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Arteriovenous malformations (AVMs) are defects of the circulatory system that are generally believed to arise during embryonic or fetal development or soon after birth. They are comprised of snarled tangles of arteries and veins, called lesions.
How do AVMs damage the brain and spinal cord?
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. 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 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 blood pressure inside the vessels located in the central portion of an AVM directly adjacent to the fistula-an area doctors refer to as the nidus, from the Latin word for nest-to rise to dangerously high levels. The arteries feeding blood into the AVM often become swollen and distorted; the veins that drain blood away from it often become abnormally constricted (a condition called stenosis). 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 neurological AVMs due to this structural weakness.
Bleeding can result from this combination of high internal pressure and vessel wall weakness. Such hemorrhages are often microscopic in size, causing limited damage and few significant symptoms. Even many nonsymptomatic AVMs show evidence of past bleeding. But massive hemorrhages can occur if the physical stresses caused by extremely high blood pressure, rapid blood flow rates, and vessel wall weakness are great enough. If a large enough volume of blood escapes from a ruptured AVM into the surrounding brain, the result can be a catastrophic stroke. AVMs account for approximately 2 percent of all hemorrhagic strokes that occur each year.
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 lesion, the greater the amount of pressure it exerts on surrounding brain or spinal cord structures. 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.
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
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 to
the spinal cord, 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 and pial AVMs can appear anywhere on the surface of the brain. Those located on the surface of the cerebral hemispheres-the uppermost portions of the brain-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 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. 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 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 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. 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.
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 clinically 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 clinically significant 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, or parenchyma, of the brain typically causes more severe neurological damage than does hemorrhage by lesions that have formed in the dural or pial 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.) Thus, location is an important factor to consider when weighing the relative risks of surgical versus non-surgical treatment of AVMs.
Besides AVMs, three other main types of vascular lesion can arise in the brain or spinal cord: cavernous malformations, capillary telangiectases, and venous malformations. These lesions may form virtually anywhere within the central nervous system, but unlike AVMs, they are not caused by high-velocity blood flow from arteries into veins. In contrast, cavernous malformations, telangiectases, and venous malformations are all low-flow lesions. Instead of a combination of arteries and veins, each one involves only one type of blood vessel. These lesions are less unstable than AVMs and do not pose the same relatively high risk of significant hemorrhage. In general, low-flow lesions tend to cause fewer troubling neurological symptoms and require less aggressive treatment than do AVMs.
Source: Information provided courtesy of the National Institute of Neurological Disorders and Stroke (NINDS), a division of the National Institutes of Health (NIH).
NINDS health-related material is provided for information purposes only and does not necessarily represent endorsement by or an official position of the National Institute of Neurological Disorders and Stroke or any other Federal agency. Advice on the treatment or care of an individual patient should be obtained through consultation with a physician who has examined that patient or is familiar with that patient's medical history.
o Arteriovenous Malformation- Overview
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