Arteriovenous fistulas (AVFs) are abnormal connections between an artery and a vein..In certain contexts, these may also be referred to as arteriovenous malformations. AVFs can exist almost anywhere in the body, depending on the etiology. These can be divided into two groups, acquired or congenital. Acquired fistulas can be further subdivided into surgically created, as in for hemodialysis, or secondary to trauma, whether accidental or procedure-related.
The anatomy of the fistula depends on location in the body. AVFs for hemodialysis are typically created in the extremities, with the upper extremity being generally preferred over the lower extremity by vascular surgeons. The cephalic and basilic veins are often used for the surgical creation of an AVF. The radial artery at the volar wrist and the brachial artery at the antecubital fossa and medial upper arm are typical anatomical locations for fistula creation, although the radio-cephalic AVF is the preferred initial access for hemodialysis.
Two types of lower extremity surgical AVFs for hemodialysis have been described in the literature. The superficial femoral vein or popliteal vein can be mobilized from the knee to anastomose with the superficial femoral artery; this is called an SFV transposition. The saphenous vein can be used to create a loop AVF on the anterior thigh, anastomosed with the common femoral artery.
Although no type of congenital AV fistula is common, reported locations for congenital AVFs include pulmonary, aortocaval, dural, carotid-cavernous, coronary, and hepatic. While the majority of neck fistulas do occur secondary to trauma, congenital vertebrovertebral fistulas and carotico-jugular fistulas have been described in children.
AVFs as a result of iatrogenic injury are typically as a result of surgical procedure, invasive line placement, or needle biopsy. The literature demonstrates multiple reports of iatrogenic injury resulting years after surgical procedures. Traumatic AVF can essentially occur anywhere there is resulting trauma, and these can also have a late presentation. Greater than 50% of traumatic AVFs happen in the lower extremity, and about one-third occur in the femoral vessels, while 15% take place in the popliteal vessels.
Arteriovenous fistulas can be surgically created for hemodialysis access, can occur as a result of a congenital anomaly, or be secondary to iatrogenic injury or trauma. Penetration of any mixed-type vasculature can ultimately result in the healing of arteries and veins together, bypassing downstream arteriole and capillary system.
Iatrogenic fistulas are most commonly reported as a result of percutaneous access of the femoral vein and femoral artery during cardiac catheterization; although, subclavian and carotid fistulas have been reported in association with the placement of central lines. The most common AVFs described as a result of percutaneous biopsy are renal; however, these are typically self-limited, and very few require intervention.
Traumatic fistulas are often associated with direct arterial trauma and long bone fractures, especially where an artery and vein are in close communication. Ninety percent of traumatic AVFs are due to penetrating trauma, the majority of which are gunshot wounds. A small portion of neck AVFs can occur in association with a hyperextension injury or spine surgery. Carotid-cavernous fistulas, typically due to trauma, can be fatal and are most often associated with a basilar skull fracture, penetrating trauma to the area, and ruptured aneurysms. Two-thirds of traumatic AVFs are diagnosed within one week of the injury; however, some may present weeks to years after the event.
Congenital fistulas are not well-understood. Central nervous system congenital AVFs can be dural or carotid cavernous. AVFs of the neck are mostly due to trauma; however, fibromuscular dysplasia, neurofibromatosis, and other types of collagen disorders have been associated. Pulmonary vascular malformations are typically simple and more similar to AVFs than to a true malformation. Other types of AVFs are very rare.
Arteriovenous fistulas were originally described by William Hunter as early as 1757. Much of the experience in managing AVFs originated out of traumatic injuries resulting in AVFs from the Second World War, the Korean War, and Vietnam War. 215 AVFs and aneurysms were reported as a result of the Korean War. In the civilian population, traumatic AVFs of the abdomen and extremities are equally distributed, unlike the military, where the majority of traumatic AVFs occur in the extremities. This difference is likely on account of body armor worn by the military. Demonstrably, traumatic fistulas are much more common than congenital fistulas and are more frequently discussed. Congenital fistulas are generally rare and found in case reports and small studies.
The National Institute of Diabetes and Digestive and Kidney Diseases reported as of 2013, over 468,000 patients were on hemodialysis. Twenty percent of those patients receive dialysis via a surgically created AVF. In the HEMO Study published in 2000, AVF prevalence varied between dialysis centers from 4-77%. The authors discovered a decreased prevalence of AVF compared to other means of HD in females, blacks, obese, elderly, and in patients with peripheral arterial disease. Geographically, the rate of AVF creation varies greatly, with the highest rates being in the Northeast and the lowest in the Southwest of the United States.
The presentation of arteriovenous fistulas can vary depending on the location and etiology. Patients with an AVF for hemodialysis will present with evidence of a surgical incision on the lateral wrist, volar forearm, or upper arm. A working AVF will have a palpable thrill and continuous bruit. A patient with an AVF with an outflow obstruction may present with a pulsatile fistula or prolonged bleeding from a puncture site from hemodialysis. Superficial fistulas have a palpable thrill, a bruit, or even a pulsatile mass. It may be possible to auscultate a machinery-like murmur over the fistula.
Fistulas of the extremities, regardless of etiology, may present with signs of venous hypertension, including varicosities, pain, and swelling—if a long-standing fistula, there may be significant size discrepancy between the two limbs. If the patient reports a history of trauma— anywhere from weeks to years after injury—in particular with long bone fractures or ongoing neurologic deficits, index of suspicion for AVF should be raised even in the event of a clinically normal exam.
Patients with congenital AVF may not present until later in life, and a history of trauma should be ruled out. Depending upon location, these fistulas may be low-flow at birth and become high-flow lesions in adulthood. Patients with brain arteriovenous malformations can present with headache, neurologic deficits, seizures, or a combination. These patients can also be at risk for hemorrhage or ischemia to the parenchyma surrounding the AVM/AVF due to steal syndrome.
In severe, chronic, or high flow fistulas, patients can present in high output cardiac failure, which results in shunting of oxygenated blood back to the right heart. Due to the shortcut that the arterial blood takes through the venous system, this results in decreased peripheral resistance. In order to maintain blood pressure, the total circulating blood volume is increased, leading to heart failure. The Nicoladoni-Israel-Branham sign is a finding of reflex bradycardia with compression of the fistula due to increased afterload.
Beyond the clinical evaluation from history and physical, a duplex ultrasound (US) is a non-invasive and inexpensive way to confirm the diagnosis of AVF, given that the AVF is superficial. A duplex US will demonstrate low resistance flow in the feeding artery. At the anastomosis or fistula, turbulence and high-velocity flow are demonstrated. Thickened walls and high-velocity flow will be seen in the dilated draining veins or venous plexus of the AVF. A pseudoaneurysm, venous aneurysm, or a dilated feeding artery may also be demonstrated on the ultrasound exam.
Computed tomography angiography (CTA) and magnetic resonance angiography (MRA) both show early contrast filling in the arterial phase in the involved vein. While MRA may not be an option in trauma or post-trauma patient due to residual metal, CTA is reliable, non-invasive, and accessible as an initial diagnostic test. Disadvantages of CTA include streak artifact from metallic objects—which can be an issue in penetrating trauma—motion artifact, and reliance on contrast timing in the fistula.
Selective angiography is the gold standard and has shown better accuracy over CTA. It is the most invasive evaluation of an AVF but provides the exact point of arteriovenous communication surrounding vascular anatomy, flow dynamics, and mechanism for treatment. Cost, procedural delay, additional arterial or venous access, and the need for a specially trained team are disadvantages of angiography.
Patients presenting in need of vascular access for ESRD need to be evaluated for optimal placement of an AVF. Given the preference of the upper extremity, non-dominant over dominant, forearm over the upper arm, all patients should be evaluated with a duplex ultrasound scan, including patients presenting for evaluation of an existing AVF. Criteria have been developed for optimal results and primary patency rates. Clinical exam, in addition to vein mapping, has been shown to decrease primary failure rates and decrease negative surgical exploration in first attempts to create AVFs.
Most cases of arteriovenous fistulas in the past were treated conservatively in wartime and, later on, were surgically managed if needed. Early intervention, however, may deter complications of AVFs that can be avoided, and post-traumatic fistulas should be closed as close to diagnosis as possible. The goal of AVF treatment is to isolate and close the fistula while attempting to maintain essential blood flow. Repair may be completed by direct primary repair, reconstruction (autogenous or prosthetic graft, or bypass), or endovascular.
Indications for treatment are simple.
Endovascular management is the preferred method of management for AVFs.
Open surgery is an option for repair when endovascular management fails.
While some congenital arteriovenous fistulas can be fatal, leading to failure to survive, the overall prognosis is good. Peripheral arteriovenous fistula does not typically incur systemic hemodynamic effects, and around 15% of all AVFs do.
The Schobinger Classification is a clinical staging system that serves to predict the success of treatment. The stage of quiescence (I) is described as cutaneous blush and skin warmth at the site of the fistula. The second stage demonstrates darkened skin, a pulsatile lesion with a bruit on auscultation, or palpable thrill. This stage is called expansion. Stage III, the destruction phase, is essentially steal syndrome and is characterized by skin changes, ulceration, and distal ischemia. Stage four is the decompensated phase, characterized by high output heart failure. While it has been shown that decompensated heart failure can be completely reversed after a high-flow AVF is closed, the prognosis is much better in patients who present earlier.
The complications of arteriovenous fistulas are as multifold:
The consultation need for management of the arteriovenous fistula depends on the location and nature of the fistula. Specialties required to be involved can include:
While AVFs may be unavoidable for patients with end-stage renal disease, the best way to prevent the development of an AVF is to avoid unnecessary trauma or procedures that may lead to their development. Congenital AVF etiology is poorly understood, and thus, there is little to do in terms of prevention.
Patients who are diagnosed with an AVF or who think they may have an AVF should see their primary care doctor first. After a thorough work-up, these patients may be referred to see one of the specialists mentioned above for management and potential repair of the fistula. Important things to write down before any appointment include any symptoms experienced, any changes in the extremity or area of concern, and any significant history, such as trauma or procedures in the same area. A fistula can arise from prior injury at any point in your lifetime.
Expectations from your doctor can include further labs and imaging, including an ultrasound, a CT, or MRI. Your doctor will look at, listen to, and feel the area to start the evaluation. In addition to your questions and the provision of symptoms, your doctor will likely ask you further questions to help with the diagnosis of this problem.
Patients presenting with signs and symptoms of arteriovenous fistulas require a multidisciplinary approach to the workup. As with any condition, this starts with a thorough history and physical exam. Involving the appropriate surgical team early in the process can help direct the evaluation such that imaging obtained can be used in surgical planning. Early warnings to trauma patients with injuries to the extremities, particularly involving an arterial or venous injury, should help to prevent late presentations of high-flow AVFs. Surgeons, interventionalist, and intensivists should be aware of arteriovenous fistulas as a potential complication of bedside procedures for venous access and biopsies.
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