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Arteriovenous Malformations (AVM's) of the Brain are
relatively rare disorders affecting approximately 300,000
Americans. The estimated incidence of AVM in the general
population of the United States is 0.14% (140 cases per
100,000 persons or 1 case per 700 persons). This is approximately
one fifth to one seventh the incidence of intracranial
Aneurysms. They occur in males and females of all racial or
ethnic backgrounds at roughly equal rates.
These defects of the circulatory system consist of an abnormal
connection between the arterial system (which normally has a higher
intravascular pressure) and the lower pressure venous pathways.
Arteries carry oxygen-rich blood away from the heart to the body's cells;
veins return oxygen-depleted blood to the lungs and heart. Under ordinary
circumstances there is a pressure regulatory system that incorporates
progressively smaller arterial vessels in order to reduce the arterial
blood pressure until it reaches the vein collection system.
This pressure regulatory system relies upon a special ultra-small
vessel called "capillaries". This capillary network is
vital in order to permit the transfer of nutrients and oxygen to the
surrounding tissue as well as reducing the intravascular pressure
before the blood is transferred to the relatively thin walled veins.
An AVM can occur when the intervening capillary system is absent.
There are several potential consequences to the presence of an AVM.
Hemorrhage into surrounding tissue is a real threat since the
thin walled vein portion of this malformation may be
unable to sustain the higher intravascular blood pressure
that is directly shunted from the thicker muscular walled arteries.
Hemorrhage can be as little as a "leak" or as
destructive as an explosive disruption within the tissue that
surrounds it. Ischemia (lack of blood born oxygen) is a
frequent problem in the absence of the capillary system
which is designed to release oxygen and nutrients to these tissues.
Ischemia can lead to the death of Brain cells in the region of an
AVM. As pressure within the venous system dilates, blood flow is
slowed. The surrounding tissue is subjected to localized pressure upon
its neurons and fibre tracts from the dilated veins. The "pressure"
related injury can add to the localized Brain tissue ischemic
damage which then predisposes this area to hemorrhage.
AVMs can form wherever arteries and veins exist in the Brain. 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.)
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 can also be located deep within the interior of the Brain.
These deep 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 (structures that surround the thalamus and coordinate complex
movements); and the Hippocampus (another vital structure that plays
a major role in memory and emotions).
AVMs can affect other parts of the Brain besides the Cerebral
Hemispheres. The back portion of the Brain (Hindbrain) is
formed from two major structures: the Cerebellum (which helps to
control balance and coordination), and the Brainstem (which serves
as the bridge linking the upper portions of the Brain with the Spinal
Cord.) These structures work together to 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.
Arteriovenous Malformations (AVMs), comprising snarled
tangles of arteries and veins, are generally believed to
arise during embryonic (fetal) development or soon after
birth. Nevertheless, the precise cause of
Arteriovenous Malformations has never been clearly defined.
There is scientific information to suggest that AVMs often result from
embryonic or fetal development mistakes that are
linked, in some instances, to genetic
mutations. There are several types of vascular
malformations that are known to be hereditary and
therefore clearly consequent to a genetic error. Some AVMs
that occur later in life are the result of Brain trauma.
There is a normal process both during fetal development,
as well as during early human body growth where
changes, growth and disappearance of new blood vessels are a
continuous process. These changes in the Brain's
vascular structure are controlled by angiogenic factors (chemical
agents produced by the body that stimulate new blood vessel formation
and growth.)
There is one type of Cavernous Malformation involving
multiple lesion formation that is caused by a genetic
mutation in chromosome 7. Although this genetic mutation appears
in many ethnic groups, it is especially frequent in a large
population of Hispanic Americans living in the Southwest portion of the
USA all of whom share a common ancestor in whom the genetic change
occurred.
Additionally there are some other hereditary types of vascular
defects of the Central Nervous System (CNS) that are part
of larger medical syndromes. AVMs may present as part of a
Neurocutaneous Syndrome, such as Osler-Weber-Rendu disease
(hereditary hemorrhagic telangiectasia) and Sturge-Weber syndrome.
Other genetic diseases associated with AVMs include: Klippel-Trenaunay
syndrome, Parkes-Weber syndrome, and Wyburn-Mason syndrome.
Rare case reports describe multiple intracranial AVMs and/or
concomitant intracranial and intraspinal AVMs. These instances are
too rare to be well characterized.
Patients who harbor AVMs have an increased risk of developing an
intracranial Aneurysm which is seen with a frequency of
approximately 7.5% in these patients. On the other hand only
about 1% of patients with an Aneurysm are found to have an
AVM. Most commonly, Aneurysms are found on arteries feeding
the AVM.
Most people with Brain AVMs never experience any significant
symptoms. Asymptomatic cases of AVM can be discovered
incidentally either during treatment for an unrelated disorder or
as a result of a neuroimaging study done for some other reason (such as a
CT scan done in a hospital's Emergency Department in a patient who has
had a head injury.)
These abnormalities cause symptoms that vary greatly in severity for
about 12 percent of the affected population (about 36,000 of the
estimated 300,000 Americans with AVMs). The related symptoms are
severe enough to become debilitating or even life-threatening for only a
small percentage of the individuals within this group. There is a
calculated mortality rate of about 1 percent per year as a
direct result of the AVM.
AVM associated symptoms can appear at any age although they
are most often noticed when people are in their twenties, thirties,
or forties since these abnormalities tend to result from a
slow buildup of neurological damage over time. In patients
where the AVMs do not become symptomatic by the time people reach
their late forties or early fifties, they are highly unlikely to produce
symptoms. During pregnancy, however, where there are
increases in blood volume and blood pressure there may be a
sudden onset or worsening of symptoms, due to these cardiovascular
changes.
Seizures (epilepsy) and headaches are the most common and
generalized symptoms of AVMs, although there is no
particular type of seizure or headache pattern. Seizures can be
partial or total, involving a loss of control over movement, convulsions,
or a change in a person's level of consciousness. The location of the
head pain is not specific to the size or location of the AVM. Headache
may involve most of the head and can vary greatly in frequency, duration,
and intensity. Occasionally the pain is similar to and as severe as in
migraines.
Hemorrhage is more likely to be caused by small lesions,
while seizures are more likely to be caused by large lesions.
There is a 2-3% annual risk of intracranial hemorrhages
associated with AVMs and a 10% mortality rate associated with the
initial bleed. The mortality rate associated with the second
bleed is 13% with the rate increasing to 20% for each
subsequent haemorrhage. There is, in addition, a 50%
likelihood of new neurological deficit occurring with each bleed.
The location and size of each patient's lesion greatly affects their
risk of morbidity and mortality. These statistics are generalized
for all AVMs.
There is a wide variety of more specific neurological symptoms that
vary from person to person, depending upon the location of the AVM as well
as the age of the patient. Subtle learning or behavioral disorders during
childhood or adolescence may be the consequence of an AVM. These
subtleties can occur long before more obvious symptoms become evident.
Paralysis or weakness in one part of the body; loss of
coordination (ataxia) and balance (dysequilibrium);
difficulty walking (gait disturbance); difficulties carrying
out tasks that require planning; dizziness (vertigo); visual
disturbances (visual field impairment); double vision
(diplopia); communication problems involving the use of or
understanding language (aphasia or dysphasia); abnormal sensations
such as numbness, tingling, or spontaneous pain (paresthesia or
dysesthesia); memory deficits; and mental confusion,
hallucinations, or dementia are among the many signs
and symptoms found in AVM patients.
There is one distinctive peculiarity that patients may notice that
indicates the presence of an AVM. The patient (or more likely a suspicious
examining physician) may recognize a sound similar to that made by
a torrent of water rushing through a narrow pipe. This
"whooshing" sound (called a bruit) caused by
excessively rapid blood flow through the arteries and veins of an
AVM can sometimes be audible to patients, particularly at
night when the surrounding environment is quiet. The bruit
may compromise hearing, disturb sleep, or cause significant psychological
distress.
An accurate diagnosis is important since these lesions often represent
a reversible cause of Brain dysfunction, seizures and prevention of
haemorrhage.
The patient may experience the "bruit" referred to above or a
physician may hear it on examination. Any of the symptoms referred to
previously may lead the patient to a physician who institutes an
investigation.
MRI (including MR Angiography) as well as CT Angiography
are among the initial neuroimaging investigations that help to
identify these problems. Ultimately Cerebral Angiography is
prerequisite to accurately and definitively identify the precise
anatomy, configuration of both the lesion as well as the feeding and
draining vessels (See Figures 1 A&B).
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Figure 1A (Left): Right Carotid Cerebral Angiogram (lateral view)
demonstrating a Posterior Frontal Right Cerebral Hemisphere AVM
Figure 1B (Right): Right Carotid Cerebral Angiogram
(antero-posterior view) demonstrating additional details of the
Posterior Frontal Right Cerebral Hemisphere AVM.
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A "grading" system known as the Spetzler-Martin grading
system (see Table 1) can be used to assess the angiographic
details of an AVM and then relate this to an anticipated
treatment outcome. The system can be applied reliably to most
AVMs with good agreement among the Neuroradiologist observers, but some
unusual AVMs expose this system's imprecision and subjectivity.
Interobserver variability can affect the reporting of results, surgical
risk assessment, and patient selection. "Undergrading" may encourage
borderline surgical candidates to choose surgery and obtain results below
their expectations.
| Small (<3 cm) |
1 |
Noneloquent |
0 |
Superficial only |
0 |
| Medium (3-6 cm) |
2 |
Eloquent |
1 |
Deep component |
1 |
| Large (>6 cm) |
3 |
... |
... |
- Measure the largest diameter of the nidus of the lesion
on angiography.
- Eloquent areas include sensorimotor, language, visual,
thalamus, hypothalamus, internal capsule, brain stem, cerebellar
peduncles, and deep cerebellar nuclei.
- The lesion is considered to be "superficial" (on the brain's
surface) only if all venous drainage is via the cortical drainage
system.
There is considerable information to support the conclusions that the
most important and significant factors associated with higher risk of
future hemorrhagic events relates to large size AVMs and those that
reside deep within the brain. These are different factors than those
found to be associated with initial hemorrhagic presentation. This
finding stresses the importance of distinguishing between presentation
and natural history when making therapeutic decisions, because the two do
not necessarily coincide.
There are no effective pharmacological means available to
treat Arteriovenous Malformations.
It is common to use "Steroids" in an attempt to acutely improve the
patient's neurological function since steroids can decrease the swelling
(edema) that often accompanies an injury to the Brain. There is no
place for the long-term use of steroids since they do nothing to treat
the underlying pathology of the disorder and have deleterious side effects
such as gastric ulceration, elevated blood glucose levels, and suppression
of the immune system.
Treatment must be custom tailored to each patient since
each Arteriovenous Malformation is a unique lesion. The present surgical
treatment options include open microsurgical ligation and/or
resection of the malformation.
The advantage to direct resection of an AVM is that immediate
and permanent cure is possible after complete resection by
craniotomy. Surgery is generally recommended for grade 1, 2, and 3
lesions, sometimes for grade 4 lesions, and not for grade 5
lesions.
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Figure 2A (Left): Operative Photo (same patient as Figures 1A&B).
Compare the larger draining vein (Arrow) on the angiogram to this
photograph.
Figure 2B (Right): Same patient after complete resection of the
AVM. Note the change in the dilated arteries and veins in the
post-resection picture to the pre-resection one in Figure 2A.
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Among the disadvantages to a direct surgical approach are:
ischemic stroke, the potential for significant intraoperative
bleeding, and damage to adjacent neural tissue. In
most AVM cases, the arteries that supply the AVM also supply intact
neural tissue. These must not be destroyed while attempting to
selectively interrupt the arterial supply to the AVM. There is the
additional risk for "perfusion-breakthrough bleeding". This
is a dreadful complication resulting in post-operative hemorrhage
into the healthy part of the Brain caused by sudden hemodynamic
shifts. These "shifts" result from the removal of a large AV
malformation which had previously been "shunting" blood rapidly.
Once that "shunt" is removed the subsequent increased flow to the
previously underperfused Brain can result in this type of
haemorrhage.
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Figure 3A (Left): Operative Photo. Large partially calcified AVM
Left Temporal Lobe in a patient with Intractable Epilepsy.
Figure 3B (Right): Post-excision of the partially calcified AVM.
This is only a portion of the AVM resected where the deep (medial)
surface has been cut to demonstrate the extent of the lesion and
variation in size of the AVM vessels.
Clinical Note: The patient tolerated the resection well without
additional neurological deficit and the seizures were well
controlled.
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Endovascular occlusion (using Interventional Neuroradiology
techniques) incorporates various methods of "embolization". These
treatments (which involve the obliteration of the "feeding" vessels
with glues or particles delivered via arterial catheter in the
angiography suite) are common and often preferential
management methods for many of these lesions.
The primary advantage of this treatment option is the significant
reduction of pathologic blood flow through the lesion. Its
main use is as adjuvant therapy prior to craniotomy to decrease
intraoperative bleeding and technical difficulty. It has also
been used to decrease the size of an AVM to make it sufficiently compact
for effective targeting by Stereotactic Radiosurgery.
Nevertheless this is also an invasive procedure with risks to
ischemia and hemorrhage that are similar to those
for open surgery. The most significant risk is for
ischemic stroke by occluding a feeding vessel that also supplies normal
Brain. Local tissue hemodynamic alterations that
occur after embolization can cause rupture of the AVM,
resulting in new neurological deficit from hemorrhage into the
Brain, similar to the "perfusion-breakthrough bleeding" described
above. Endovascular embolization is not normally used by
itself, since it rarely achieves complete obliteration of
the AVM. Additionally, even when the vessels are
successfully occluded, they are likely to recanalize over time.
Radiosurgery (also known as Stereotactic Radiosurgery) is an
important method of treatment for some lesions since it is
noninvasive and can access all anatomic
locations of the Brain. Unfortunately this technique can only
treat smaller lesions (<3 cm in diameter) and requires 2 or more
years for its full destructive effect. Radiosurgery is believed to
"work" by initiating an "inflammatory" response in the
pathological blood vessels which ultimately results in their
progressive narrowing and ultimate closure. This process is
relatively slow and the risk for hemorrhage is not reduced during this
lag time. There is the added risk of radiation necrosis (death) of
adjacent healthy brain tissue or brain cyst formation.
Total eradication of the AVM may require more than one treatment
modality in some patients. It is well known that partial treatment of
an AVM may increase the risk of hemorrhage. Endovascular embolization
can be performed prior to a planned direct microsurgical excision in
order to reduce the difficulty of surgery. This may also be
appropriate prior to Radiosurgery in an effort to bring the size of the
lesion to within the limits of this Stereotactic technology.
Radiosurgery may be useful as well in instances where small
residual disease has been left after an attempted microvascular resection
of the AVM. Leaving a portion of AVM may be part of a planned
procedure when there is some significant microsurgical technical
difficulty or when the AVM involves "eloquent" or vital Brain structures.
http://www.emedicine.com/RADIO/topic93.htm
http://www.avmsupport.org.uk/brain_avms.php
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Cavernous Angiomas (also known as Cavernous Malformations,
Cavernous Hemangiomas or Cavernomas) are being discovered
with increased frequency due to the accuracy and availability of
MRI and CT scanning. These malformations are well-defined structural
lesions which frequently reach a significant size and can be
confused with a Brain tumor. Structurally these malformations
consist of enlarged capillaries with areas that are
dilated and which lack the smooth muscle and elastic fibers of normal
vessels. They can occur anywhere in the Central Nervous
System (CNS). A small proportion of Cavernous Angiomas are
hereditary.
One of the unique characteristics of these lesions is that
the Cavernous Angiomas vessels are directly adjacent to one another
without having any normal Brain tissue between them. These
abnormalities are not associated with enlarged feeding arteries or
draining veins. Blood flow within the lesion is quite slow and often
stagnated. The lesions are known to become clotted and may
calcify as part of a progressive "degenerative" process.
Cavernous Angiomas are known to occur with a higher
frequency in Mexican-American families (who also have a
higher incidence of multiple lesions) compared to the
general population. These lesions are often identified on
neuroimaging studies with small hemorrhages that also tend to leave some
residual products of that bleeding called "hemosiderin".
This hemosiderin is a "breakdown" product from the blood
and includes some "iron salts" that are precipitated
in the adjacent Brain tissue. These "left over" products of
the haemorrhage can result in changes that incite a scar
reaction (gliosis) within the Brain. The "gliotic" areas can then
become a focus for abnormal electric activity that causes
Epilepsy.
The statistical likelihood for any lesion to haemorrhage is less
than 1%; however, the re-haemorrhage rate dramatically increases in the
event that the lesion enlarges within one year of initial
identification.
Cavernous Angiomas are also referred to as Cryptic
Arteriovenous Malformations or Occult Malformations because
they usually do not show up on routine cerebral angiograms.
In some patients who have suffered an intracerebral haemorrhage,
neuroimaging investigations (including angiography) may not be able to
identify the source of the bleeding. In fact, the "source" (which was a
"Cryptic AVM") "obliterated" itself with the haemorrhage.
Approximately 75% of patients with a Cavernous Angioma never
develop any symptoms. Of those who do develop problems, the
commonest symptoms are epilepsy, hemorrhage and headache
consequent to a "mass" lesion (blood clot). Once a patient
becomes symptomatic they are likely to remain
symptomatic.
CT scans often show evidence of focal calcification, recent
hemorrhage and/or mass effect from a blood clot. There may be
faint "enhancement" when intravenous "contrast" material is
given as part of the neuroimaging study. Cerebral angiograms are
often entirely normal whereas MRI scans reveal
well-defined lesions on T1 and T2 sequences. The hemosiderin
ring is best seen on T2 sequences. Evidence of previous bleeding is often
seen in the center of the lesion as well as local Brain swelling which
results from the hemorrhage.
Surgical removal is appropriate for patients with intractable
seizures, patients who have experienced an increase in lesion size on MRI
scan and/or where there has been a significant gross hemorrhage.
The advantages to surgical removal in symptomatic patients include:
definitive removal of the lesion as well as the blood clot (mass);
elimination of any subsequent risk of hemorrhage; removal of the seizure
focus in cases where the lesion is associated with Epilepsy.
Approximately 50% of patients experience elimination of seizures
with the remainder having a decreased frequency of seizures. Elimination
of seizures is more likely if the patient has not had seizures for many
years.
The role of Radiosurgery is not well defined in the absence
of scientific evidence supporting its use for these lesions. There is
limited scientific information regarding large numbers of
patients treated by Radiosurgery and followed with neuroimaging
studies over an extended period of time.
http://www.ninds.nih.gov/disorders/cavernous_malformation/cavernous_malformation.htm
http://www.angiomaalliance.org/cainfo.html
http://ghr.nlm.nih.gov/condition=cerebralcavernousmalformation (GENETICS information)
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Venous Angiomas (Venous Malformations) are the most common
type of intracranial vascular malformation with an estimated
prevalence of approximately 2.5% of the general population.
A Venous Angioma is characterized by an enlarged
collection of veins which, other than their size, are microscopically
normal. These enlarged veins receive drainage from adjacent
healthy Brain. The veins have a characteristic appearance of
a radial arrangement, all converging on an enlarged central venous
trunk. This trunk drains into healthy superficial or deep
venous systems. Venous Angiomas are not associated with
abnormal arteries and are generally regarded as anomalies
rather than pathological structures.
Most patients that are identified with Venous Angiomas are consequent
to an incidental neuroimaging study.
Most patients with a Venous Angioma are completely
asymptomatic and have no need for any
treatment.
While it is generally accepted that Venous Angiomas have a very low
risk of bleeding, intracranial hemorrhage may be the presenting
symptom for a small percentage of patients. Patients may also
present with new onset seizure or focal neurological deficit.
Posterior Cranial Fossa (back & lowest "chamber" of the interior
of the Skull) lesions are more likely to result in some form of
neurological deficit.
Angiograms usually have a "hydra" or "caput medusae"
appearance consequent to the image of small radial veins converging
on a central draining venous trunk. CT or MRI Scans usually
demonstrate a curvilinear structure that resembles the spokes of a
wheel.
Venous Angiomas are usually part of the normal venous drainage system
for normal Brain. Therefore, surgical removal can lead to a disastrous
venous infarction of the surrounding and adjacent brain resulting in
significant morbidity and mortality. Excision or ablation of venous
Angiomas is to be avoided except for very unusual circumstances.
Surgical intervention may occasionally be appropriate in a
patient with a Venous Angioma to remove a clot after a
hemorrhage; however, it is more likely that the bleeding
originated from a Cavernous Angioma since the two appear to be
related pathophysiologically.
In a case of a hemorrhage associated with a Venous Angioma,
it is imperative to investigate the patient for an adjacent Cavernous
Angioma. If a Cavernous Angioma is found, surgical removal of
the clot and the Cavernous Angioma is appropriate while leaving the
Venous Angioma.
http://www.angiomaalliance.org/venous_angioma.html
http://www.emedicine.com/radio/byname/brain-venous-vascular-malformations.htm
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Capillary Telangiectasias are small (0.3 to 1.0 cm) lesions
composed of capillary vessels with saccular or fusiform dilations.
These tiny blood vessels lack muscular and elastic components in
their walls. The vessels are separated from each
other by normal appearing brain tissue without "gliosis"
(scarring of the Brain). These lesions are rarely symptomatic
and not likely to cause bleeding. Capillary Telangiectasias are
almost always clinically silent.
Most Capillary Telangiectasias are found incidentally on
autopsy. There are very few reports of concerning significant
hemorrhage from these structures. There is some scientific
speculation suggesting that Capillary Telangiectasias are
pathophysiologically related to Cavernous Angiomas.
Capillary Telangiectasia can be identified as a tiny area of
hypointensity on T2-weighted MRI scans although these hypodensities
may represent previous subclinical hemorrhage.
http://www.emedicine.com/RADIO/topic94.htm
http://cme.emedicine.com/wc.dll
http://www.emedicine.com/med/topic3469.htm
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The Vein of Galen is a major draining vein of the
Brain and is located deep within the center region of the
Brain. This lesion is usually identified on neuroimaging studies
as a large vascular "mass" in the region of the Pineal Gland. "Vein
of Galen Aneurysms" are not true aneurysms but more
accurately described as Galenic Arteriovenous Malformation.
Aneurysms of the Vein of Galen usually produce symptoms at,
or very soon after, birth. This condition is frequently
associated with Hydrocephalus (an accumulation of fluid within
the Ventricles of the Brain where Cerebrospinal Fluid [CSF] is
produced.)
The affected child will have visible enlargement of the head,
swollen veins on the scalp, seizures, failure to thrive, and congestive
heart failure. Children born with this condition who survive past infancy
often remain developmentally impaired.
Dr. Lazar has reported some of his experience with this difficult problem
in the scientific literature.
Vein of Galen aneurysm: successful excision of a completely thrombosed
aneurysm in an infant. Surg Neurol. 1974 Jan; 2(1):22-4. Lazar, ML
(There are multiple references to this article in the medical
literature regarding Vein of Galen Aneurysms.)
http://www.ijppediatricsindia.org/article.aspn
http://www.emedicine.com/neuro/topic538.htm
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This page last edited on 5/9
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