Fundamental Structural Differences Between Aneurysms and AVMs
An aneurysm is a focal outpouching or dilation of an arterial wall, while an arteriovenous malformation (AVM) is a tangle of abnormal blood vessels (nidus) that directly connects arteries to veins without an intervening capillary bed. 1
Key Distinguishing Features
Aneurysm Characteristics
- Localized arterial wall weakness resulting in a balloon-like expansion of a single vessel 1
- Typically occurs at arterial bifurcations or branch points
- Does not involve abnormal arteriovenous shunting
- Presents as a discrete, saccular or fusiform dilation 1
AVM Characteristics
- Complex vascular nidus consisting of abnormal dilated vascular channels that directly shunt high-pressure arterial blood into low-pressure venous system 1
- Lacks normal capillary bed intermediary between arteries and veins 1
- Involves multiple feeding arteries and draining veins 1
- Creates high-flow hemodynamic state with altered regional blood flow 1
Clinical Significance of Their Coexistence
These lesions frequently occur together, with aneurysms found in 7-17% of AVM patients, making their distinction clinically critical 1. The aneurysms associated with AVMs are classified into distinct subtypes:
- Flow-related aneurysms on feeding arteries (proximal or distal) that develop due to hemodynamic stress from the high-flow AVM 1
- Intranidal aneurysms within the AVM nidus itself 1
- Remote aneurysms at typical Circle of Willis locations unrelated to AVM hemodynamics 1
Critical Management Distinction
When both lesions coexist and cause hemorrhage, the aneurysm is usually the bleeding source and carries higher morbidity and mortality than AVM rupture, necessitating aneurysm treatment first or simultaneously 1. Flow-related aneurysms may spontaneously involute after AVM treatment due to normalized hemodynamics, while remote aneurysms require separate definitive treatment 1.
Hemorrhage Risk Profiles
Aneurysm Rupture
- Intraoperative hemorrhage risk of 7-41% during AVM surgery due to hemodynamic changes 1
- Higher immediate mortality when ruptured compared to AVM hemorrhage 1
AVM Rupture
- Annual rupture risk: 1.3% for unruptured AVMs, 4.8% for previously ruptured lesions 1, 2
- First hemorrhage mortality: 10-30% with 10-20% long-term disability 2
- Higher risk features include intranidal aneurysms, deep venous drainage, deep location, or venous outflow obstruction 1, 2
Diagnostic Imaging Approach
Digital cerebral angiography remains the gold standard for distinguishing these lesions due to superior spatial and temporal resolution 1, 2. Angiography definitively demonstrates:
- Aneurysm: focal arterial dilation with defined neck and dome
- AVM: complex nidus with early venous filling, multiple feeding arteries, and abnormal arteriovenous shunting 1
CTA demonstrates 90% sensitivity for AVM detection and 88% for associated aneurysms, but angiography is essential for treatment planning 1.