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What is an AVM?
Arteriovenous malformations, or AVM’s, are a collection of abnormal vessels
which may occur within virtually any part of the brain tissue. Blood
normally flows from the heart through arteries to supply all organs
including the brain. Four major arteries supply the brain, two carotid
arteries in the front of the neck and two vertebral arteries in the back of
the neck.
Normally blood is pumped from the heart into the large arteries at
relatively high pressure and speed which decreases as the blood flow reaches
smaller and smaller arteries. Eventually the blood reaches the smallest
vessels called capillaries. Capillaries are smaller than the diameter of a
human hair and as the blood flows slowly through these tiny vessels, it
gives up oxygen and nutrients to the brain tissue and receives carbon
dioxide and other waste products. The blood then enters tiny veins at very
low pressure, joining larger veins to return to the heart and lungs.
Pressure and speed of blood flow in the veins are normally very low compared
to that of the arteries. The walls of the veins are therefore relatively
thin and delicate compared to those of the arteries.
AVM’s are abnormal communications or "short circuits" which directly connect
relatively large arteries within the brain directly to veins. The blood is
delivered at relatively higher pressure with more rapid flow directly into
the veins. Because the veins were never designed to take the higher
pressures and flows, they expand, pushing against the neighboring areas of
normal brain. This may damage the normal brain causing weakness, numbness,
loss of vision, or seizures. Often the supplying arteries, the AVM itself,
or the enlarged veins rupture, resulting in the most common presentation of
an AVM- intracranial hemorrhage, a type of stroke.
"Stroke" is a general term meaning interference with brain function caused
by damage to the brain’s arteries or veins. Strokes may result from
hemorrhage, as mentioned above, when a vessel breaks or ruptures allowing
blood to escape, similar to a leak in a pipe. In hemorrhage, the escaping
blood may damage the surrounding brain directly and the tear in the vessel
may prevent brain supplied by that vessel from receiving its normal blood
flow. Hemorrhage or hemorrhagic stroke is the type of stroke most often
caused by AVM’s.
How does one get an AVM?
AVM’s are most often congential (present at birth or early in life and
slowly develop over time) and usually no specific cause for the AVM can be
identified. In the vast majority of cases, AVM’s are not inherited and other
members of the family are not at increased risk for having an AVM.
How are AVM’s diagnosed?
AVM’s are usually diagnosed by medical imaging studies, which are often
ordered after a patient develops symptoms. Symptoms of an AVM may include
headache, weakness, numbness, visual problems, seizures, or most often, the
abrupt onset of stroke.
The first imaging study in patients who are suspected to have an AVM is
usually a CT (computed tomography) scan or MRI (magnetic resonance imaging).
These studies are quite good at identifying an AVM and are relatively
noninvasive, requiring only an injection of contrast material into a small
vein during the study.
Further identification of the vessels involved in the AVM requires a test
called an angiogram or arteriogram. Angiography is the only test currently
available which provides sufficiently detailed information on which to plan
and carry out therapy of most AVM’s.
What is an angiogram?
Angiography is a study of the brain blood vessels. It is performed by a
physician experienced and trained in performing and interpreting blood
vessel examinations of the brain, using X-rays to map the specific blood
vessels involved in the AVM. A small tube or catheter is inserted into the
femoral artery in the groin and maneuvered into the vessels in the neck
supplying the brain. Material called contrast or "dye" which is visible on
the X-ray is injected into the vessel and images are taken as the liquid
flows through the AVM. The angiogram gives a detailed picture of the vessels
involved and the flow through the AVM. Based on the information gained by
the angiogram, treatment options can be determined for each patient.
How are AVM’s treated?
There are three major treatment methods that may be useful either alone or
in combination to treat an AVM. The specific treatment for an individual is
based on that patient’s history, symptoms, and anatomy of the AVM including
its size, feeding arteries, draining veins, and location within the brain.
Treatments include: endovascular embolization (closure of the AVM from
within the blood vessels), open surgical removal of the AVM, radiosurgery,
or a combination of techniques.
Open surgical treatment involves removing a portion of the skull so that
surgical instruments can be inserted to remove the AVM. Surgical treatment
is often performed after embolization has closed portions of the AVM. The
combination of embolization followed by surgical resection is frequently
safer than surgical resection alone in treating an AVM.
Radiosurgery is a technique which uses focused beams of radiation to treat
AVM’s which are sufficiently small and located in appropriate areas of the
brain. Despite the name, no opening of the skull is required; instead the
radiation causes scarring in blood vessels of the AVM, thereby eliminating
it. After treating the AVM with radiosurgery, a period of two to three years
is required for the full effect of the treatment to be determined. In over
80% of cases where the AVM is sufficiently small, there is complete
obliteration of the AVM.
What is embolization?
Embolization is an endovascular technique, (performed from within the blood
vessels) to block vessels of the AVM. Embolization is performed using
catheters and angiographic techniques similar to those discussed above. For
the embolization procedure, a very tiny catheter is threaded from the groin
directly into the AVM vessels within the brain. Under X-ray guidance,
material is injected through the catheter to permanently block and close off
the vessels of the AVM. Materials used might include particles, small
platinum coils, and/or liquid embolic agents similar to glue.
Embolization of an AVM is usually performed before treatment by either
surgery or radiosurgery. Embolization is often able to decrease the size of
the AVM making the surgery or radiosurgery much safer than would otherwise
be the case. However, certain AVM’s blood flow may be totally blocked by
embolization techniques, and no further therapy may be required.
Who performs endovascular embolization procedures?
Endovascular treatment is a relatively new type of treatment requiring
specialized training. Most physicians who perform endovascular embolization
are neuroradiologists or neurosurgeons who have received several years of
additional training in navigating inside the brain blood vessels to treat a
variety of blood vessel disorders.
What do I need to know before having an embolization?
The patient should not eat or drink anything after midnight on the night
before the procedure. This is very important in that any food in the stomach
will make the patient nauseated during the procedure and may cause vomiting
with severe consequences. Patients should take their normal medications the
morning of the procedure with a small amount of water. Embolizations are
performed either under sedation or under general anesthesia. Procedures
frequently take 4 to 6 hours and the time necessary is often not predictable
before the procedure begins.
After the procedure, the patient will need to remain still and avoid bending
the hip where the puncture occurred for at least 6-8 hours. This means lying
flat and is often the portion of the entire experience that is most
uncomfortable. The period of rest is very important since it allows the
small puncture hole in the artery to heal. The potential complications and
risks of the procedure should be discussed with your doctor.
Patients usually remain in the hospital for about several days following an
embolization procedure. The first night is usually spent in the neurological
intensive care unit for close monitoring, this is followed by transfer to a
regular hospital room for 24 hours before discharge. If surgery is to follow
the embolization procedure, it may be performed on the day after
embolization. Sometimes multiple embolization procedures are necessary in
order to get the maximum closure of the AVM safely, these are usually
scheduled several weeks to months apart.
Will all my symptoms be cured by treatment of the AVM?
Treatment of an AVM is directed toward preventing brain injury including
that which might result from bleeding or rebleeding. No treatment currently
exists which can repair damage already done to the brain by the AVM. This
means for example, that seizures might continue after embolization or even
complete removal of the AVM. Patients with neurological deficits resulting
from AVM hemorrhage would likely still have the deficits after treatment of
the AVM, although improvement may occur.
For more information go to: www.asitn.org
Figure 1: AP angiograms demonstrate a
large arteriovenous malformation of the left parietal region. The film on
the left demonstrates the feeding arteries from the middle cerebral artery
system with a large draining vein medially to the sagittal sinus. The film
on the right demonstrates several large feeding vessels from the posterior
cerebral artery on the left side.
Figure 2: The AVM of the patient shown in
Figure 1 after endovascular embolization using N-butyl-cyanoacryalate glue
for part of the AVM. As can be seen, the AP angiogram on the carotid
circulation (left) shows a greatly diminished arterial to venous shunt. As
well, the film of the right demonstrates decreased filling from the
posterior cerebral supply to the AVM.
Figure 3: AP angiograms following
surgical resection of the large AVM of the patient shown in Figures 1 and 2.
Preoperative embolization made the operation safer with a minimum of blood
loss at the time of operation. As can be seen on the carotid injection (left
film). there is no arterial to venous shunting. The vertebral
injection (right film) shows no AV shunting. The patient made an excellent
recovery from surgery with no neurologic deficits
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