Pulmonary arteriovenous malformations (AVMs) are rare vascular anomalies of the lung, in which abnormally dilated vessels provide a pulmonary artery-to-pulmonary vein right-to-left shunt. They are generally considered direct high flow, low-resistance fistulous connections between the pulmonary arteries and veins.
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Epidemiology
There is a recognized female predilection with F:M ratios ranging from 1.5 to 1.8:1. The estimated incidence is around 2-3 per 100,000 11.
Clinical presentation
Despite most patients being asymptomatic, the connection between the venous and arterial system can lead to dyspnea (due to right-to-left shunting) and embolic events (due to paradoxical emboli). Although it is assumed that vascular defects are present at birth, they seldom manifest clinically until adult life, when the vessels have been subjected to pressure over several decades. Clinically, a murmur or bruit may be audible over the lesion (especially if peripheral). There is a highly variable age of presentation from infant to old age, although most present within the first three decades of life.
Pathology
In congenital cases, they are considered to result from a defect in the terminal capillary loops, which causes vascular dilatation and the formation of thin-walled vascular sacs. They can be multiple in around one-third of cases.
Classification
Pulmonary AVMs are classified as simple, complex or diffuse 17,19:
simple type: commonest; has a single segmental artery feeding the malformation; the feeding segmental artery may have multiple subsegmental branches that feed the malformation but must have only one single segmental level
complex type: have multiple segmental feeding arteries (~20% 7)
diffuse type: rare (~5% of lesions); the diffuse form of the disease is characterized by hundreds of malformations; some patients can have a combination of simple and complex AVMs within a diffuse lesion
An older embryological based classification proposed by Anatwabi et al. in 1965 does not aid in their management 11,18:
group I: multiple small arteriovenous fistulas without an aneurysm
group II: large arteriovenous aneurysm
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group III
large arteriovenous aneurysm (central)
large arteriovenous aneurysm with anomalous venous drainage
multiple small arteriovenous fistulae with anomalous venous drainage
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group IV
large venous aneurysm with systemic arterial communication
large venous aneurysm without fistula
group V: anomalous venous drainage with fistulae
Location
These are often unilateral. Although they can potentially affect any part of the lung, there is a recognized predilection towards the lower lobes (50-70%) 7.
Associations
PAVMs have been described in association with a number of conditions.
hereditary hemorrhagic telangiectasia (HHT) frequently have PAVMs 1; it is reported that at least 33% of those with a single PAVM and at least 50% of those with multiple PAVMs have HHT 6
In addition, PAVMs have been found in:
hepatic cirrhosis (as part of the hepatopulmonary syndrome)
trauma
previous cardiac surgery (e.g. Glenn and Fontan procedures for cyanotic congenital heart disease) 7
metastatic thyroid carcinoma
tuberculosis 13,14 (Rasmussen aneurysm)
Radiographic features
A number of modalities are available for the diagnosis of PAVMs, including contrast echocardiography, radionuclide perfusion lung scanning, computed tomography (CT), magnetic resonance imaging (MRI), and the gold standard, pulmonary angiography 2.
Plain radiograph
A dilated pulmonary vessel may be apparent as a non-specific soft tissue mass, often with a relatively unusual orientation compared to adjacent vessels. More than one raises the possibility of hereditary hemorrhagic telangiectasia 12.
CT
CT is often the diagnostic imaging modality of choice. The characteristic presentation of a PAVM on non-contrast CT is a homogeneous, well-circumscribed, non-calcified nodule up to several centimeters in diameter or the presence of a serpiginous mass connected with blood vessels 3. Occasionally, associated phleboliths may be seen as calcifications. Contrast injection demonstrates enhancement of the feeding artery, the aneurysmal part, and the draining vein on early-phase sequences.
MRI
Three-dimensional contrast-enhanced MR angiography is considered the MR technique of choice for imaging vascular structures in the thorax 10. Most lesions within the lung have relatively long relaxation time and produce medium to high-intensity signals. Lesions with rapid blood flow within resulting in a signal void and produce low-intensity signals.
Treatment and prognosis
Treatment options include:
trans-catheter coil embolization
surgery (historically treated with surgery)
Treatment is indicated in cases with a feeding artery diameter greater than 3 mm 16. Once successfully treated (embolotherapy, surgical resection), the prognosis is generally good for an individual lesion.
Complications
cyanosis (due to the right to left shunt)
the development of pulmonary arterial hypertension for an unrelated cause may increase the size of pre-existing pulmonary arteriovenous malformations 1
History and etymology
The first description of pulmonary arteriovenous malformation was reported by T Churton in 1897.
Differential diagnosis
Possible imaging differential considerations can be divided into vascular and non-vascular lesions 16 and include:
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vascular
abnormal systemic vessels
highly vascular parenchymal mass
other congenital or acquired pulmonary arterial or venous lesions (e.g. pulmonary varix)
pulmonary artery pseudoaneurysm
hepatopulmonary vessel
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nonvascular
bronchoceles: on contrast scans
mucoceles
granulomas
atelectasis
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