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Table of Contents (click to jump to sections) What
are arteriovenous malformations (AVMs)? What are arteriovenous
malformations (AVMs)? AVMs
of the brain or spinal cord (neurological AVMs) are believed to affect
approximately 300,000 Americans. They occur in males and females of all
racial or ethnic backgrounds at roughly equal rates. What are the symptoms? Seizures
and headaches are the most generalized symptoms of AVMs, but no particular
type of seizure or headache pattern has been identified. Seizures can be
partial or total, involving 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. Sometimes 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
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 muscle weakness or paralysis in one part of the body; a
loss of coordination (ataxia) that can lead to such problems as gait
disturbances; apraxia, or difficulties carrying out tasks that require
planning; dizziness; visual disturbances such as a loss of part of the visual
field; an inability to control eye movement; papilledema (swelling of
a part of the optic nerve known as the optic disk); various problems using or
understanding language (aphasia); abnormal sensations such as
numbness, tingling, or spontaneous pain (paresthesia or dysesthesia);
memory deficits; and 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 an
auditory phenomenon called a bruit, coined from the French word
meaning noise. (A sign is a physical effect observable by a
physician, but not by a patient.) Doctors use this term to describe the
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-that is, an effect experienced by a patient-when it is especially
severe. When audible to patients, the bruit may compromise hearing, disturb
sleep, or cause significant psychological distress. 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. In
contrast to the vast majority of neurological AVMs, one especially severe
type causes symptoms to appear at, or very soon after, birth.Called a vein
of Galen defect 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
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. Where do neurological AVMs tend to
form? 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. What are the health consequences of
AVMs? 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. What other types of vascular lesions
affect the central nervous system?
What causes vascular lesions? 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 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. By
studying patterns of familial occurrence, researchers have established that
one type of cavernous malformation involving multiple lesion formation is
caused by a genetic mutation in chromosome 7. This geneticmutation 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
(also known as Osler-Weber-Rendu disease), Sturge-Weber syndrome,
Klippel-Trenaunay syndrome, Parkes-Weber syndrome, and Wyburn-Mason
syndrome. How are AVMs and other vascular
lesions detected? Two
of the most frequently employed noninvasive imaging technologies used to
detect AVMs are computed axial tomography (CT) and magnetic
resonance imaging (MRI) scans. CT scans use X-rays to create a series of
cross-sectional images of the head, brain, or spinal cord and are especially
useful in revealing the presence of hemorrhage. MRI imaging, however, offers
superior diagnostic information by using magnetic fields 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 as well as the flow of cerebrospinal fluid
throughout the brain and spinal cord. CT, MRI, and MRA can provide
three-dimensional representations of AVMs by taking images from multiple
angles. How can AVMs and other vascular
lesions be treated? 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, left untreated, 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 percent 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. Today,
three surgical options exist for the treatment of AVMs: conventional
surgery, endovascular embolization, and radiosurgery. The
choice of treatment depends largely on the size and location of an AVM. Conventional
surgery involves entering the brain or spinal cord and removing the central
portion of the AVM, including the fistula, 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 generally cannot be approached through conventional surgical techniques
because there is too great a possibility that functionally important brain
tissue will be damaged or destroyed. Endovascular
embolization and radiosurgery are less invasive than conventional surgery and
offer safer treatment options for some AVMs located deep inside the brain. 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
substance that will plug the fistula, correcting the abnormal pattern of
blood flow. This process is known as embolization because it causes an embolus
(a blood clot) to travel through blood vessels, eventually becoming lodged in
a vessel and obstructing blood flow. The materials used to create an
artificial blood clot in the center of an AVM include fast-drying
biologically inert glues, fibered titanium coils, and tiny balloons. Since
embolization usually does not permanently obliterate the AVM, it is usually
used as an adjunct to surgery or to radiosurgery to reduce the blood flow
through the AVM and make the surgery safer. Radiosurgery
is an even 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 blood vessels making up 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. Embolization
frequently proves incomplete or temporary, although in recent years 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.
Moreover, 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 now offer the
possibility of treating deeply situated AVMs that had previously been
inaccessible. And in many patients, staged embolization followed by
conventional surgical removal or by radiosurgery is now performed, resulting
in further reductions in mortality and complication rates. Because
so many variables are involved in treating AVMs, doctors must assess the
danger posed to individual patients largely on a case-by-case basis. The
consequences of hemorrhage are potentially disastrous, 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 resultant neurological damage. What research is being done? In
partnership with the medical school of Columbia University, the NINDS has
established a long-term Arteriovenous Study Group to learn more about the
natural course of AVMs in patients and to improve the surgical treatment of
these lesions. Another
group of NINDS -sponsored researchers is currently studying large populations
of patients with AVMs to formulate criteria that will allow doctors to
predict more accurately the risk of hemorrhage in individual patients. Of
particular importance is the role that high blood pressure within the lesion
plays in the onset of hemorrhage. Other scientists are examining the genetic
basis of familial cavernous malformations and other hereditary syndromes that
cause neurological vascular lesions, including ataxia telangiectasia. Other
scientists are seeking to refine the techniques now available to treat AVMs.
Radiosurgery is a special area of interest because this technology is still
in its infancy. An ongoing study is closely examining the precise effects
that radiation exposure has on vascular tissue in order to improve the
predictability and consistency of treatment results. Finally,
several ongoing studies are devoted to developing new noninvasive
neuroimaging technologies to increase the effectiveness and safety of AVM
surgery. Some scientists are pioneering the use of MRI to measure amounts of
oxygen present in the brain tissue of patients with vascular lesions in order
to predict the brain's response to surgical therapies. Others are developing
a new micro-imager that may be inserted into catheters to increase the
accuracy of angiography. In addition, new types of noninvasive imaging
devices are being developed that detect functional brain activity through
changes in tissue light emission or reflectance. This technology may prove
more sensitive than MRI and other imaging devices currently available, giving
surgeons a new tool for improving the efficacy and safety of AVM surgery.
For
more information on neurological disorders or research programs funded by the
National Institute of Neurological Disorders and Stroke, contact the
Institute's Brain Resources and Information Network (BRAIN) at: BRAIN Information also is
available from: National
Organization for Rare Disorders (NORD) International
Radiosurgery Support Association (IRSA) Or follow our Resources link for more
useful online information. |
Leslie on Lake George Summer 2004 |