

Thoracic Aortic Aneurysms

• 60% occur in the ascending aorta 1
• 10% in the aortic arch 1
• 30-40% in the descending thoracic aorta 1

Abdominal Aortic Aneurysms (AAA)

Most commonly affect the infrarenal aorta and represent a major cause of mortality, with 150,000-200,000 deaths annually worldwide. 5

Cerebral Aneurysms

Predominantly occur at branching points of the Circle of Willis, with 55-77% involving the middle cerebral artery. 1, 4

Peripheral Aneurysms

Can affect extremity vessels and typically present as asymptomatic incidental findings or with local compression symptoms. 6

Risk Factors and Epidemiology

The most important modifiable risk factors are:

• Hypertension (present in 80% of thoracic aortic aneurysm cases) 1
• Smoking (strongest environmental risk factor) 7, 2
• Male sex (3:1 male predominance for atherosclerotic aneurysms) 1, 2
• Age >60-65 years 7, 2

Women are more susceptible to cerebral aneurysms, while men predominate for aortic aneurysms. 1

Genetic factors account for approximately 20% of thoracic aortic aneurysms, with familial predispositions requiring evaluation. 1

Clinical Presentation

Asymptomatic Phase

The majority of aneurysms remain asymptomatic until catastrophic rupture occurs. 1, 7, 2 Approximately 30% of abdominal aortic aneurysms are discovered incidentally as a pulsatile abdominal mass on routine examination. 7

Symptomatic Presentations

For cerebral aneurysms:

• Severe headache, nausea, vomiting, photophobia, nuchal rigidity when ruptured 1
• Altered consciousness, focal weakness, cranial nerve deficits 1
• Subarachnoid hemorrhage is the typical presentation of ruptured saccular aneurysms 3

For aortic aneurysms:

• Back pain (particularly with contained rupture or vertebral erosion) 1
• Abdominal pain with potential thrombosis or embolization 7
• Pulsatile abdominal mass 1, 7

Rupture Risk

Rupture risk increases dramatically with aneurysm diameter, with mortality rates of 65-85% for ruptured abdominal aortic aneurysms. 2 The risk of rupture is influenced by size, expansion rate, continued smoking, and persistent hypertension. 7

Diagnostic Approach

Ultrasonography is the screening modality of choice for abdominal aortic aneurysms due to high sensitivity, specificity, safety, and lower cost. 7

CT angiography (CTA) and MRI are used to define anatomy, characterize size and thrombus, and assess rupture risk. 6, 2

Physical examination alone misses more than 60% of aneurysms, making imaging essential for diagnosis. 8

Size Thresholds and Clinical Significance

In clinical practice, aortic root dilatation is suspected when:

• Diameter >40 mm in males 1
• Diameter >36 mm in females 1
• Indexed diameter/BSA (aortic size index) >22 mm/m² 1

Treatment thresholds for aortic aneurysms are typically:

• ≥5.5 cm for men 3
• ≥5.0 cm for women 3

Critical Pitfalls

Aneurysms can be easily confused with other pathology including hematomas, abscesses, or neoplastic processes, particularly when causing vertebral body erosion. 1, 8

Mycotic (infectious) aneurysms represent a distinct entity caused by bacterial infection of the arterial wall, most commonly associated with infective endocarditis, and require different management strategies. 1

Pseudoaneurysms are fundamentally different, representing full-thickness arterial wall disruption with extravascular blood containment rather than true vessel wall expansion. 8