Cranial
Anatomy - Arteriovenous Malformation
What is Arteriovenous
Malformation?
Arteriovenous malformations
(AVMs) are defects of the circulatory system that are
generally believed to arise during embryonic or fetal
development or soon after birth. Although AVMs can develop
in many different sites, those located in the brain
or spinal cord can have especially widespread effects
on the body. Most people with neurological AVMs experience
few, if any, significant symptoms. 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.
Normally, arteries carry blood containing oxygen from
the heart to the brain, and veins carry blood with less
oxygen away from the brain and back to the heart. When
an arteriovenous malformation (AVM) occurs, a tangle
of blood vessels in the brain or on its surface bypasses
normal brain tissue and directly diverts blood from
the arteries to the veins.
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Normal Blood
Vessels
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Abnormal Connection
of Blood Vessels
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What symptoms can it cause?
Seizures and headaches are the most generalized symptoms.
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, loss of coordination,
difficulties carrying out tasks that require planning,
dizziness, visual disturbances, problems using or understanding
language, abnormal sensations (such as numbness, tingling,
or spontaneous pain), memory deficits, mental confusion,
hallucinations, or dementia.
Is there any treatment?
Medication can often alleviate general symptoms such
as headache, back pain, and seizures caused by AVMs
and other vascular lesions. However, the definitive
treatment for AVMs is either surgery or focused irradiation
therapy. The decision to perform surgery on any individual
with an AVM requires a careful consideration of possible
benefits versus risks. 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.
How is an AVM evaluated?
Physicians now use an array of traditional and new
imaging technologies to uncover the presence of AVMs.
Angiography provides the most accurate pictures of blood
vessel structure in AVMs. The technique requires injecting
a special water-soluble dye, called a contrast agent,
into an artery. The dye highlights the structure of
blood vessels so that it can be recorded on conventional
X-rays. Although angiography can record fine details
of vascular lesions, the procedure is somewhat invasive
and carries a slight risk of causing a stroke. Its safety,
however, has recently been improved through the development
of more precise techniques for delivering dye to the
site of an AVM. Superselective angiography involves
inserting a thin, flexible tube called a catheter into
an artery; a physician guides the tip of the catheter
to the site of the lesion and then releases a small
amount of contrast agent directly into the lesion.
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 technologymagnetic 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.
A detailed history-taking of
the patient's symptoms and a physical examination are
done first, followed by any of several tests, such as
x-ray studies, Computerised Tomography (CT) scans, Magnetic
Resonance Imaging (MRI), and angiograms. All findings
are used to evaluate the patient's symptoms, determine
the tumour's exact location, and provide the physician
with a tentative diagnosis of the tumour type. During
surgery, ultrasound imaging may be used to pinpoint
the tumour's precise location and help the surgeon plan
his approach for its removal. If an emergency craniotomy
is required, an extensive workup may not be possible.
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