What is an arteriovenous malformation?


An arteriovenous malformation (AVM) is an abnormal tangle of blood vessels made up of arteries and veins. In a normal circulatory system, blood flows from the heart through arteries, into tiny capillaries that supply oxygen and nutrients to tissues, and then back to the heart through veins. In an AVM, the blood flows directly from arteries to veins, bypassing the capillaries and the surrounding tissue.

Because of this abnormal connection, the blood vessels in an AVM are often twisted, disorganized, and fragile. They do not supply nutrients to nearby tissues the way normal vessels do, and they can cause problems if they bleed or press on surrounding structures.

Where do arteriovenous malformations occur?

AVMs can develop in any part of the body, but the most common and potentially serious locations are:

  • Brain

  • Spinal cord

  • Liver

  • Lungs

  • Skin or soft tissue

AVMs may be present at birth (congenital) or develop over time. In many cases, the exact cause is unknown.

What are the symptoms of an arteriovenous malformation?

The symptoms of an AVM depend on its location, size, and whether it is causing pressure on nearby tissues or bleeding. Some AVMs cause no symptoms and are discovered by chance on imaging tests.

Possible symptoms include:

  • Headaches, seizures, or neurological changes (for AVMs in the brain)

  • Back pain, numbness, or weakness (for spinal cord AVMs)

  • Abdominal pain or swelling (for liver AVMs)

  • Bleeding or visible swelling under the skin (for skin or soft tissue AVMs)

In some cases, bleeding from an AVM can be life-threatening, especially if it occurs in the brain or spinal cord.

Why are arteriovenous malformations important?

AVMs are important because they can lead to serious complications, including:

  • Bleeding (hemorrhage) – Fragile AVM vessels are more likely to rupture and cause internal bleeding

  • Tissue damage – Nearby tissues may not get enough oxygen or nutrients, leading to damage or dysfunction

  • Pressure effects – Large AVMs can press on surrounding structures and cause pain or neurological symptoms

  • Seizures or stroke-like symptoms – Particularly in brain AVMs

The risks depend on the location, size, and whether the AVM has bled in the past.

How is an arteriovenous malformation diagnosed?

AVMs are often diagnosed using imaging tests, including:

  • MRI or CT scans – to detect the AVM and look for signs of bleeding or pressure on nearby tissues

  • Angiography – a special imaging test that uses dye to show how blood flows through the vessels

  • Ultrasound – useful for AVMs in soft tissue or superficial locations

In some cases, a biopsy may be performed if the AVM is found in an unexpected area or needs to be confirmed.

What does an arteriovenous malformation look like under the microscope?

When examined under the microscope, an arteriovenous malformation (AVM) appears as a tangled mass of abnormal blood vessels. Unlike normal tissue, which has a clear separation between arteries, capillaries, and veins, an AVM shows direct connections between arteries and veins with no capillaries in between.

The blood vessels in an AVM often have:

  • Thick, irregular walls – from high-pressure blood flow.

  • Unusual shapes and sizes – including large, dilated spaces or tightly coiled vessels.

  • Fibrosis or scarring – may be seen in older lesions, especially if there has been bleeding or repeated trauma

  • Inflammation or clotting – in cases where the AVM has bled or caused irritation.

These features help the pathologist confirm that the tissue is consistent with an AVM and not a tumour or another type of vascular lesion.

How are arteriovenous malformations treated?

Treatment depends on the AVM’s location, size, and whether it is causing symptoms or has a risk of bleeding. Options may include:

  • Monitoring only – for small AVMs that are not causing symptoms.

  • Surgery – to remove the AVM if it can be done safely.

  • Embolization – a procedure that blocks the blood vessels feeding the AVM using a special substance.

  • Radiation therapy – used in some brain AVMs to shrink the malformation over time.

Your doctor will work with a team of specialists to decide on the safest and most effective treatment.

Questions to ask your doctor

  • What symptoms should I watch for?

  • What are the risks of bleeding or other complications?

  • Do I need treatment now, or can the AVM be monitored?

  • What are my treatment options, and what are the risks and benefits?

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