Abnormal patterns of blood flow with shunting of blood from the high-resistance arterial system to the low-resistance venous system, bypassing the capillary beds, is common to all AVF, although it may be less prominent in some extratruncular AVMs. The normal arterial and venous flow, as well as the abnormalities that accompany an AVF, are depicted in Figs. 1 and 2. The symptoms that result from this phenomenon depend on the level at which the fistula exists and the size of the abnormal connection. The more central the fistula and the larger the degree of shunt, the more likely it is to become symptomatic. As a general rule, the multiple small communications associated with congenital AVMs lead to a more indolent clinical course than is associated with large acquired AVF. However, inexorable progression, albeit at varying rates, is the clinical pattern in all of these conditions, which has important implications for therapy. As a rule, arteriovenous shunting is greatest through acquired AVFs and truncular AVMs, and least through extratruncular AVMs.

Fig. 1 Normal physiology of an artery and vein without fistula. (a) and (b) depict normal blood flow through the artery. (c) and (d) illustrate normal venous return with presence of functioning valves.

Fig. 2 Altered physiology in the presence of an AVF. (a) Absence of reversal of flow during diastole. (b) Reversal of flow in the distal vessel, resulting in steal. (c) Dilatation of the proximal venous segment, venous hypertension. (d) The resulting incompetent valves.

Increased flow through the arterial system associated with reduced resistance in the venous system leads to loss of the normal reversal of arterial flow during diastole with continuous antegrade flow via the proximal artery into the AVF (see Fig. 2a). There is also an increase of blood flow in the artery proximal to the AVF. These hemodynamic changes lead to increased shear stress on the arterial wall. To accommodate that shear stress, the proximal artery dilates. In extreme cases, this dilation may become significant and the artery may become attenuated and/or aneurysmal. Initially blood flow is maintained in the distal arteries but, as the fistulous connections enlarge, peripheral arterial perfusion decreases and in some cases may be reversed. This process can lead to peripheral arterial ischemia, known as steal (see Fig. 2b) [4].

Blood flow also increases in the central vein. The shear stress of the pulsatile arterial blood flow through the vein leads to thickening, or arterialization, of the vessel wall, as well as dilatation and elongation. Moreover, with increased venous volume and pressure in the proximal vein, distal venous blood flow slows, resulting in valvular incompetence, reversal of venous blood flow, and venous hypertension. (see Fig. 2c, d) [5].

Systemic effects on the circulation become manifest as the fistula enlarges. There is an overall increase in the volume of blood in the venous system caused by the large reserve venous capacity. This increase results in increased return of blood to the heart, a decrease in peripheral blood pressure, and a subsequent increase in cardiac output by increasing both the heart rate and the stroke volume [6]. Because there is increased venous blood volume and a subsequent reduction of arterial blood volume, the renin-angiotensin-aldosterone system is activated, resulting in a further increase in blood volume via retention of sodium and water [4]. If the heart is unable to compensate for the increase of blood volume, high-output heart failure may result.

The signs and symptoms from AVFs are the result of increased shunting, venous hypertension, arterial ischemia, and compression of or impingement on adjacent structures. The severity of the signs and symptoms are related to the location and the size of the abnormality. Peripherally located fistulae more often result in locally recognized effects, whereas centrally located fistulae are linked to systemic symptoms. The signs and symptoms from AVF are listed in Box 1.

The most common presenting symptom of an AVF is a palpable thrill or audible bruit. There may be warmth of the skin overlying the fistula as well as a palpable mass [5]. As the vessels dilate and the size of the fistula increases, an increased amount of blood is shunted directly from the arterial system into the venous system. If a significant amount of blood is shunted away from the peripheral tissues, arterial insufficiency distal to the fistula may occur. This insufficiency can result in arterial insufficiency of varying degrees, from intermittent claudication, rest pain, tissue loss, and even gangrene. In addition to the complications of decreased distal arterial blood flow, there are complications of increased venous pressure. Extremities can develop evidence of chronic venous hypertension and insufficiency as seen by peripheral edema, skin hyperpigmentation, venous varicosities, and even venous ulceration [4].

A patient with a high-flow central AVF may initially present with signs and symptoms such as dyspnea on exertion, fatigue, and peripheral edema, which all indicate high-output cardiac failure. Brewster and colleagues [6] found symptoms of congestive heart failure in one-third of patients with aortocaval or iliocaval fistula. Renal insufficiency, hematuria, and peripheral edema are also common in this clinical scenario [1,6]. Patients may also present with lower extremity edema, venous hypertension, and pulsating varicose veins [1].

AVMs often present with signs and symptoms of venous dilatation, venous hypertension, and/or compression of adjacent structures. Peripheral AVMs usually present as a mass or cluster of dilated veins. Children with AVMs may develop discrepant limb growth. Patients with intra-abdominal or pelvic AVMs may present with pelvic pain or congestion, hematuria, or even rectal bleeding. Lesions in the central nervous system may manifest as neurologic deficits resulting from compression or edema of the brain or spinal cord or evidence of ocular engorgement in the case of a carotid cavernous fistula.

Careful history and physical examination can often identify the existence of an AVF, but additional imaging is required for confirmation as well as for planning of treatment. Imaging allows the practitioner to evaluate the size of the fistula, assess hemodynamic consequences, and define the relationship to adjacent structures.