Arteriovenous malformation (AVM) is a rare condition where blood vessels form a tangled web, creating abnormal connections between arteries and veins without the usual network of capillaries in between. This unusual structure can occur anywhere in the body but is most commonly found in the brain or spinal cord, where it poses the greatest health risks.
Understanding Arteriovenous Malformation
In a healthy body, blood flows in an organized circuit. Arteries carry oxygen-rich blood from the heart to organs and tissues. Veins carry blood back to the heart and lungs after oxygen has been used. Between arteries and veins lie capillaries, tiny blood vessels that slow down blood flow so that organs like the brain can absorb oxygen and nutrients.[1]
When an arteriovenous malformation exists, these capillaries are missing. Blood rushes directly from arteries into veins at a much faster rate than normal. This abnormal connection looks like a bird’s nest or tangled web of blood vessels. Because veins are not built to handle the high pressure that comes from arteries, the walls of these blood vessels can weaken over time. This creates a risk that the vessels may burst or leak blood into surrounding tissues.[2]
The missing capillaries mean that nearby tissues do not receive the oxygen they need. Over time, this can lead to tissue damage and the death of nerve cells and other cells. The force of high-pressure blood flowing through the AVM can also cause the malformation to grow larger as more blood flows through it.[3]
Types of Arteriovenous Malformations
There are two main types of AVMs. Brain arteriovenous malformations develop within the brain tissue, on the surface of the brain, in the brainstem, or in the spinal cord. These are the most common locations for AVMs to occur. Peripheral arteriovenous malformations can form anywhere else in the body’s extensive network of blood vessels, including the face, arms, legs, and internal organs such as the heart, liver, or lungs.[3]
Epidemiology
Arteriovenous malformations are rare conditions. They are present in approximately 1 in 100,000 people in the general population. This makes them an uncommon finding, though they can have serious consequences when they do occur.[3]
Although anyone can be born with an AVM, healthcare providers most commonly discover them in people between the ages of 20 and 40. The risk of experiencing symptoms from an AVM is highest between ages 30 and 50. The most common age for brain AVMs to be diagnosed is in a person’s 30s to 40s, though they can be found in children as well as older adults.[3][5]
Men and women appear to be equally likely to have an arteriovenous malformation. There is no known difference in occurrence between sexes.[5]
Most AVMs are discovered accidentally during medical imaging performed for other reasons. Many people with AVMs never experience symptoms. However, when symptoms do appear, bleeding is often the first sign. About 50 percent of people with a brain AVM experience a brain bleed as their initial symptom. The most common first symptom caused by an AVM is bleeding in the brain, which occurs in 70 percent of symptomatic patients.[3][5]
Causes
Scientists are not entirely sure what causes arteriovenous malformations to form. The prevailing belief is that most people are born with them, meaning they develop during fetal development in the womb. AVMs are therefore considered congenital, or present from birth, even if they are not discovered until much later in life.[1][3]
The exact reason why the blood vessels fail to form properly during early development is not well understood. During early stages of pregnancy, a failure occurs in the orderly resorption of primitive blood vessels. Instead of developing the normal capillary network that should connect arteries to veins, a direct connection forms between these two types of vessels.[10]
In most cases, AVMs are not inherited from parents. They are not typically passed down through families. However, in rare situations, arteriovenous malformations may have a hereditary component and run in families. But this is the exception rather than the rule.[1][3]
Though AVMs are usually present at birth, they often do not become clinically evident until childhood, adolescence, or adulthood. They commonly become more noticeable during periods of significant body change, such as puberty or pregnancy, when hormonal shifts and increased blood volume may affect the malformation.[10]
Risk Factors
Because the cause of arteriovenous malformations is not fully understood, there are no clearly defined risk factors that increase a person’s likelihood of developing one. AVMs are not caused by anything a person does or does not do. They are not the result of lifestyle choices, behaviors, or environmental exposures.[3]
Since most AVMs are congenital, the main risk factor is simply being born with one. In the rare cases where AVMs appear to be hereditary, having a family history of the condition may increase risk, though this accounts for only a small percentage of cases.[1]
Once an AVM has been discovered, certain factors may increase the risk of it causing problems. If an AVM has already bled once, the risk of it bleeding again is higher. Other factors that may increase the risk of a second rupture include deep venous drainage, association with an aneurysm (a weakened, bulging area in a blood vessel wall), or location deep within the brain or spinal cord.[18]
The best available data suggests that once discovered, AVMs have a 1 to 3 percent per year risk of bleeding. This means that over time, there is an ongoing risk that the malformation may rupture, though the exact timing cannot be predicted.[5]
Symptoms
Symptoms of arteriovenous malformation can vary greatly from person to person. In fact, many people with AVMs experience no symptoms at all. Up to 15 percent of people with AVMs do not have any noticeable symptoms. These individuals often only discover they have an AVM when medical imaging is performed for an unrelated health concern.[3]
When symptoms do occur, they can range from mild to severe. Often, the first sign that someone has an AVM is after it begins to bleed. A hemorrhage, or bleeding into the brain, can cause sudden and serious symptoms that require immediate medical attention.[1]
Seizures are one of the most common symptoms of AVMs. These can be focal, affecting only a small part of the brain, or generalized, involving widespread brain activity. Seizures may include convulsions, loss of control over movement, or changes in consciousness. In about 25 percent of symptomatic cases, seizures are the first symptom experienced. The average age of patients who first experience seizures from an AVM is 25 years.[2][5]
Headaches are another frequent symptom. These can vary greatly in how often they occur, how long they last, and how intense they are. Sometimes headaches can become as severe as migraines. The pain may occur on one side of the head or both sides. When headaches consistently occur in the same location, this may indicate where the AVM is positioned in the brain. Only 5 to 15 percent of patients complain of long-standing headaches before their AVM is detected, though it should be noted that most people with chronic headaches do not have an AVM.[2][5]
Muscle weakness or even complete paralysis can occur, affecting one part of the body or spreading to multiple areas. If the AVM is in the spinal cord, it can lead to degeneration of nerve fibers below the level of the malformation, causing widespread paralysis. Back pain that is sudden and severe, or weakness extending from the hips down through the legs to the toes, may indicate a spinal cord AVM.[2][3]
Visual problems can develop when an AVM is located near the optic nerve or in the part of the brain that processes images. These problems may include loss of part of the visual field, inability to control eye movements, or swelling of the optic nerve.[2]
Other symptoms can include numbness or tingling sensations, dizziness, nausea and vomiting, problems with movement, speech, memory, thinking, balance, or vision. Some people experience mental confusion, hallucinations, or symptoms of dementia. If the AVM is in the lungs, symptoms may include shortness of breath during physical activity or coughing up blood. AVMs on the arms, legs, or trunk may appear as lumps and cause pain and swelling.[2][3]
Prevention
Because arteriovenous malformations are congenital conditions that develop before birth, there are no known ways to prevent them from forming. Since the exact cause is not fully understood and AVMs are not related to lifestyle factors or behaviors, there are no preventive measures that can stop an AVM from developing in the first place.[3]
However, for people who have been diagnosed with an AVM, there are steps that can be taken to reduce the risk of complications. Regular monitoring through medical imaging and clinical follow-up is essential. This allows healthcare providers to track any changes in the AVM over time and intervene if the risk of bleeding increases.[9]
Taking care of overall health is important for people living with an AVM. Maintaining a healthy diet and engaging in regular exercise can support general well-being. However, specific activity restrictions may be recommended depending on the location and characteristics of the AVM. It is important to discuss with healthcare providers what activities are safe.[15]
For individuals with AVMs that cause seizures, medications can help control these episodes and reduce the risk of injury during a seizure. Similarly, medications may be prescribed to manage headaches or other symptoms associated with the malformation.[9]
Staying informed about the condition and maintaining open communication with healthcare providers is one of the most important aspects of living with an AVM. Understanding the risks, knowing what symptoms to watch for, and having a clear plan for when to seek emergency care can help ensure prompt treatment if complications arise.[15]
Pathophysiology
The pathophysiology of arteriovenous malformation involves significant changes to normal blood flow and the structure of blood vessels. In a healthy vascular system, blood pressure gradually decreases as blood moves from arteries through capillaries and into veins. Capillaries serve as a buffer, slowing the flow and allowing for the exchange of oxygen and nutrients with tissues.[1]
In an AVM, the absence of capillaries means that high-pressure arterial blood flows directly into veins. Veins have thinner walls than arteries and are not designed to withstand such high pressure. The sudden influx of high-pressure blood causes several problems. First, the veins may dilate and weaken over time. Second, the rapid flow through the AVM creates turbulent blood flow, which can be detected as a bruit, a whooshing sound that can sometimes be heard with a stethoscope. This sound resembles water rushing through a narrow pipe.[3][12]
The abnormal blood flow pattern disrupts the typical process of how blood circulates through tissues. Because blood rushes quickly from arteries to veins without passing through capillaries, the surrounding tissue is deprived of oxygen. This phenomenon is sometimes called a “steal” effect, as blood is essentially stolen away from tissues that need it. Over time, this oxygen deprivation can lead to tissue damage and cell death, particularly affecting sensitive nerve cells in the brain or spinal cord.[1][4]
As blood continues to flow through the AVM, the malformation may grow larger. The increased blood volume and pressure can cause the tangle of vessels to expand. The walls of these abnormal vessels become progressively thinner and weaker. Eventually, the structural integrity of the blood vessels may fail, leading to rupture and bleeding into the surrounding tissue.[1]
When an AVM bleeds, blood escapes from the vessels and enters the brain or spinal cord tissue. Because these structures are enclosed in rigid bone with limited space for expansion, even a small amount of blood can create significant pressure on nerve tissue. This pressure damages neurons and other cells, disrupts normal brain or spinal cord function, and can lead to stroke, paralysis, or other serious neurological problems.[3]
The location of an AVM within the brain or body determines which functions are affected when the malformation causes symptoms. AVMs in different regions of the brain will impact different abilities, such as movement, speech, vision, or memory, depending on what functions those brain regions control. Similarly, AVMs in the spinal cord affect the body below the level of the malformation, while peripheral AVMs affect the specific area of the body where they are located.[2]
The risk of bleeding from an AVM is related to several factors. The size of the malformation, its location, the pattern of blood flow through it, and whether it has bled before all influence the likelihood of future hemorrhage. Once an AVM has ruptured and bled once, the risk of rebleeding increases significantly compared to AVMs that have never bled.[5]
