Arteriovenous fistula

An arteriovenous fistula is an abnormal connection between an artery and a vein. The most important examples include carotid-cavernous fistula and pulmonary arteriovenous fistula. A carotid-cavernous fistula is an abnormal communication between a carotid artery and the cavernous sinus. It is most commonly caused by trauma. This fistula leads to a high-pressure inflow of arterial blood into the venous sinuses, resulting in compression and damage to adjacent structures. The main symptom is diplopia, which is caused by compression injury of the oculomotor nerves. Other common symptoms include pulsatile tinnitus, exophthalmos, and headache. Diagnosis is established based on typical CT/MRI or angiography findings (e.g., enlarged cavernous sinus). The preferred treatment method is endovascular occlusion of the fistula with balloons or coils. A pulmonary arteriovenous fistula is an abnormal communication between the pulmonary artery and veins, and it leads to a high-pressure inflow of arterial blood into the venous system, causing venous congestion. The main symptom is dyspnea; other common symptoms include hemoptysis, platypnea and orthopnea, chest pain, and cyanosis. Diagnosis is established based on typical transthoracic contrast echocardiography, CT, and angiography findings (e.g., mass with feeding vessels). Treatment is only required for symptomatic patients, and the preferred treatment method is embolotherapy.

• Definition: an abnormal communication between the carotid artery and the cavernous sinus
• Classification

• Pathophysiology: : congenital malformation or trauma → arteriovenous fistula formation → arterial blood flowing into venous system → venous congestion → compression of cavernous sinus structures (i.e., CN III, CN IV, CN VI, CN V1, and CN V2
• Clinical features ^[2]
  • Headache
  • Orbital pain
  • Diplopia, blurred vision
  • Pulsatile tinnitus (fistula bruit)
  • Signs of congestion
    • Unilateral or bilateral, pulsatile exophthalmos
    • Chemosis
    • Massive conjunctival congestion and hemorrhage, with potential bleeding in the retina and vitreous humor
    • Increase in intraocular pressure
    • Optic disc swelling
• Diagnostics
  • CT/MRI (with or without angiography) may show:
    • Proptosis
    • Enlargement of the cavernous sinus
    • Expansion of the superior ophthalmic veins
    • Skull fractures (if fistula is due to trauma)
    • Abnormal cavernous sinus flow
  • Ultrasound with transcranial Doppler: shows increased blood flow
  • Cerebral angiography
    • Gold standard for diagnosis (and treatment)
    • Visualization of feeding vessels and blood flow
• Treatment
  • Embolization using balloons or coils
    • Direct fistulas: transarterial approach
    • Indirect fistulas: transvenous approach
  • Neurosurgery
    • Indicated if endovascular interventions fail or are not possible
    • Occlusion of the fistula via suturing or packing

If not treated swiftly, carotid-cavernous fistulas may result in cerebral hemorrhage or infarction, intracranial hypertension, vision loss, or even death.

• Definition: an abnormal communication between pulmonary arteries and veins
• Etiology
  • Can be acquired (e.g., trauma, surgery)
  • Frequently associated with hereditary hemorrhagic telangiectasia
• Pathophysiology: arteriovenous fistula formation → flow of arterial blood into venous system → venous congestion → pulmonary symptoms
• Clinical features
  • Dyspnea (most common symptom)
  • Hemoptysis (due to PAVF rupture)
  • Platypnea, orthodeoxia
  • Chest pain, cough
  • Cyanosis, clubbing
  • Physical examination may show murmurs/bruits (loudest during inspiration).
• Diagnostics ^[3]
  • CT: test of choice for initial diagnosis
    • Multiple or single, round, dense nodules or masses with well-demarcated, smooth borders and a sac-like appearance
    • Lobulation may be visible.
    • Feeding vessels often appear as tubular structures that terminate at the sac.
  • Contrast-enhanced pulmonary angiography: gold standard for defining the anatomy of PAVF and for definitive diagnosis
  • Transthoracic contrast echocardiography: to evaluate the right-to-left shunt (shunt fraction assessment)
  • Chest radiography: feeding vessels that have the appearance of smooth nodules with linear parallel shadows
  • Laboratory studies
    • CBC: anemia or polycythemia
    • Arterial blood gas analysis: hypoxemia (↓ SaO₂)
• Differential diagnosis
  • Highly vascular parenchymal tumor
  • Hepatopulmonary syndrome
  • Atrial septal defects
  • Nasal telangiectases, endobronchial telangiectases
• Management ^[4][5]
  • Not all PAVFs require intervention.
  • In symptomatic PAVFs, embolotherapy is preferred.
• Complications
  • Stroke, brain abscess (both due to paradoxical embolism)
  • Hemothorax (due to rupture of a subpleural PAVF)
  • During pregnancy, especially during the third trimester, pulmonary hemorrhage may occur and can be fatal.
  • Polycythemia (due to chronic hypoxemia) or anemia (due to blood loss)
  • Pulmonary hypertension