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What are arteriovenous malformations?
What causes arteriovenous malformations?
What are the symptoms of arteriovenous malformations?
How do arteriovenous malformations cause symptoms?
Where do arteriovenous malformations tend to form?
How are arteriovenous malformations diagnosed?
How are arteriovenous malformations treated?
What are arteriovenous malformations?
Arteriovenous malformations (AVMs) are tangles of abnormal arteries and veins that compromise oxygen delivery to the brain or spinal cord by altering normal patterns of blood flow. Arteries and veins are normally interconnected by a series of progressively smaller blood vessels that control and slow the rate of blood flow. Oxygen delivery to surrounding tissues takes place through the thin, porous walls of the smallest of these interconnecting vessels, known as capillaries, where the blood flows most slowly. The arteries and veins that make up AVMs, however, lack this intervening capillary network. Instead, arteries dump blood directly into veins through a passageway called a fistula. The flow rate is uncontrolled and extremely rapid -- too rapid to allow oxygen to be dispersed to surrounding tissues. When starved of normal amounts of oxygen, the cells that make up these tissues begin to deteriorate, sometimes dying off completely.
This abnormally rapid rate of blood flow frequently causes increased blood pressure inside the vessels located in the central portion of an AVM directly adjacent to the fistula -- an area referred to as the nidus. The arteries feeding blood into the AVM often become swollen and distorted; the veins that drain blood away from it often become abnormally constricted. Moreover, the walls of the involved arteries and veins are often abnormally thin and weak. Aneurysms (balloon-like bulges in blood vessel walls that are susceptible to rupture) may develop in association with approximately half of all brain AVMs due to this structural weakness.
Although AVMs can develop in many different sites, those located in the brain or spinal cord can have serious effects on the body. This disorder, which is thought to arise during fetal development or shortly after birth, is believed to affect approximately 1 in 1000 individuals. They occur in males and females of all racial or ethnic backgrounds at roughly equal rates.
What causes arteriovenous malformations?
Although the cause of these vascular anomalies of the central nervous system is not yet well understood, scientists believe that they most often result from mistakes that occur during 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.
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, which are chemicals that are produced by the body that stimulate new blood vessel formation and growth. Researchers have recently 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.
What are the symptoms of arteriovenous malformations?
Most people with brain or spinal cord AVMs experience few, if any, significant symptoms, and the malformations tend to be discovered only incidentally, usually either at autopsy or during treatment for an unrelated disorder. But for about 12 percent of the affected population these abnormalities cause symptoms that vary greatly in severity and each year about 1% of those with AVMs will die as a direct result of the AVM.
Seizures and headaches are the most generalized symptoms of AVMs, but no particular type of seizure or headache pattern has been identified. Seizures can involve a loss of control over movement, convulsions, or a change in a person’s level of consciousness. Headaches can vary greatly in frequency, duration, and intensity, sometimes becoming as severe as migraines. At times a headache consistently affecting one side of the head may be closely linked to the site of an AVM. More frequently, however, the location of the pain is not specific to the lesion and may encompass most of the head.
AVMs can also cause more specific neurological symptoms which depend upon the location of the AVM. Such symptoms may include muscle weakness or paralysis in one part of the body; a loss of coordination, gait disturbances, difficulties with carrying out tasks that require planning, dizziness, visual disturbances, various problems with using or understanding language, numbness, tingling, memory deficits, mental confusion, hallucinations, or dementia. Researchers have recently uncovered evidence that AVMs may also cause subtle learning or behavioral disorders in some people during their childhood or adolescence, long before more obvious symptoms become evident.
One of the more distinctive signs indicating the presence of an AVM is a phenomenon called a bruit, which is a rhythmic, "whooshing" sound caused by excessively rapid blood flow through the arteries and veins of an AVM that can be heard by placing a stethoscope on the scalp over the AVM, and in some cases can even be heard by the patient.
Symptoms caused by AVMs can appear at any age, but because these abnormalities tend to result from a slow buildup of neurological damage over time they are most often noticed when people are in their twenties, thirties, or forties. If AVMs do not become symptomatic by the time people reach their late forties or early fifties, they tend to remain stable and rarely produce symptoms. In women, pregnancy sometimes causes a sudden onset or worsening of symptoms, due to accompanying cardiovascular changes, especially increases in blood volume and blood pressure.
One especially severe type of AVM, called vein of Galen malformation, causes symptoms at, or very soon after, birth. This lesion is located deep inside the brain, and 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 arteriovenous malformations cause symptoms?
AVMs become symptomatic only when the damage they cause to the brain or spinal cord reaches a critical level. This is one of the reasons why a relatively small fraction of people with these lesions experiences significant health problems related to the condition. In addition to compromising blood flow to tissues, AVMs also damage the brain or spinal cord through bleeding (hemorrhage) into surrounding tissues and by compressing or displacing parts of the brain or spinal cord.
The greatest potential danger posed by AVMs is hemorrhage. Researchers believe that each year between 2 and 4 percent of all AVMs 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. The present state of knowledge does not permit doctors to predict whether or not any particular person with an AVM will suffer an extensive hemorrhage. The lesions can remain stable or can suddenly begin to grow. In a few cases, they have been observed to regress spontaneously. 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.
A few physical characteristics appear to indicate a greater-than-usual likelihood of significant hemorrhage. Smaller AVMs have a greater likelihood of bleeding than do larger ones. Impaired drainage by unusually narrow or deeply situated veins also increases the chances of hemorrhage. Pregnancy also appears to increase the likelihood of hemorrhage, mainly because of increases in blood pressure and blood volume. Finally, AVMs that have hemorrhaged once are about nine times more likely to bleed again during the first year after the initial hemorrhage than are lesions that have never bled.
The damaging effects of a hemorrhage are related to lesion location. 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 on the surface of the brain or spinal cord. Thus, location is an important factor to consider when weighing the relative risks of surgical versus non-surgical treatment of AVMs.
Even in the absence of bleeding or significant oxygen depletion, large AVMs can damage the brain or spinal cord simply by their presence. They can range in size from a fraction of an inch to more than 2.5 inches in diameter, depending on the number and size of the blood vessels making up the lesion. The larger the AVM, the greater the amount of pressure it exerts on surrounding brain or spinal cord. The largest lesions may compress several inches of the spinal cord or distort the shape of an entire hemisphere of the brain. Such massive AVMs can constrict the flow of cerebrospinal fluid -- a clear liquid that normally nourishes and protects the brain and spinal cord -- by distorting or closing the passageways and open chambers (ventricles) inside the brain that allow this fluid to circulate freely. As cerebrospinal fluid accumulates, hydrocephalus results. This fluid buildup further increases the amount of pressure on fragile neurological structures, adding to the damage caused by the AVM itself.
Where do arteriovenous malformations tend to form?
AVMs can form virtually anywhere in the brain or spinal cord. Some are even formed from blood vessels located in the dura mater, which is the outermost membranes surrounding the brain and spinal cord. AVMs affecting the spinal cord are of two types, AVMs of the dura mater, which affect the function of the spinal cord by transmitting excess pressure to the venous system of the spinal cord, and AVMs of the spinal cord itself, which affect the function of the spinal cord by hemorrhage, by reducing blood flow, or by causing excess venous pressure. Spinal AVMs frequently cause attacks of sudden, severe back pain, often concentrated at the roots of nerve fibers where they exit the vertebrae; the pain is similar to that caused by a slipped disk. These lesions also can cause sensory disturbances, muscle weakness, or paralysis in the parts of the body served by the spinal cord or the damaged nerve fibers. Spinal cord injury by the AVM by either of the mechanisms described above 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.
Dural AVMs can appear anywhere on the surface of the brain, where they can exert pressure on the cerebral cortex, the brain’s “gray matter.” Depending on their location, these 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 brain, may cause a variety of visual disturbances.
AVMs can also form from blood vessels located deep inside the interior 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 the hippocampus (which plays a major role in memory).
AVMs can also affect the cerebellum, which is nestled under the rear portion of the brain, and the brainstem, 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. Damage to these parts can result in dizziness, vomiting, a loss of the ability to coordinate complex movements such as walking, or uncontrollable muscle tremors.
How are arteriovenous malformations diagnosed?
An array of traditional and new imaging technologies are available to uncover the presence of AVMs. These include angiography, computed tomography (CT), magnetic resonance imaging (MRI), and magnetic resonance angiography (MRA).
Angiography provides the most accurate pictures of blood vessel structure in AVMs and is often the gold standard in detection of these lesions; however, it is somewhat invasive and carries a slight risk of causing a stroke. Two of the most frequently employed non-invasive imaging technologies used to detect AVMs are CT and MRI scans. A CT is especially useful in revealing the presence of hemorrhage, while an MRI offers superior diagnostic information to detect subtle changes in neurological tissues. A recently developed application of MRI technology -- magnetic resonance angiography (MRA) -- can record the pattern and velocity of blood flow through vascular lesions and may be a useful adjunct in the diagnosis of AVMs. as well as the flow of cerebrospinal fluid throughout the brain and spinal cord.
How are arteriovenous malformations treated?
Medication can often alleviate general symptoms such as headache, back pain, and seizures caused by AVMs; however, the definitive treatment for AVMs is either surgery or focused radiation therapy (radiosurgery) -- with endovascular embolization as an adjunct to either treatment. The choice of therapy depends largely on the size and location of an AVM.
The decision to perform surgery on any individual with an AVM requires a careful consideration of possible benefits versus risks. The natural history of an individual AVM is difficult to predict; however, if not treated, they have the potential of causing significant hemorrhage, which may result in serious neurological deficits or death. On the other hand, surgery on any part of the central nervous system carries its own risks as well -- AVM surgery is associated with an estimated 8% risk of serious complications or death. There is no easy formula that can allow physicians and their patients to reach a decision on the best course of therapy -- all therapeutic decisions must be made on a case-by-case basis.
Conventional surgery involves entering the brain or spinal cord and removing the AVM while causing as little damage as possible to surrounding neurological structures. This surgery is most appropriate when an AVM is located in a superficial portion of the brain or spinal cord and is relatively small in size. AVMs located deep inside the brain sometimes cannot be approached through conventional surgical techniques because there is too great a possibility that functionally important brain tissue will be damaged or destroyed.
Radiosurgery is a less invasive therapeutic approach. It involves aiming a beam of highly focused radiation directly on the AVM. The high dose of radiation damages the walls of the abnormal blood vessels in the lesion. Over the course of the next several months, the irradiated vessels gradually degenerate and eventually close, leading to the resolution of the AVM.
In endovascular embolization the surgeon guides a catheter though the arterial network until the tip reaches the site of the AVM. The surgeon then introduces a glue-like substance that will plug the AVM, correcting the abnormal pattern of blood flow in a process known as embolization. This method, however, usually does not permanently obliterate the AVM; therefore, it is generally used as an adjunct to conventional surgery or radiosurgery to reduce the blood flow through the AVM and make the surgery safer.
Radiosurgery can have incomplete results, particularly when an AVM is large, and it poses the additional risk of radiation damage to surrounding normal tissues. Moreover, even when successful, complete closure of an AVM takes place over the course of many months following radiosurgery; and during that period, the risk of hemorrhage is still present. However, both radiosurgery and embolization techniques now offer the possibility of treating deeply situated AVMs that had previously been inaccessible. And in many patients, several serial embolizations followed by conventional surgical removal or by radiosurgery is now performed, resulting in further reductions in mortality and complication rates.
