Arteriovenous Malformations (AVMs)

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Arteriovenous Malformations (AVMs)

• Definition, Presentation, Treatment, and
  Histology
• By Dr. Martin Mihm, Jr.
• and Linda Rozell-Shannon, M.S.
• March 2005

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What are AVMs?

• Arteriovenous Malformations (AVMs) belong to a
  group of disorders known as vascular
  malformations.
• AVMs are defects of the circulatory system that
  generally arise during embryonic or fetal
  development or soon after birth.
• They consist of masses of abnormal blood vessels.
  
• There are three major groups of AVMs Truncal,
  Diffuse and Localized.
• Truncal common in the head, neck, upper limb and
  lower limb and pelvis (trunk area).
• Diffuse common in the lower limbs
• Localized common in any organ
• Lesions may be located superficially with only
  minimal arterio-venous shunting or more deeply
  with significant, high flow, AV shunting.
• AVMs consist of a blood vessel nidus (nest)
  through which arteries connect directly to veins,
  instead of through the elaborate collection of
  very small vessels called capillaries.

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Understanding AVMs

• In 1968 Holman gave us the single most important
  contribution to understanding these lesions.
• A proximal AVM with a significant shunt may cause
  an increase in cardiac output that in turn may
  lead to high output cardiac failure.
• Distal shunting, on the other hand, has the
  propensity to significantly reduce the flow rate
  beyond the shunt and so induce peripheral
  ischemia (steal syndrome) without adversely
  affecting the cardiac output.
• Because an AVM constitutes a high to low pressure
  shunt, an increased flow in the afferent artery
  may be seen due to the decreased peripheral
  resistance

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Understanding AVMs (continued)

• Increased flow will, in turn, cause dilation and
  tortuousity of the afferent artery with
  subsequent thickening of the wall due to
  hypertrophy in the media.
• In the face of a large shunt across an ARV, the
  arterial flow distal to the shunt may reverse
  direction, thus causing distal ischemia, or the
  steal syndrome.
• The decrease in arterial pressure also encourages
  the development of an extensive collateral
  circulation.
• An increased flow through the AVM may also dilate
  the efferent venous system with consequent
  thickening of the wall due to hypertrophy of the
  media.

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Histological Analysis of AVMs

• Histological analysis of AVMs has shed some light
  on their pathogenesis
• Examination of specimens revealed that a nidus is
  made up of a bed of dilated capillaries.
• As the lesion matures, the degree of ectasia
  increases, and the development of venous dilation
  and arterial hypertrophy becomes apparent.
• The primary abnormality or nidus, therefore,
  appears to be an ectatic capillary bed.
• Arterial hypertrophy and venous dilation are
  secondary phenomena that result from the increase
  flow across the nidus.

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Histological Analysis of AVMs (continued)

• Because the nidus is simply an ectatic capillary
  bed and because the precapillary sphincters
  regulate the blood flow through the capillary
  bed, we believe that arteriovenous malformations
  result from an abnormality at the level of the
  precapillary sphincter.
• An absence of autonomic nerve supply to the
  sphincters, an absence of the actual sphincters,
  or some deficiency in the neuroreceptors at this
  level will result in free flow across that
  particular capillary bed.
• In time, the vessels in the bed dilate, and
  eventually the area supplying the arteries
  enlarge and the veins dilate.
• This absence of capillary sphincter control may
  be absolute or relative, hence the variation in
  age of presentation and speed of progression.

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AVMs Growth and Bleeding Cycle?

• Some people are born with the nidus. As years go
  by, it tends to enlarge as the pressure of the
  arterial vessels cannot be handled by the veins
  that drain out of it.
• Most of these malformations bleed between the
  ages of 10-55 after 55, the chances of bleeding
  diminishes rapidly. Before 55, the likelihood of
  hemorrhaging is between 3-4 per year (with a
  death incidence of about 1).
• Once a patient has hemorrhaged, the risk of
  having another one may approach 20 during the
  first year, and will gradually lessen to about
  3-4 over the next few years.

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What Are Some AVM Statistics

• AMVs affect approximately 300,000 Americans.
• In the Netherlands between 1980 and 1990, the
  annual incidence of symptomatic AVMs was 1.1 per
  100,000 population.
• They occur equally in males and females from all
  ethnic and racial backgrounds.
• They are more prevalent in late childhood (over 9
  years of age) than early childhood, although they
  can occur at any age.
• More than 50 present with intracranial AVMs.
• About 12 of the affected population will present
  with symptoms that vary greatly in severity.
• Each year about 1 of those with AVMs will die as
  a direct result of the AVM.

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Presentation of an AVM

• Some AVMs of the body (not internal) have an
  overlying port wine stain.
• Exterior (non brain) AVMs are usually noticeable
  at birth because even the deeper lesions cause
  cutaneous blushing and localized warmth.
• Ulceration of the overlying skin and skeletal
  hypertrophy are all clinical signs of AVMs of the
  head and body.
• AVMs of the brain usually present with seizure
  and have an 2 rate of hemorrhage.
• AVMs are firmer to palpation than venous
  malformations (VM) and do not empty as readily as
  VMs.
• Once compressed, AVMs rapidly refill.
• A bruit is often felt with an AVMs and is easily
  detected by a doppler.
• Large facial AVMS are problematic because they
  are so disfiguring and can cause severe
  psychological problems.

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Diagnosing an AVM

• Clinical exam and patient history are usually
  adequate for accurate diagnosis
• The initial cutaneous blushing may mislead the
  clinician to diagnose a superficial hemangioma,
  however, the absence of a rapid growth phase
  during the first month of life suggests that the
  lesion is a vascular malformation since true
  hemangiomas display a proliferative phase during
  the first weeks after birth.
• Physical exam is usually adequate to separate
  arteriovenous malformations from venous
  malformations as the presence of a thrill, bruit,
  or pulsation indicates the presence of an
  arterial component to the lesion.
• Doppler or ultrasound is useful in diagnosis, and
  they will appear as heterogeneous lesions with
  large vessels and multiple sites of pulsatile AV
  shunting.
• MRI/MRA is excellent for evaluating the extent of
  the lesion and for separating them from venous
  lesions.

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Schobingers Natural History of AVM

• Quiescence Cutaneous blush and increased warmth
  of skin (Many AVMS never progress past the
  Quiescence stage).
• Expansion Darkening cutaneous blush, enlarging
  soft tissue mass and increased cardiac output
  AV fistulas enlarges (Expansion starts in
  childhood or early adolescence often in response
  to puberty or trauma).
• Destruction Arterial steal worsens, causing
  distal ischemia, pain, and necrosis. Increased
  total blood flow causes edema, dystrophic skin
  changes, ulceration, bleeding. Soft tissue and
  bone hypertrophy may occur.
• Decompensation High output cardiac failure
  (Only the largest AVMs reach this stage).

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Treatment Options For Most Head, Neck And Body
AVMs

• AVMs are complex lesions which require a
  multi-disciplinary team approach.
• Treatment starts with determining whether the
  goal is to cure, manage pain, ulcers or bleeding
  or to treat functioning disabilities (vision,
  breathing, feeding, etc.)
• Noninvasive imaging such as computed tomographic
  scan and MRIs can localize the main mass effect,
  but an intravenous injection of contrast material
  is required.
• The complexity of these lesions requires
  complementation with super selective angiography.
• Proper planning for treatment can only be done
  with an understanding of the functional vascular
  anatomy including the dominant blood supply and
  potential collaterals of a lesion.

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More Information on Treatment

• Selective intra-arterial embolization with fine
  catheters and direct lesional embolization
• Judicious resection and reconstruction with local
  or expanded tissue flaps
• Careful follow-up with serial examinations, and
  arteriography.
• Treatment is difficult because it must be safe
  and result in an optimal aesthetic outcome while
  minimizing the chance for recurrence.
• Ligation of bilateral external carotid arteries
  (proximal arterial ligation or embolization) has
  not been successful but it resulted in
  enlargement of the malformation and severe
  internal carotid artery atherosclerosis.
• Selective embolization of the nidus has been
  shown to successfully collapse an AVM for a
  temporary period.
• Selective embolization combined with subsequent
  radical resection can lead to complete resolution
  of some AVMs.

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Imaging and Other Successful Management
Techniques

• Arteriography demonstrates dilated feeding
  arteries high flow arteriovenous shunts, direct
  venous draining and stealing.
• The use of a reconstruction algorithm can be
  followed to achieve optimal surgical results.
• Communication between the endovascular team and
  the surgical team is mandatory from the beginning
  of the treatment plan.

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Use of the Spetzer-Martin 5-Point Scale for Brain
AVMS?

• A score of 4 or 5 points higher risk of
  persistent neuro deficit after surgery
• Size of lesion (maximal diameter)
• lt 3 cm 1 point
• 3-6 cm 2 points
• gt 6 cm 3 points
• Location
• Noneloquent 0 point
• Sensorimotor, language visual cortex,
  hypothalamus, internal capsule, brainstem,
  cerebral peduncle, cerebellar nuclei 1 point
• Patterns of Venous Drainage
• Superficial 0 point
• Deep 1 point

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AVM - Summary

• AVMs are complicated vascular malformations which
  can grow in the brain, head and neck area, limbs,
  or even in organs.
• An understanding of the functional vascular
  anatomy is critical to successful treatment.
• Successful treatment requires a
  multi-disciplinary team that has constant
  interaction.
• The combination of pre-surgery embolization
  followed by surgical resection is currently
  yielding the most successful results for head,
  neck and body lesions.
• A combination of surgery, embolization and/or
  radiation has the highest success rate for
  treating brain AVMs.
• Each AVM is unique and complicated. A treatment
  algorithm can be followed to achieve optimal
  results.