Arteriovenous fistula

# Overview
An arteriovenous fistula (AVF) is an abnormal connection or passageway between an artery and a vein. It may be congenital, surgically created for hemodialysis treatment, or acquired due to a pathologic process, such as trauma or erosion of an arterial aneurysm. These communications can occur at any point in the vascular system; varying in size, length, location, and number.

# Historical Perspective
- Arteriovenous fistula (AVF) was first discovered by William Hunter, in 1758, during a venipuncture with accidental piercing of the artery.
- Congenital arteriovenous fistula (AVF) of the coronary arteries, a rare cardiac anomaly, was first described by Krause W. Uber, in  1865.

# Classification
- Arteriovenous fistula (AVF) can be classified into:
- Spontaneous, which can be further subdivided into cirsoid/congenital and idiopathic/aneurysmal.
- Acquired, which can be the result of a trauma or part of treatment for an underlying disorder.

# Pathophysiology
- The abnormal communication causes shunting of blood from high-pressure arterial side to the low-pressure venous side.
- An arteriovenous fistula (AVF) involving a major artery such as the abdominal aorta can lead to a large decrease in peripheral resistance, which in turn causes the heart to increase cardiac output in order to maintain proper blood flow to all tissues.
- The physical manifestations of this would be a relatively normal systolic blood pressure with a decreased diastolic blood pressure resulting in a wide (large) pulse pressure.
- Large arteriovenous fistulae (AVF) can lead to congestive heart failure associated with increased activity of vasoconstrictor neurohormonal systems, the renin-angiotensin, sympathetic nervous system, endothelin system, and Arginine vasopressin.
- The increased activity of vasoconstrictor neurohormonal systems is in concurrence with compensatory activation of the vasodilating systems, the atrial natriuretic peptide (ANP) and nitric oxide (NO).
- A fistula can progress to an aneurysm, usually in the setting of a trauma, carrying a risk of rupture and necessitating surgical intervention.
- The malformation can result in hemorrhage and the risk is significantly higher with small size malformation because of the significantly higher feeding artery pressures.

# Clinical Features
- In the case of a coronary arteriovenous fistula (AVF), patients can be asymptomatic or can present with:
- Continuous murmur
- Cardiomegaly
- Chest pain
- Fatigue
- Palpitation
- Angina
- Bacterial endocarditis
- Systemic hypotension
- Fistulae involving the spinal vasculature can present with neurologic symptoms of the lower extremities which may include weakness and sensory disturbance.
- Acquired arteriovenous fistula (AVF) can result in digital clubbing.
- Patients on hemodialysis with arteriovenous fistula (AVF) can develop pseudo-Kaposi's sarcoma.
- Abdominal arteriovenous fistulae can present with:
- Lower extremity edema with cyanosis
- Pulsatile varicose veins
- Scrotal edema
- Ilio-iliac arteriovenous fistula (AVF) presents with progressive abdominal distention, dyspnea, and edema of the leg, which makes it difficult to distinguish from deep venous thrombosis (DVT) and can lead to a delay in the diagnosis.
- Tentorial dural arteriovenous fistula (AVF) can rarely manifest as trigeminal neuralgia.

# Differentiating Arteriovenous Fistula (AVF) from other Conditions
- It is important to differentiate an arteriovenous fistula (AVF) from other conditions that cause a hyperdynamic circulation such as:
- Cirrhosis
- Graves ophthalmopathy
- Various sarcomas such as, angiosarcoma and leiomyosarcoma.
- Paget's disease of bone
- An ilio-iliac arteriovenous fistula (AVF) can lead to a misdiagnosis of deep venous thrombosis (DVT)

# Epidemiology and Demographics
- In a Scottish population-based study, the incidence of dural arteriovenous fistulae in adults was 0.16 per 100 000 per year.
- In an Olmsted County, Minnesota-based study, the incidence and prevalence of dural arteriovenous fistulae from 1965-1992 are as follows:
- From 1965-1992, the incidence was estimated to be 1.84 cases per 100,000 individuals.
- From 1985-1992, the incidence was estimated to be 2.75 cases per 100,000 individuals.
- On January 1, 1990, the prevalence of identified cases was 19.0 per 100,000 individuals.

## Age
- Individuals of all age groups may acquire the condition.
- Congenital arteriovenous fistula (AVF) is present at birth and may manifest early or later in life.

## Gender
- Arteriovenous fistula (AVF) is observed in both men and women equally.

## Race
- There is no racial predilection for an arteriovenous fistula (AVF).

# Risk Factors
- Common risk factors for the development of an arteriovenous fistula (AVF):
- Arterial catheterization
- Venous catheterization
- Hypertension
- Intravenous (IV) drug abuse

# Natural History, Complications and Prognosis
- Patients can remain asymptomatic or can present with symptoms depending on the involved vasculature.
- Early clinical features include hypotension with hypoperfusion, skin changes, and can progress to include cardiac manifestations.
- If left untreated, a large arteriovenous fistula (AVF) can lead to compensated/decompensated heart failure.
- Complications of an arteriovenous fistula (AVF) include:
- High-output compensated heart failure
- Decompensated heart failure
- Aneurysm formation
- Hemorrhage
- Portal hypertension
- Venous stasis and thrombosis
- Venous stenosis
- Intracranial hypertension
- Infective endocarditis
- Ischemic heart disease and myocardial infarction
- Cardiac arrhythmias
- Paradoxical embolism (causing stroke and pulmonary embolism)
- Angiosarcoma (at the arteriovenous fistula site for hemodialysis)
- Pseudo-Kaposi's sarcoma
- Steal phenomenon
- Leiomyosarcoma
- Prognosis of an arteriovenous fistula (AVF) is dependent on the system involved with many congenital fistulae undergoing regression and large fistulae may progress to cardiac decompensation and death.

# Diagnosis

## Diagnostic Criteria
- An arteriovenous fistula (AVF) can be detected by a stethoscope and can present with a machinery-like murmur.
- A more superficial fistula can also be detected by palpation, presenting as continuous vibration.
- Imaging is the modality of choice to effectively diagnose an arteriovenous fistula (AVF) and it includes:
- Duplex ultrasound
- Computed tomography angiography (CTA)
- Magnetic resonance angiography (MRA)

## Symptoms
- An arteriovenous fistula (AVF) can be asymptomatic.
- Symptoms, when present, may include:
- Low blood pressure and dizziness
- Swelling in limbs/body
- Pain at the acquired arteriovenous fistula (AVF) site
- Dyspnea and fatigue
- Skin changes
- Palpable thrill
- Melena
- Hemoptysis

## Physical Examination
- Patients with an arteriovenous fistula (AVF) usually appear normal.
- Physical examination may be remarkable for:
- Increased size of the limb
- Skin temperature and color alteration
- Prominent veins
- Audible murmur
- Palpable thrill
- Nicoladoni-Branham sign (a decrease in pulse and increase in blood pressure that immediately follows the sudden occlusion of an arteriovenous fistula )
- Mass effect
- Pain and numbness
- Extremity weakness
- Cyanosis
- Clubbing

## Laboratory Findings
- Laboratory findings associated with an arteriovenous fistula (AVF) can include the following:
- Polycythemia
- Decreased arterial PO2
- Decreased oxygen saturation
- Increased venous oxygen content
- Decreased platelets and fibrinogen in the case of large fistulae
- Swan-Ganz catheter findings in pulmonary arteriovenous fistula (AVF) include decreased arterial oxygenation and shunting of cardiac output.
- Swan-Ganz catheter findings in aorto-caval fistula include higher vena caval pressure and higher than normal oxygen content.
- Patients on hemodialysis suffering from occlusive and thrombotic events of the arteriovenous fistula (AVF) may exhibit significant increase of platelet surface glycoproteins GPIb and GPIIb/IIIa.
- Large fistulae can also present with laboratory findings of consumptive coagulopathy such as, low platelets, increased bleeding time, elevated prothrombin time (PT) and partial thromboplastin time (PTT), and signs of enhanced fibrinolysis.

## Imaging Findings
- Computed tomography angiography (CTA) and magnetic resonance angiography (MRA) are the imaging modalities of choice for an arteriovenous fistula (AVF) with modest evidence that computed tomography angiography (CTA) is better than magnetic resonance angiography (MRA).
- On plain radiograph, an arteriovenous fistula (AVF) is characterized by a round or oval lobulated homogeneous mass and the identification of feeding and draining vessels can also be possible.
- On ultrasound, an arteriovenous fistula (AVF) is characterized by a pseudoaneurysm, high flow velocity, increased diastolic arterial flow, abnormal arterial pulsitility in the vein, and enlargement of the downstream vein.
- On computed tomography angiography (CTA) and magnetic resonance angiography (MRA), an arteriovenous fistula (AVF) is characterized by abnormal dilatation along with early and flow-related enhancements.
- On digital subtraction arteriography (DSA), an arteriovenous fistula (AVF) is characterized by an abnormal early filling of an adjacent vein in the region of the fistula.

## Other Diagnostic Studies
- An arteriovenous fistula (AVF) involving the heart and pulmonary vasculature in some cases, can also be diagnosed via echocardiography.
- Findings on the echocardiography may include dilated lumen and abnormal unidirectional continuous flow signals with broad velocity spectra
- An aorto-caval fistula may also be diagnosed using Swan-Ganz catheter, demonstrating higher vena caval pressure and higher than normal oxygen content.

# Treatment

## Medical Therapy
- There is no medical treatment for an arteriovenous fistula (AVF).
- Iatrogenic arteriovenous fistulae, presenting only as pain, can resolve after a trial of compression and observation.
- Asymptomatic fistulae can resolve spontaneously and most lesions can be monitored after detection especially if the size of the lesion is small and they do not cause adverse effects.
- Conservative management has been tried with a successful outcome in the case of an arteriovenous fistula (AVF) as a complication of vascular catheterization.

## Surgery
- Surgery is the mainstay of therapy for an arteriovenous fistula (AVF).
- Endovascular embolization is the most common approach to the treatment of an arteriovenous fistula (AVF), where a catheter is inserted into an artery and, guided by fluoroscopic or x-ray imaging, contrast is injected to visualize the vasculature followed by the injection of an embolization material (such as, particles, liquid embolics, embolization glue, detachable balloons, vascular plugs, and coils) into the exact location where the artery and the vein meet.
- Microsurgery, with or without endovascular embolization, is the most appropriate treatment for dural, brain, or spinal arteriovenous fistulae, where a titanium clip is placed to cut off the connection between the artery and the vein.
- Stereotactic radiosurgery is the treatment of choice to successfully obliterate an arteriovenous fistula (AVF) with low complication rates and hence preferred over endovascular embolization and microsurgery in the case of an arteriovenous fistula (AVF) close to important brain structures.
- Open surgical repair

## Prevention
- There are no specific primary prevention measures available for an arteriovenous fistula (AVF).
- The acquired type, caused by trauma, can be prevented by avoiding trauma and also using caution while performing procedures involving vascular access.