Sep. 05, 2020
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Arteriovenous malformations (AVMs) are abnormal, snarled tangles of blood vessels that cause multiple irregular connections between your arteries and veins. These malformations most often occur in the spinal cord and in any part of your brain or on its surface but can develop elsewhere in the body.
Normally, arteries carry oxygen-rich blood away from your heart to the body's cells, organs, and tissues; veins return blood with less oxygen to the lungs and heart. But in an AVMs, the absence of capillaries—a network of small blood vessels that connect arteries to veins and deliver oxygen to cells—creates a shortcut for blood to pass directly from arteries to veins and bypass tissue, which can lead to tissue damage and the death of nerve cells and other cells. Over time, some AVMs get progressively larger as the amount of blood flow increases.
In most cases, people with neurological AVMs experience few, if any, significant symptoms. In some cases, a weakened blood vessel may burst, spilling blood into the brain (hemorrhage) that can cause stroke and brain damage. Most malformations tend to be discovered only incidentally, usually during treatment for an unrelated disorder or at autopsy.
Treatment options depend on your type of AVM, its location, noticeable symptoms, and your general health.
Symptoms, which vary greatly in severity, may include.
AVMs also can cause a wide range of more specific neurological symptoms that vary from person to person, depending primarily upon the location of the AVM. Such symptoms may include:
Symptoms caused by AVMs can appear at any age. Because the abnormalities tend to result from a slow buildup of neurological damage over time, they are most often noticed when people are in their twenties or older. If AVMs do not become symptomatic by the time people reach their late forties or early fifties, they tend to remain stable and are less likely to produce symptoms. Some pregnant women may experience a sudden onset or worsening of symptoms due to accompanying cardiovascular changes, especially increases in blood volume and blood pressure.
Although most neurological AVMs have very few, if any, significant symptoms, one particularly severe type of AVM causes symptoms to appear at, or very soon after, birth. Called a vein of Galen defect (named after the major blood vessel involved), this lesion is located deep inside the brain. It is frequently associated with hydrocephalus (an accumulation of fluid within certain spaces in the brain, often with visible enlargement of the head), swollen veins visible on the scalp, seizures, failure to thrive, and congestive heart failure. Children born with this condition who survive past infancy often remain developmentally impaired.
How do AVMs damage the brain and spinal cord?. AVMs damage the brain or spinal cord through three basic mechanisms: by reducing the amount of oxygen reaching neurological tissues; by causing bleeding (hemorrhage) into surrounding tissues; and by compressing or displacing parts of the brain or spinal cord.
AVMs can form virtually anywhere in the brain or spinal cord—wherever arteries and veins exist. Some are formed from blood vessels located in the dura mater or in the pia mater, the outermost and innermost, respectively, of the three membranes surrounding the brain and spinal cord. (The third membrane, called the arachnoid, lacks blood vessels.)
AVMs in the spinal cord. AVMs can affect how the spinal cord functions by causing hemorrhage, by reducing blood flow to the spinal cord, or by causing excess pressure in the blood vessels. Spinal AVMs frequently cause attacks of sudden, severe back pain and can also cause sensory disturbances, muscle weakness, or paralysis in the parts of the body served by the spinal cord or the damaged nerve fibers. A spinal cord AVM can lead to degeneration of the nerve fibers within the spinal cord below the level of the lesion, causing widespread paralysis in parts of the body controlled by those nerve fibers.
AVMs in the brain. AVMs on the surface of the cerebral hemispheres—the uppermost portions of the brain—exert pressure on the cerebral cortex, the brain's “gray matter” that is made up mostly by nerve cells. AVMs may damage portions of the cerebral cortex involved with thinking, speaking, understanding language, hearing, taste, touch, or initiating and controlling voluntary movements. AVMs located on the frontal lobe close to the optic nerve or on the occipital lobe (the rear portion of the cerebrum where images are processed) may cause a variety of visual disturbances.
AVMs also can form from blood vessels located deep inside the interior of the cerebrum (the main portion of the brain). These AVMs may compromise the functions of three vital structures: the thalamus, which transmits nerve signals between the spinal cord and upper regions of the brain; the basal ganglia surrounding the thalamus, which coordinate complex movements and plays a role in learning and memory; and the hippocampus, which plays a major role in memory.
AVMs can affect other parts of the brain besides the cerebrum. The hindbrain is formed from two major structures: the cerebellum, which is nestled under the rear portion of the cerebrum, and the brain stem, which serves as the bridge linking the upper portions of the brain with the spinal cord. These structures control finely coordinated movements, maintain balance, and regulate some functions of internal organs, including those of the heart and lungs. AVM damage to these parts of the hindbrain can result in dizziness, giddiness, vomiting, a loss of the ability to coordinate complex movements such as walking, or uncontrollable muscle tremors.
Health consequences of AVMs. The greatest potential danger posed by AVMs is hemorrhage. Most episodes of bleeding remain undetected at the time they occur because they are not severe enough to cause significant neurological damage. But massive, even fatal, bleeding episodes do occur.
A few physical characteristics appear to indicate a greater-than-usual likelihood of clinically significant hemorrhage:
Bleeding from AVMs located deep inside the interior tissues of the brain typically causes more severe neurological damage than does hemorrhage by lesions that have formed in the membranes or on the surface of the brain or spinal cord. (Deeply located bleeding is usually referred to as an intracerebral or parenchymal hemorrhage; bleeding within the membranes or on the surface of the brain is known as subdural or subarachnoid hemorrhage.)
Other vascular lesions that affect the central nervous system. Besides AVMs, three other main types of vascular lesion can arise anywhere in the brain or spinal cord. Unlike AVMs, these lesions involve only one type of blood vessel and do not pose the same relatively high risk of significant hemorrhage. In general, low blood low lesions tend to cause fewer troubling neurological symptoms and require less aggressive treatment than do AVMs.
Who is more likely to get arteriovenous malformations?
It is unclear why AVMs form. Most often AVMs are congenital (you are born with them), but they can appear shortly after birth or later in life. In some cases, the AVM may be inherited, but it is more likely that other inherited conditions increase the risk of having an AVM. It is estimated that brain AVMs occur in less than one percent of the general population; each year about one percent of those with AVMs will die as a direct result of the AVM.
Causes of vascular lesions. The cause of vascular anomalies of the central nervous system is not yet well understood. Scientists believe the anomalies most often result from mistakes that occur during embryonic or fetal development. These mistakes may be linked to genetic mutations in some cases. A few types of vascular malformations are known to be hereditary and thus are known to have a genetic basis. Some evidence also suggests that at least some of these lesions are acquired later in life as a result of injury to the central nervous system.
During fetal development, new blood vessels continuously form and then disappear as the human body changes and grows. These changes in the body's vascular map continue after birth and are controlled by angiogenic factors, chemicals produced by the body that stimulate new blood vessel formation and growth. Researchers have identified changes in the chemical structures of various angiogenic factors in some people who have AVMs or other vascular abnormalities of the central nervous system. However, it is not yet clear how these chemical changes actually cause changes in blood vessel structure.
By studying patterns of occurrence in families, researchers have established that one type of cavernous malformation involving multiple lesion formation is caused by a genetic mutation in chromosome 7. This genetic mutation appears in many ethnic groups, but it is especially frequent in a large population of Hispanic Americans living in the Southwest; these individuals share a common ancestor in whom the genetic change occurred. Some other types of vascular defects of the central nervous system are part of larger medical syndromes known to be hereditary. They include hereditary hemorrhagic telangiectasia, Sturge-Weber syndrome, and Klippel-Trenaunay syndrome.
How are arteriovenous malformations diagnosed and treated?
Diagnosing AVMs. One of the more distinctive signs clinicians use to diagnose an AVM is an auditory phenomenon called a bruit—a rhythmic, whooshing sound caused by excessively rapid blood flow through the arteries and veins of an AVM. The sound is similar to that made by a torrent of water rushing through a narrow pipe. A bruit can sometimes become a symptom when it is especially severe. When audible to individuals, the bruit may compromise hearing, disturb sleep, or cause significant psychological distress.
An array of imaging technologies can be used to uncover the presence of AVMs.
Treating AVMs. Whenever an AVM is detected, the individual should be carefully and consistently monitored for any signs of instability that may indicate an increased risk of hemorrhage.
There are several options for treating AVMs. Although medication can often lessen general symptoms such as headache, back pain, and seizures caused by AVMs and other vascular lesions, the definitive treatment for AVMs is either surgery or focused radiation therapy. Venous malformations and capillary telangiectases rarely require surgery. Cavernous malformations are usually well defined enough for surgical removal, but surgery on these lesions is less common than for AVMs because they do not pose the same risk of hemorrhage.
Because so many variables are involved in treating AVMs, doctors must assess the danger posed to individuals largely on a case-by-case basis. A hemorrhage from an untreated AVM can cause serious neurological deficits or death, leading many clinicians to recommend surgical intervention whenever the physical characteristics of an AVM appear to indicate a greater-than-usual likelihood of significant bleeding and subsequent neurological damage. However, surgery on any part of the central nervous system carries some risk of serious complications or death. There is no easy formula that can allow physicians and individuals to reach a decision on the best course of therapy.
An AVM grading system developed in the mid-1980s can help health care professionals estimate the risk of surgery based on the size of the AVM, location in the brain and surrounding tissue involvement, and any leakage.
Three surgical options are used to treat AVMs: conventional surgery, endovascular embolization, and radiosurgery. The choice of treatment depends largely on the size and location of an AVM. Endovascular embolization and radiosurgery are less invasive than conventional surgery and offer safer treatment options for some AVMs located deep inside the brain.
Embolization frequently proves incomplete or temporary, although new embolization materials have led to improved results. Radiosurgery often has incomplete results as well, particularly when an AVM is large, and it poses the additional risk of radiation damage to surrounding normal tissues. Even when successful, complete closure of an AVM takes place over the course of many months following radiosurgery. During that period, the risk of hemorrhage is still present. However, both techniques can treat deeply situated AVMs that had previously been inaccessible. And in many individuals, staged embolization followed by conventional surgical removal or by radiosurgery is now performed, resulting in further reductions in death and complication rates.
What are the latest updates on arteriovenous malformations?
In addition to NINDS, other NIH institutes and centers support research relevant to understanding, treating, or preventing arteriovenous malformations and vascular lesions. More information is available through the NIH RePORTER, a searchable database of current and previously funded research, as well as research results and publications.
For research articles and summaries on AMVs, search PubMed, which contains citations from medical journals and other sites.
How can I or a loved one help improve care for people with arteriovenous malformations?
Consider participating in a clinical trial so clinicians and scientists can learn more about AVMs and vascular lesions. Clinical research uses human volunteers to help researchers learn more about a disorder and perhaps find better ways to safely detect, treat, or prevent disease.
All types of volunteers are needed—those who are healthy or may have an illness or disease—of all different ages, sexes, races, and ethnicities to ensure that study results apply to as many people as possible, and that treatments will be safe and effective for everyone who will use them.
For information about participating in clinical research visit NIH Clinical Research Trials and You. Learn about clinical trials currently looking for people with AVMs at Clinicaltrials.gov, a database of past and current clinical studies.
Where can I find more information about arteriovenous malformations?
The following organizations and resources help people living with AVMs and their families, friends, and caregivers:
Brain Aneurysm Foundation
Phone: 781-826-5556 or 888-272-4602
National Library of Medicine (NLM)
Phone: 301-496-6308 or 888-346-3656
National Organization for Rare Disorders (NORD)
Phone: 203-744-0100 or 800-999-6673; 844-259-7178 Spanish
Content source: https://www.ninds.nih.gov/health-information/disorders/arteriovenous-malformations-avms Accessed July 10, 2023.
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