What is an arteriovenous malformation (AVM)?

What is an Arteriovenous Malformation (AVM)?

An arteriovenous malformation is an abnormal tangle of blood vessels that creates a direct connection between arteries and veins, bypassing the normal capillary bed and resulting in high-flow arteriovenous shunting of blood. 1

Structural Definition

AVMs are direct connections of artery to vein via abnormal dilated vascular channels without a normal intermediary capillary bed, creating a high-flow, low-resistance shunt between the arterial and venous systems. 1, 2

• The abnormal dilated vascular channels are known as the nidus, which represents the core tangle of dysplastic vessels 1, 3
• This pathological arteriovenous shunting fundamentally alters hemodynamics by allowing blood to bypass the capillary network entirely 2, 4
• The absence of the normal capillary bed means blood flows directly from high-pressure arteries into low-pressure veins, creating a fast-flow lesion 1

Diagnostic Gold Standard

Four-vessel cerebral contrast angiography is the diagnostic gold standard for brain AVMs, demonstrating the arteriovenous shunting with high spatial and temporal resolution. 1

• Cervicocerebral angiography remains the reference standard for imaging cerebrovascular AVMs, providing critical characterization of feeding vessels, nidus architecture, and venous drainage patterns 1, 2
• MRI typically shows an inhomogeneous signal void on T1- and T2-weighted sequences, often with hemosiderin deposits suggesting prior hemorrhage 5
• Doppler ultrasound demonstrates the hallmark finding of fast-flow with arterialized venous waveforms, distinguishing AVMs from other vascular lesions like venous varices 2

Key Distinguishing Features from Other Vascular Lesions

AVMs must be distinguished from other vascular anomalies based on their high-flow characteristics and direct arteriovenous connection. 1, 2

• AVMs are not cavernous malformations, dural arteriovenous fistulas, venous malformations, venous varices, or vein of Galen malformations 1
• Unlike venous varices (which are simply dilated veins), AVMs demonstrate high-velocity arterialized flow on Doppler evaluation 2
• AVMs are classified as vascular malformations (abnormal vessel formation) rather than vascular tumors (true neoplasms with increased mitotic activity) 1

Clinical Presentation and Natural History

Brain AVMs most commonly present with hemorrhage (50-60% of cases, >75% in children), representing the primary cause of mortality and morbidity. 6

• The annual hemorrhage risk is approximately 1-3% per year for previously unruptured brain AVMs 5, 6, 3
• After initial rupture, the hemorrhage risk increases fivefold to 4.8% annually 1, 6
• Mortality from first hemorrhage ranges 10-30%, with 10-20% of survivors experiencing permanent disability 5, 6
• Other presentations include seizures, focal neurological deficits, or incidental discovery on imaging 1

Location and Prevalence

AVMs can occur in multiple organ systems, with brain and pulmonary locations being most clinically significant. 1

• The asymptomatic prevalence of brain AVMs on MRI is estimated at 0.05% 1
• Pulmonary AVMs occur in 70-90% of patients with hereditary hemorrhagic telangiectasia (HHT), an autosomal dominant disorder 5, 6
• Between 10-20% of HHT patients will develop at least one AVM during their lifetime 1, 2
• Soft tissue AVMs represent approximately 8% of all vascular malformations 1

Risk Factors for Hemorrhage

Prior hemorrhage is the strongest predictor of future bleeding in patients with AVMs. 5, 6

• Small AVM size paradoxically increases hemorrhage risk 5, 6
• Presence of intranidal aneurysms significantly elevates rupture risk 1, 5, 6
• Deep venous drainage patterns and deep location (periventricular or intraventricular) increase hemorrhage risk 1, 5, 6
• Venous outflow obstruction is associated with higher bleeding rates 1

Associated Conditions

The majority of pulmonary AVMs (70-90%) are associated with hereditary hemorrhagic telangiectasia. 5, 6

• HHT type 1 (endoglin mutation) presents with cerebral and pulmonary AVMs 6
• HHT type 2 (ALK1 mutation) presents with pulmonary hypertension and hepatic AVMs 6
• Multiple AVMs in different organ systems strongly suggest hereditary HHT rather than sporadic disease 2
• Other causes of AVMs include trauma, surgery, certain infections, and hepatopulmonary syndrome 5

Common Pitfalls

Never perform liver biopsy in patients with proven or suspected HHT due to catastrophic hemorrhage risk from hepatic vascular malformations. 2

• Pregnancy dramatically increases the risk of AVM hemorrhage due to hormonal and hemodynamic changes, requiring heightened surveillance 5, 6
• 25-58% of patients are completely asymptomatic before catastrophic complications occur, emphasizing the critical importance of screening in high-risk populations 6
• Vessel loops and infundibular origins can lead to false-positive diagnoses of AVMs on MRA 1