1. Arteriovenous Malformations (AVMs)
Definition, Presentation, Treatment, and Histology
By Dr. Martin Mihm, Jr.
and Linda Rozell-Shannon, M.S.
March 2005

2. 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.

3. 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

4. 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.

5. 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.

6. 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.

7. 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.

8. 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.

9. 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.

10. 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.

11. Schobinger’s 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).

12. 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.

13. 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.

14. 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.

15. 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):
< 3 cm = 1 point
3-6 cm = 2 points
> 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

16. 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.