Arteriovenous Fistulas ?>

Arteriovenous Fistulas

Arteriovenous Fistulas
Introduction
Background

Vascular birthmarks encompass a group of vascular disorders that for a long time included conditions of various presentation and etiology. In 1996, the Workshop of the International Society for the Study of Vascular Anomalies (ISSVA), with a subsequent modification, in 1998 presented a new classification of this vast group of conditions dividing them into vascular tumors and vascular malformations.1 This allowed for more systematic classification and management with less confusion in diagnosis of these often complex lesions.

Arteriovenous fistula per definition describes an abnormal communication between an artery and a vein. These communications are congenital; can occur at any point in the vascular system; and vary in size, length, location, and number. Arteriovenous fistula (AVF) is a term reserved for a singular communication between an artery and a vein that usually has an acquired etiology.

The first recorded case of an arteriovenous malformation (AVM) was in the late 16th century. In 1757, William Hunter described an arteriovenous fistula (AVF) as an abnormal communication between an artery and a vein. Krause in 1862 used injection studies of an amputated specimen to characterize the abnormal vasculature. In 1875, Nicoladoni described the reflex slowing of the pulse following occlusion of an artery proximal to an arteriovenous malformation (AVM). In 1920, Halsted contended that an arteriovenous malformation (AVM) could produce cardiac enlargement and observed that a congenital arteriovenous fistula (AVF) without a nevus is rare. In 1936, Emile Holman published a text describing the pathophysiology and natural history of arteriovenous malformations (AVMs). This publication forms the basis for today’s knowledge. In 1967, Fontaine observed that puberty or pregnancy can cause enlargement of arteriovenous malformations (AVMs).

Pathophysiology

Classification of vascular anomalies has been recently revised, resulting in a division being made between tumors and malformations. The first group is characterized by high turnover of endothelium, whereas the latter has presence of dysmorphogenesis with no evidence of abnormal epithelial turnover.

Vascular tumors

The most common vascular tumor is infantile hemangioma. It is seen shortly after birth, is more common in girls and in whites, and is congenital. These tumors are usually solitary but not infrequently they may be multiple and involving the liver, GI tract, and CNS. They are characterized by 3 phases of growth-proliferation, involution, and involuted phase.

The first phase is characterized by rapid growth of the tumor, bright red or bluish in color, firm, and tense in appearance. They may develop surface ulcerations and episodes of bleeding.

The involuting phase usually begins at the end of first year of life and is characterized by decreased tempo of the tumor growth, with the gradual color fading from the center of the mass and less tense consistency.

Transition to the involuted phase occurs during the second half of the first decade of life. Normal skin is restored in half of patients, and persistent skin changes such as telangiectasias, skin thinning and scarring may occur.

Other congenital tumors include tufted angioma, infantile fibrosarcoma, infantile myofibromatosis and kaposiform hemangioepithelioma.

Vascular malformations

Generally, congenital vascular malformations are in-born errors in embryologic development. Woollard described the development of the vascular system in 3 stages.

The first step is the condensation of undifferentiated cells into capillary blood spaces. The next stage involves the formation of a retiform plexus in which blood flows from an arterial to a venous side. The channels in the retiform plexus combine and regress to form the vessels in the vascular system. The last stage involves the development of axial arteries in the limb buds.

The final product is a complex interplay among genetic, hormonal, biochemical, and chemical factors. Developmental arrest can occur at any point during vessel formation, creating different types of vascular malformations. The exact cause for the arrest is not entirely known. All arteriovenous malformations (AVMs) are present at birth, but they are not always clinically evident. Stimuli during puberty or pregnancy or following minor trauma can precipitate clinical features of the malformation.

Vascular malformations are characterized by normal epithelial turnover and are usually sporadic; however, some of them have been linked to genetic disorders.

They can be divided based on the type and size of vascular bed involved, as well as the rate of flow through them.

Capillary malformations

Capillary malformations are usually sporadic. They can be divided into slow-flowing Klippel-Trenaunay, Maffucci, and Proteus syndromes and fast-flowing Parkes Weber and Bannayan-Riley-Ruvalcaba syndromes.

They are frequently associated with dilated capillary vessels in the dermis, asymmetric overgrowth of the involved limbs, and sometimes multiple soft tissue tumors, abnormal development of venous and arterial systems, and the presence of port-wine stains.

Lymphatic malformations

Lymphatic malformations are usually related to genomic mutations and include Milroy disease, Meige syndrome, yellow-nail syndrome, and Noonan syndrome. They are usually characterized by abnormal development of different portions of the lymphatic system.

Venous malformations

Venous malformations are the most common vascular malformations. They are usually sporadic and may be present at birth but are not always clinically obvious and predominantly occur in the skin and soft tissue. They include glomuvenous malformation and blue rubber bleb nevus syndrome.

Arteriovenous malformations

Arteriovenous malformations have the presence of arteriovenous shunts in multiple capillary beds both on the skin but also involving internal organs. They are usually accompanied by a bruit and hyperemia with prominent venous outflow. They are represented by hereditary hemorrhagic telangiectasia syndrome, also known as Rendu-Osler-Weber disease.

Although the pathogenetic mechanisms of arteriovenous malformations (AVMs) are not completely understood, the hemodynamic alterations that lead to the clinical manifestations of arteriovenous malformations (AVMs) have been described well.
An abnormal communication causes shunting of blood from the high-pressure arterial side to the low-pressure venous side. This creates an abnormal low-resistance circuit that steals from the high-resistance normal capillary bed.
Blood follows the path of least resistance. Flow in the afferent artery and efferent vein increases, causing dilatation, thickening, and tortuosity of the vessels.
If the resistance in the fistula is low enough, the fistulous tract steals from the distal arterial supply, actually causing a reversal of arterial flow in the segment distal to the arteriovenous fistula (AVF). This is known as a parasitic circulation. The parasitic circulation causes decreased arterial pressures in the distal capillary beds and can cause tissue ischemia.
The increased flow into the venous circulation does not necessarily cause higher venous pressures. However, it can cause vessel wall abnormalities such as thickening of the media and fibrosis of the wall. These changes are known as arterialization.
The blood flow into the venous circulation causes turbulence, which is responsible for the palpable thrill. The thrill is dependent on the geometry of the fistula and does not represent volume of flow accurately.
In addition to the decreased distal arterial pressures, which might cause distal ischemia, peripheral venous pressures are increased, leading to swelling, visible veins (varicosities), and even ulcers in the limb.
The heart responds to the decreased peripheral vascular resistance by increasing stroke volume and cardiac output. This leads to tachycardia, left ventricular dilatation, and, eventually, heart failure.

Other classification systems for arteriovenous malformations (AVMs) have been suggested. Mulliken describes a classification system based on structural criteria.
Truncal arteriovenous malformations (AVMs) are usually hemodynamically active and tend to present in the upper limb, head and neck, and pelvis. The lesions are localized and composed of a mass of enlarged vessels.
Diffuse arteriovenous malformations (AVMs) are usually found in the limbs, more frequently in the lower limbs than the upper limbs. In contrast to the truncal type, the connections are small and numerous. They are hemodynamically less active.
Localized arteriovenous malformations (AVMs) are usually inconsequential hemodynamically because of higher resistance in the connections. The lesions are composed mainly of abnormal intercalated tissues, not masses of enlarged vessels.

Any structural type of arteriovenous malformation (AVM) can be hemodynamically active or stable.
Mortality/Morbidity

Many congenital arteriovenous malformations (AVMs)/arteriovenous fistulas (AVFs) may regress spontaneously. The large ones, over the years, may lead to cardiac decompensation and death. In acquired arteriovenous fistulas (AVFs), death can occur from cardiac failure, infection (bacterial endocarditis), or rupture, if the arteriovenous fistula (AVF) is between a large artery and veinlike iliac or renal arteriovenous fistulae or aortocaval fistula.
Sex

Arteriovenous malformations (AVMs) occur with equal frequency among males and females.
Age

All arteriovenous malformations (AVMs) are present at birth, but they are not always clinically evident. Stimuli during puberty or pregnancy or following minor trauma can precipitate clinical features of the malformation.
Clinical
History
Cutaneous malformations can present with a mass, pink stain, dilated veins, unequal limb length and girth, or skin ulceration.
Patients may experience limb heaviness that is aggravated with dependency and relieved with elevation.
One half of patients experience pain. The pain may be caused by tissue ischemia or by mass effect on local nerves. Some lesions, such as glomuvenous malformations, can be tender to palpation.2
The increased blood flow to the limb in congenital arteriovenous malformation (AVM) or arteriovenous fistula (AVF) may result in increased growth of the limb (one leg may be larger and longer than the other). In acquired arteriovenous fistulas (AVFs), a history of trauma (eg, gunshot wound, stab wound, or even blunt trauma and fractures) can exist. Arteriovenous fistula (AVF) can also occur after medical diagnostic or interventional procedures like angiography or even after operative procedures that have caused inadvertent trauma to the artery and vein.
Physical

Small arteriovenous fistula (AVF) and arteriovenous malformation (AVM) may be totally asymptomatic and may be incidentally discovered. Large arteriovenous fistula (AVF) may present with increased size of the limb, mild discoloration, swelling, or prominent veins with audible murmur or palpable thrill.
The lesion may be pulsatile.
Branham sign may be present (slowing of the heart rate upon compression proximal to arteriovenous malformation).
Patients may develop hyperhidrosis, hypertrichosis, hyperthermia, or a palpable thrill or bruit over the lesion.
Patients may have functional impairment of limbs or joints from mass effect or gangrene from prolonged tissue ischemia.
Visceral arteriovenous malformations (AVMs) can present with hematuria, hematemesis, hemoptysis, or melena.
Rarely, patients present initially with signs of congestive heart failure (eg, dyspnea, leg edema). This is particularly common when the communication is between a very large artery and a vein.
Causes

The majority of arteriovenous malformations (AVMs) are developmental errors that occur between the 4th and 10th weeks of embryogenesis. The factors that cause these errors are unknown. Potential exogenous causes, such as viral infections, toxins, and drugs, have been implicated but not yet proven. Almost all arteriovenous malformations (AVMs) are sporadic and nonfamilial, although a few syndromes (eg, Sturge-Weber, Klippel-Trenaunay) include inherited vascular abnormalities.

The most common etiology for acquired arteriovenous fistulas (AVFs) is penetrating trauma. AVFs also can occur from nontraumatic causes, such as erosion of an aneurysm into a neighboring vein or following surgery for therapeutic purposes (eg, access for hemodialysis).

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