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Vascular Surgery > MEDICAL TOPICS
Arteriovenous Fistulas
Article Last Updated: Jun 22, 2006
AUTHOR AND EDITOR INFORMATION
Section 1 of 10  
Author: Sateesh C Babu, MD, Professor of Clinical Surgery, New York Medical College; Associate Director of Vascular Surgery, Co-Chief of Endovascular Surgery, Department of Surgery, Westchester Medical Center
Sateesh C Babu is a member of the following medical societies: American College of Surgeons, American Heart Association, American Institute of Ultrasound in Medicine, American Medical Association, Eastern Vascular Society, International Society of Endovascular Specialists, New York Academy of Sciences, Royal Society of Medicine, Society for Vascular Surgery, and Stroke Council of the American Heart Association
Coauthor(s):
Mark D Morasch, MD, Clinical Practice Director, Division of Vascular Surgery, Assistant Professor of Surgery, Department of Surgery, Northwestern University Feinberg School of Medicine;
Dipen Maun, MD, Staff Physician, Department of Surgery, Mount Sinai School of Medicine
Editors: William H Pearce, MD, Chief, Division of Vascular Surgery, Violet and Charles Baldwin Professor of Vascular Surgery, Department of Surgery, Northwestern University School of Medicine; Francisco Talavera, PharmD, PhD, Senior Pharmacy Editor, eMedicine; Vincent Lopez Rowe, MD, Assistant Professor of Surgery, Department of Surgery, Division of Vascular Surgery, University of Southern California Medical Center; Paolo Zamboni, MD, Professor of Surgery, Chief of Day Surgery Unit, Chair of Vascular Diseases Center, University of Ferrara, Italy; William H Pearce, MD, Chief, Division of Vascular Surgery, Violet and Charles Baldwin Professor of Vascular Surgery, Department of Surgery, Northwestern University School of Medicine
Author and Editor Disclosure
Synonyms and related keywords:
vascular malformations, AVF, acquired singular communication between an artery and a vein, arteriovenous malformations, AVM, congenital abnormal communication between an artery and a vein
Background
An arteriovenous malformation (AVM) is 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 AVM was in the late 16th century. In 1757, William Hunter described an 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 AVM. In 1920, Halsted contended that an AVM could produce cardiac enlargement and observed that a congenital AVF without a nevus is rare. In 1936, Emile Holman published a text describing the pathophysiology and natural history of AVMs. This publication forms the basis for today's knowledge. In 1967, Fontaine observed that puberty or pregnancy can cause enlargement of AVMs.
Pathophysiology
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 AVMs are present at birth, but they are not always evident clinically. Stimuli during puberty or pregnancy or following minor trauma can precipitate clinical features of the malformation.
Although the pathogenetic mechanisms of AVMs are not completely understood, the hemodynamic alterations that lead to the clinical manifestations of 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 will steal from the distal arterial supply, actually causing a reversal of arterial flow in the segment distal to the 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.
Various classification systems for AVMs have been suggested, but none are accepted as definitive. Mulliken describes a classification system based on structural criteria.
- Truncal AVMs usually are active hemodynamically 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 AVMs usually are found in the limbs—lower limbs more frequently than upper limbs. In contrast to the truncal type, the connections are small and numerous. They are hemodynamically less active.
- Localized AVMs usually are 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 AVM can be hemodynamically active or stable.
Mortality/Morbidity
Many congenital AVM/AVF may regress spontaneously. The large ones, over the years, may lead to cardiac decompensation and death. In acquired AVFs, death can occur from cardiac failure, infection (bacterial endocarditis), or rupture, if the AVF is between a large artery and vein-like iliac or renal arteriovenous fistulae or aortocaval fistula.
Sex
AVMs occur with equal frequency among males and females.
Age
All AVMs are present at birth, but they are not always evident clinically. Stimuli during puberty or pregnancy or following minor trauma can precipitate clinical features of the malformation.
History
- Cutaneous malformations can present with a mass, pink stain, dilated veins, unequal limb length and girth, or skin ulceration.
- Patients may complain of limb heaviness that is aggravated with dependency and relieved with elevation.
- One half of patients complain of pain. The pain may be caused by tissue ischemia or by mass effect on local nerves.
- The increased blood flow to the limb in congenital AVM or AVF may result in increased growth of the limb (one leg may be larger and longer than the other). In acquired AVFs, a history of trauma (gunshot wound, stab wound, or even blunt trauma, fractures, etc.) can exist. AVF can also occur after medical diagnostic or interventional procedures like angiogram or even after operative procedures that have caused inadvertent trauma to the artery and vein.
Physical
Small AVF and AVM may be totally asymptomatic and may be discovered incidentally. Large 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 AVM).
- 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 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 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 AVMs are sporadic and nonfamilial, although a few syndromes (eg, Sturge-Weber, Klippel-Trenaunay) include inherited vascular abnormalities.
The most common etiology for acquired 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).
Cirrhosis
Other Problems to be Considered
Differential diagnosis of AVF includes other conditions that may cause hyperdynamic circulation (increased heart rate, increased cardiac output, and low peripheral resistance). Cirrhosis, hyperthyroidism, Paget disease of the bone, and, occasionally, vary large highly vascular tumors like sarcomas.
Lab Studies
- Blood gas analysis in an AVF reveals a higher oxygen saturation in the venous blood immediately distal to the fistula as compared to normal venous blood.
- Hemodynamic assessment with flow directed balloon catheter (Swan-Ganz catheter) reveals high cardiac output and low peripheral vascular resistance (PVR).
- Extremely large AVF or AVMs may present with low platelet count (due to turbulence and trapping of platelets), and occasionally, with laboratory findings of consumptive coagulopathy such as low platelets, elevated prothrombin time (PT) and partial thromboplastin time (PTT), increased bleeding time, low fibrinogen, and elevated euglobulin clot lysis time, signs of fibrinolysis.
Imaging Studies
- Plain films may demonstrate soft tissue masses or abnormalities within bony structures.
- Duplex ultrasonography can be used to characterize the direction and velocity of blood flow. The Doppler can be used preoperatively and intraoperatively. However, ultrasound does not have any therapeutic use.
Duplex scan will show reversal of flow in the artery distal to the AVF (steal phenomenon). - Contrast-enhanced CT scans are useful to locate the abnormality, to evaluate for aneurysm formation, and to identify bony involvement.
- Contrast angiography is the most important method for investigating AVMs or AVFs. It is an excellent method to delineate the number, location, and extent of the arteriovenous connections. Angiographic signs include early filling of veins, hypertrophied and tortuous arteries proximal to the malformation, and varicose and dilated veins distal to the fistula.
- MRI has become the new criterion standard in the preoperative evaluation of patients with AVMs. MRI generates multiplanar views and can be used to accurately define tissue planes and to identify critical flow characteristics. MRI is the best modality to define local soft tissue and adjacent organ involvement, which helps with preintervention planning. Magnetic resonance (MR) sequences can be postprocessed into MR angiogram images, which help to define the malformation more clearly.
- Radiolabeled studies can determine the shunt fraction, which is the proportion of blood being shunted through the fistulous tract.
Other Tests
- Plethysmography is useful for quantifying flow in a whole limb, but assessing blood flow through circumscribed areas is difficult. Ideally, plethysmography data are compared to normal data from the contralateral limb.
- Thermography determines heat loss from a region. However, results are of limited clinical value because they do not reveal the location of the lesion accurately, and the data cannot be used to differentiate among various types of vascular malformations.
- In AVF of limbs, segmental limb pressure measurements can document a significant drop in pressure distal to the fistula. This can be used before and after surgical correction of the fistula to confirm that the fistula has been eliminated
Procedures
- Percutaneous biopsy never is indicated in the workup of a known vascular malformation; bleeding that is difficult to control may result. Open biopsy can be helpful if a lesion is suspected to be sarcomatous or if the clinical impression is unclear.
- Invasive and noninvasive cardiac evaluation may be indicated in patients with congestive heart failure because cardiac output can be markedly elevated in patients with large proximal AVMs. Cardiac output is best measured with invasive right heart catheter techniques but can be evaluated noninvasively with echocardiography. In order to document success, measurement of cardiac output is indicated before and after surgical or interventional procedures to reduce the size of these larger AVMs.
Histologic Findings
Histology documents arterialization of the thickening of the wall of the vein, including its muscular layer, and thinning of the artery in large long standing arteriovanous fistulas.
Medical Care
The majority of arteriovenous malformations can be managed and controlled medically. Only a few AVMs demonstrate progressive growth and require surgical intervention. Most of the symptoms of AVMs (pain, heaviness, swelling) are due to venous hypertension. The cornerstone approach in managing lower extremity symptoms is elastic support hose. An elastic support stocking that provides 30-40 mm Hg of compression usually is sufficient to relieve leg symptoms.
Alcohol sclerotherapy may shrink the size of the AVM, but this treatment also places the patient at risk for peripheral nerve injury. The treatment of large AVMs with alcohol needs to be performed by an experienced interventional radiologist, and these risks must be explained to patients when they consent to undergo therapy.
Surgical Care
Indications for surgical intervention of AVMs include hemorrhage, painful ischemia, congestive heart failure, nonhealing ulcers, functional impairment, or limb-length inequality.
Embolization is an option for treatment and should be considered when conservative measures have failed or when the vascularity of the malformation needs to be reduced prior to surgical resection. The procedure involves the percutaneous placement of a vascular catheter and the injection of particulate matter into the malformation. Passage of emboli into the normal circulation occurs but usually only poses a problem if it enters the cerebral or mesenteric vasculatures. The common adverse effects are pain and tenderness near the malformation and a transient fever and leukocytosis. More worrisome complications include necrosis of healthy adjacent tissue and neurologic injury. Thorough angiographic imaging and clear delineation of the vessels helps to minimize most of these adverse effects. Embolization can provide a promising treatment option if it is carried out by an experienced interventional radiologist.
Most AVMs are not amenable to complete surgical excision. A lesion must be well localized for a chance at complete resection. Resectability depends on the degree of extension into adjacent structures. Patients with disease that extends into the deep fascia or contiguous structures, such as muscle and bone, usually are not surgical candidates. Malformations that extend into the pelvis and gluteal region also are not surgically resectable. Those patients severely afflicted with malformations who are not candidates for local extirpation may be candidates for amputation and rehabilitation with a limb prosthesis.
- In contrast to the congenital AVMs that are difficult to treat, almost all acquired AVF are amenable for either surgical or interventional treatment. Occlusion of the feeding vessel with coils can be done. If the AVF is between a medium- or large-sized artery and vein, then occlusion of the artery may be hazardous. Surgical treatment is preferred . The fistulous communication is disconnected and repair of the defect in the artery and vein is accomplished. Recently, vascular surgeons are able to treat some of these with the minimally invasive endovascular techniques. A covered stent graft is deployed in the artery, thus covering the site of communication between the artery and vein.
Complications
- Except for very small AVFs, all other acquired AVFs need to be treated to prevent complications of distal limb ischemia, continued large flow of blood with eventual heart failure, and rarely infection, such as endocarditis. Recurrence is a complication of inadequate or incomplete treatment.
Prognosis
- Recent advancements in the diagnosis and treatment of arteriovenous malformations are promising. These advancements provide hope to patients who are affected by this sometimes debilitating disease. In acquired AVFs, prognosis is excellent once the AVF is corrected.
- Yakes et al obtained follow-up studies in 19 of 20 patients with AVMs treated with ethanol embolization. All patients showed persistent occlusion of the malformation radiographically after as long as 24 months of follow-up. Wildus et al treated 11 patients with cyanoacrylate embolization. During a 40-month follow-up period, 82% reported complete resolution of their symptoms and the remaining patients showed improvement. No patients reported worsening of their symptoms with embolization in these 2 series.
- In 1992, Pearce reported experience in 15 patients with vascular malformations treated surgically. Five patients were lost to follow-up. Assuming those patients did well and did not seek further intervention, two thirds of those undergoing excision improved. Thirteen percent were unchanged and 20% were worse after surgical excision.
Patient Education
- Physicians have to be aware of the subtle signs of AVF to make the correct diagnosis. Prominent veins in the leg in a young individual following trauma, which may be mistakenly diagnosed as simple varicose veins, and rapid onset of heart failure in an otherwise healthy person, which may be diagnosed as cardiomyopathy, are examples for the need to look further for the presence of AVFs.
Medical/Legal Pitfalls
- Alcohol sclerotherapy may shrink the size of the AVM, but this treatment also places the patient at risk for peripheral nerve injury. The treatment of large AVMs with alcohol needs to be performed by an experienced interventional radiologist, and these risks must be explained to patients when they consent to undergo therapy.
- It is easy to miss the presence of AVF after blunt or penetrating trauma, but it needs to be looked for. In cases of any gunshot wound or stab wound that traverses the course of a blood vessel, ruling out vascular injury, particularly AVF, with an imaging study (duplex scan, CT scan, MR angiogram, or even conventional angiogram) is important. Failure to diagnose may have medicolegal implications.
| Media file 3:
Upper extremity arteriovenous malformation (AVM). |
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| Media file 4:
Hypertrophied subclavian artery resulting from low-resistance high-volume flow through an upper extremity arteriovenous malformation (AVM). |
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| Media file 5:
Hand angiogram demonstrating arteriovenous connections. Note steal of blood from fingertips. |
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Arteriovenous Fistulas excerpt Article Last Updated: Jun 22, 2006
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