What is an Arteriovenous Malformation (AVM)?

Arteriovenous Malformation (AVM)

An arteriovenous malformation (AVM) is an abnormal tangle of blood vessels characterized by direct connections between arteries and veins without an intervening capillary bed, resulting in high-flow shunting of blood that can lead to hemorrhage, seizures, and neurological deficits. 1

Definition and Pathophysiology

AVMs are vascular anomalies defined by:

• Direct arteriovenous shunting (nonnutritive blood flow) between the high-pressure arterial system and low-pressure venous system 1
• Absence of the normal capillary network that typically connects arteries and veins 1, 2
• A central nidus (the abnormal dilated vascular channels where arteriovenous shunting occurs) 1

Epidemiology

• Asymptomatic prevalence on MRI is estimated at 0.05% 1
• Detection rate of symptomatic cases is approximately 1.2 per 100,000 person-years 1
• Typically diagnosed before age 40, though occasionally seen in elderly patients 1
• 10-20% of patients with hereditary hemorrhagic telangiectasia will develop at least one AVM during their lifetime 1

Clinical Presentation

AVMs commonly present with:

1. Hemorrhage (>50% of cases)

   • Intracerebral hemorrhage most common
   • Subarachnoid or intraventricular hemorrhage can also occur
   • AVM-related hemorrhage accounts for approximately 1% of all strokes 1

2. Seizures (20-25% of cases)

   • Can be focal or generalized
   • May indicate the location of the lesion 1

3. Headaches (15% of patients) 1

4. Focal neurological deficits (<5% of cases) 1

5. Other presentations:

   • Pulsatile tinnitus
   • In children <2 years: congestive heart failure, hydrocephalus, seizures 1
   • Incidental finding during imaging for unrelated conditions 1

Natural History and Risk Assessment

• Annual risk of initial hemorrhage: 2-3% per year for unruptured AVMs 1
• Mortality from first hemorrhage: 10-30% 1
• Long-term disability among survivors: 10-20% 1
• Risk of recurrent hemorrhage is elevated in the first year after initial bleeding:
  • 6-17.9% in the first year, then returns to baseline 1
  • May be even higher (25%) after second hemorrhage 1

Risk factors for hemorrhage include:

• Intranidal aneurysm
• Deep venous drainage
• Deep location
• Venous outflow obstruction 1

Diagnostic Imaging

Gold Standard

• Cerebral angiography (arteriography): Provides highest spatial and temporal resolution, critical for characterizing:
  • Nidus architecture
  • Feeding vessels
  • Intranidal aneurysms
  • Venous drainage patterns 1

Additional Imaging

• MRI: Shows inhomogeneous signal void on T1/T2-weighted sequences, often with hemosiderin suggesting prior hemorrhage 1
• MRA: Provides noninvasive vascular information but lacks detail on intranidal aneurysms and venous drainage 1
• CTA: 90% sensitivity for overall AVM detection, 100% for AVMs >3cm 1

Treatment Approaches

Treatment options include:

1. Medical management: May be appropriate for unruptured AVMs based on the ARUBA trial, which suggested medical management alone was superior to interventional therapy for preventing death or stroke in unruptured AVMs 1

2. Surgical resection: Provides immediate obliteration but carries risk of neurological morbidity 3

3. Endovascular embolization: Minimally invasive but typically achieves cure in only a minority of cases 3, 4

4. Stereotactic radiosurgery: Minimally invasive with little immediate morbidity, but obliteration occurs in a delayed fashion 3

5. Combination approaches: Often used for complex AVMs 2

Key Considerations in Management

• Risk-benefit analysis must weigh lifetime hemorrhage risk against intervention risks
• Complete nidal obliteration is the goal of AVM intervention
• Location, size, and vascular architecture influence treatment selection
• For brain AVMs, the recommendation is to obtain both MRI and 4-vessel angiogram to delineate anatomy before treatment planning 1

Complications

• Hemorrhage (most serious complication)
• Progressive neurological deficits
• Seizures
• Hydrocephalus (rare)
• High-output heart failure (in severe cases, especially in young children) 1, 5

AVMs require careful evaluation by an interdisciplinary team of experienced physicians to determine the optimal management strategy that balances the risk of intervention against the natural history of the disease.