Figure 1 and Movie 1: The thoracic aorta, proximal great vessels, left subclavian and left branchial
arteries are all patent and are without evidence of focal stenosis. There is a surgical arteriovenous fistula connecting the distal left brachial artery to the cephalic vein with a severe focal stenosis of the venous outflow from the fistula approximately 2 cm from the anastomosis site (arrow).

Arteriovenous fistula stenosis.

Arteriovenous fistulas (AVF) are abnormal communications between arteries and veins. There are two types of fistulas: acquired and congenital. The congenital type of AV fistulas are developmental abnormalities that are present from birth and most often occur in the head and neck area. Acquired AV fistulas are most often created surgically to provide long-term vascular access for hemodialysis, chemotherapy, and parenteral nutrition, or they may be iatrogenic due to a needle puncture from venipuncture, arterial blood gas sampling, or catherization. They can also be caused secondary to penetrating trauma and can be found in association with neoplasms, infection, or atherosclerotic aneurysms.

Chronic hemodialysis requires long-term access to the vascular tree. In most patients the upper extremity is the preferred site of arteriovenous fistula (AVF) construction or arteriovenous graft (AVG) placement. These surgically created fistulas are usually between the radial artery and cephalic vein. The direct arteriovenous (DAV) Brescia-Cimmo fistula is constructed between the radial artery and the cephalic vein. This AV fistula type is the best access for long-term hemodialysis and has the highest patency rate and lowest incidence of complications. For patients with no suitable vessels for forearm or antecubital DAV fistula, a straight (radial artery to antecubital vein) or looped (brachial artery to antecubital vein) forearm bridge graft can be used. Currently, expanded polytetrafluoroethylene graft (E-PTFE) is the preferred graft material with the longest shunt survival and fewer complications compared with saphenous vein or bovine carotid artery. The natural history of vascular access is that of eventual failure with each construction having variable but limited life span. The challenge is to provide access with the highest long-term patency and the least complications. The common complications of AVFs and AVGs include stenosis, thrombosis, infection and aneurysm formation. Other less common complications include steal syndrome, increased cardiac output, venous hypertension, congestive heart failure, and peripheral neuropathy.

Thrombosis is the most frequent complication in hemodialysis AV fistulas and AV grafts. The incidence of thrombosis is 0.5-2.5 per patient per year. AVF and AVG thrombosis is secondary to vascular access stenoses caused by progressive intimal hyperplasia. Stenotic lesions are more common in prosthetic graft hemodialysis fistulas than autogeneous fistulas. Intrinsic narrowing or stenosis can occur anywhere along the course of the AVF or AVG and may be focal or involve a long segment of the graft/fistula. Vascular access stenoses and subsequent thrombosis most commonly occur at several centimeters proximal to the anastomosis (on the venous side) of an AVF. In an AVG the usual site of stenosis is at the graft to vein anastomosis. The likelihood of thrombosis depends of the type of fistula or shunt constructed, site of the arteriovenous anastomosis, selection of prosthetic materials (most commonly polytetrafluoroethylene graft), and the adequacy of the patient’s vessels. The etiologies of vascular access thrombosis may be considered as those causing early ( 0 to 3 months) or late ( > 3 months) thrombosis. Early thrombosis is most commonly due to a technical error such as a choice of poor vein or narrowing of the lumen of the artery or vein during anastomosis. Occasionally failure to properly anticoagulate intraoperatively may cause clotting inside of the shunt. Thrombosis can also result from external compression for hemostasis, hypotension, dehydration, or early puncture of an immature vein. Late thrombosis can be due to repeated trauma from needle punctures with subsequent fibrosis and narrowing, hypo tension, dehydration, compression, or trauma. However, it is most commonly due to an acquired stenosis of the runoff vein, attributed to mechanical endothelial damage from the shearing effect of the high pressure or the pulsatile arterial blood flow in the venous system leading to venous intimal hyperplasia.

Early detection and treatment of hemodialysis vascular access stenoses can prevent thrombosis.
Digital subtraction angiography (DSA) is the conventional technique used to identify and grade AVF and AVG stenoses. However, this technique is invasive, has a radiation load for the patient, uses potentially nephrotoxic iodinated contrast agents, and provides limited spatial information. Analysis of DSA images can also be difficult due to vessel overlap, especially at the level of the anastomosis. Contrast enhanced magnetic resonance angiography (CE-MRA) is an alternative non-invasive technique that can help characterize complications of hemodialysis access fistulas and grafts without all the disadvantages of DSA. MRA also offers the ability to acquire 3D data sets, which can potentially help solve the problem of vessel overlap. In addition, CE-MRA can provide a means to measure access flow, which has been suggested as a better parameter for impending vascular access failure than anatomic information alone. Finally, recent studies have show that CE-MRA is also highly sensitive for the detection of flow-limiting stenoses. With implementation of fast imaging techniques, including parallel imaging, time-resolved MRA can be used for improved visualization of failing AVF or AVG.

References:

1. Machleder HI (ed). Vascular Disorders of the Upper Extremity, 3rd Ed. Armonk: Futura Publishing Company Inc., 1998.
2. Planken RN, Tordoir JHM, et al. Stenosis Detection in Forearm Hemodialysis Arteriovenous Fistulae by Multiphase Contrast-Enhanced Magnetic Resonance Angiography: Preliminary Experience. Journal of Magnetic Resonance Imaging. 2003; 17: 54-64.
3. Han KJ, Duijm LEM, et al. Failing Hemodialysis Access Grafts: Evaluation of Complete Vascular Tree With 3D Contrast-Enhanced MR Angiography with Spatial Resolution: Initial Results in 10 Patients. Radiology. 2003; 227: 601-605.
4. Smits JHM, Bos C, et al. Hemodialysis Access Imaging: Comparison of Flow-Interrupted Contrast-Enhanced MR Angiography and Digital Subtraction Angiography. Radiology. 2002; 225: 829-834.
5. Laissy JP, Menegazzo D, et al. Failing Arteriovenous Hemodialysis Fistulas: Assessment with Magnetic Resonance Angiography. Investigative Radiology. 1999; 34(3): 218-224.