Pulmonary arteriovenous malformation
Pulmonary arteriovenous malformations (PAVM's) are rare vascular anomalies of the lung, in which abnormally dilated vessels provide a right-to-left shunt between the pulmonary artery and vein. They are generally considered direct high flow, low-resistance fistulous connections between the pulmonary arteries and veins.
There is a recognised female predilection with F:M ratios ranging around 1.5 to 1.8:1. The estimated incidence is thought to be around 2-3 per 100,000 11.
Despite most patients being asymptomatic, the connection between the venous and arterial system can lead to dyspnoea (due to right-to-left shunting), as well as embolic events (due to paradoxical emboli). Although it is assumed that the vascular defects are present at birth, they are seldom manifested 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 are be an extremely variable age of presentation from infant to old age although most patient within the first three decades of life.
In congenital cases they are considered to result from a defect in the terminal capillary loops which causes dilatation and the formation of thin-walled vascular sacs. They can be multiple in around one-third of cases.
They can be classified as simple, complex or diffuse 9.
- 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 characterised by hundreds of malformations; some patients can have a combination of simple and complex AVMs within a diffuse lesion
Another older embryological based classification proposed by Anatwabi et al. in 1965 is as 11
- group I: multiple small arteriovenous fistulas without an aneurysm
- group II: large arteriovenous aneurysm
- large arteriovenous aneurysm (central)
- large arteriovenous aneurysm with anomalous venous drainage
- multiple small arteriovenous fistulae with anomalous venous drainage
- large venous aneurysm with systemic arterial communication
- large venous aneurysm without fistula
- group V: anomalous venous drainage with fistulae
These are often unilateral. Although can potentially affect any part of the lung, there is a recognised predilection towards the lower lobes (50-70%) 7.
PAVMs have been described in association with a number of conditions.
- hereditary haemorrhagic 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 PAVM's have HHT 6
In addition PAVMs have been found in:
- hepatic cirrhosis (as part of the hepatopulmonary syndrome)
- mitral stenosis
- previous cardiac surgery (e.g. Glenn and Fontan procedures for cyanotic congenital heart disease) 7
- actinomycosis: thoracic actinomycosis infection
- Fanconi syndrome
- metastatic thyroid carcinoma
- tuberculosis 13-14 (Rasmussen aneurysm)
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.
A pulmonary varix (dilated vessel) may be apparent as a non-specific soft tissue mass. These often have a relatively unusual orientation compared to adjacent vessels. More than one may be identified, such as in patient's with HHT.12
CT / CT angiography
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 centimetres 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.
MR / MR angiography
Three-dimensional contrast–enhanced MR angiography has is considered the MR technique of choice for imaging vascular structures in the thorax 10. Most lesions within the lung have a relatively long relaxation time and produce medium to high intensity signals. Lesions with rapid blood flow within result in a signal void and produce low intensity signals.
Treatment and prognosis
Treatment options include:
- trans-catheter coil embolisation
- surgery (historically treated with surgery)
Once successfully treated (embolotherapy, surgical resection), prognosis is generally good for an individual lesion.
- cyanosis (due to the right to left shunt)
- high output congestive cardiac failure
- paradoxical cerebral embolism 5
History and etymology
The first description of pulmonary arteriovenous malformation was reported by T Churton in 1897.
Possible imaging differential considerations include
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