What is an arteriovenous malformation?

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 non-nutritive blood. 1, 2

Core Structural Definition

• AVMs are structurally abnormal vascular communications where feeding arteries connect directly to draining veins through a complex vessel network (called the nidus) without the intervening normal capillary bed. 1, 2, 3

• The absence of the capillary bed fundamentally alters hemodynamics, creating pathological high-flow arteriovenous shunting that distinguishes AVMs from other vascular anomalies. 2

• AVMs demonstrate fast-flow with arterialized venous waveforms on Doppler ultrasound, which is the hallmark diagnostic finding. 2

• Four-vessel cerebral contrast angiography is considered the diagnostic gold standard for brain AVMs, while digital cerebral angiography remains the reference standard for characterizing high-flow lesions due to superior spatial and temporal resolution. 1, 4

Location and Types

• Brain AVMs (BAVMs): These are abnormal tangles of vessels in the brain that result in arteriovenous shunting demonstrated by cerebral angiography. 1

• Pulmonary AVMs (PAVMs): These are structurally abnormal vascular communications providing continuous right-to-left shunt between pulmonary arteries and veins. 1

• Head and neck AVMs: These can present region-specific clinical manifestations affecting cosmetic appearance, ingestion, respiratory, and neuronal functions. 5

• AVMs can occur in virtually any organ system, including the liver, adnexa, and other locations, though brain and lung are most clinically significant. 6, 7

Critical Clinical Significance

Brain AVMs Present Life-Threatening Risks:

• Hemorrhage is the most common initial presentation (50-60% of all cases, >75% in children), representing the primary cause of mortality and morbidity. 6

• Mortality from first hemorrhage ranges 10-30%, with 10-20% of survivors experiencing permanent disability. 6, 4

• Annual hemorrhage risk is approximately 1.3% per year for unruptured brain AVMs, increasing to 4.8% for previously ruptured lesions. 6, 4

• Recurrent hemorrhage risk is highest in the first year (6-32.9%) following initial bleeding. 6

Pulmonary AVMs Cause Paradoxical Embolic Complications:

• Transient ischemic attacks and cerebral strokes occur in 3.2-55% of patients with pulmonary AVMs due to loss of the pulmonary capillary bed's filtering function. 6

• Cerebral abscesses develop in 0-25% of cases from systemic infections bypassing pulmonary filtration. 6

• Hypoxemia occurs in 27-71% of patients, manifesting at rest or during exercise. 6

• Massive hemoptysis and hemothorax are rare (0-2%) but life-threatening emergencies. 6

• Pregnancy dramatically increases hemorrhage risk due to hormonal and hemodynamic changes. 6

Association with Hereditary Hemorrhagic Telangiectasia (HHT)

• 70-90% of pulmonary AVMs occur with HHT, an autosomal dominant disorder. 6

• Multiple AVMs in the same individual, particularly in different organ systems, strongly suggest hereditary HHT rather than sporadic disease. 2

• 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

• Between 10-20% of HHT patients will develop at least one AVM during their lifetime, most commonly in the lungs, brain, or liver. 2

Key Distinguishing Features from Other Vascular Lesions

• AVMs must be distinguished from venous varices, which are simply abnormally dilated veins without arteriovenous shunting. 2

• Venous varices show low-velocity or absent flow on Doppler evaluation, in stark contrast to the high-flow characteristics of AVMs. 2

• AVMs are distinct from cavernous malformations, dural arteriovenous fistulas, venous malformations, venous varices, and pure vein of Galen AVMs. 1

Clinical Presentation Patterns

• 25-58% of patients are completely asymptomatic before catastrophic complications occur, emphasizing the importance of screening in high-risk populations (especially HHT patients). 6

• AVMs can remain clinically silent for decades before becoming symptomatic. 6

• Most symptoms depend on size, number, type (complex versus simple), and flow through the malformations. 6

• Clinical presentation may include hemorrhage, seizures, focal neurological deficits, or incidental discovery. 1