**1. Introduction**

Pulmonary arteriovenous malformations (PAVMs) are structurally abnormal, direct vascular communications between pulmonary arteries and veins, which bypass capillary beds to create low-resistance, high-flow continuous intrapulmonary right-to-left shunts [1–3].

The majority of PAVMs (70% or more) are associated with the autosomal dominant disorder hereditary hemorrhagic telangiectasia (HHT), also known as Osler-Weber-Rendu syndrome [2,4–6]. HHT mutations, which most commonly a ffect the *ENG* gene (HHT Type 1) or *ACVRL1* gene (HHT type 2), disrupt key regulators in angiogenesis, resulting in the development of congenital PAVMs and other vascular anomalies [1,7]. PAVMs a ffect about 50% of HHT patients overall [8], with a higher incidence and number of PAVMs in patients with *ENG* mutations [9]. Acquired causes of PAVMs account for approximately 20% of cases and include trauma, cardiothoracic surgery, hepatic cirrhosis, metastatic cancer, mitral stenosis, infection, amyloidosis, and chronic thromboembolic disease [2,3]. The wide variety of conditions associated with acquired PAVM suggests a common underlying developmental mechanism which triggers an angiogenic cascade within the pulmonary vasculature [3]. The remaining minority of PAVMs which cannot be classified as congenital or acquired are categorized as idiopathic.

PAVM angioarchitecture is classified as simple or complex based on the segmental pulmonary artery anatomy, which is important for planning endovascular interventions [5,10]. Simple PAVMs are supplied by a single segmental pulmonary artery. The single segmental pulmonary artery will often branch distally into one to three subsegmental branches all supplying the PAVM [5]. Complex PAVMs are supplied by two or more segmental pulmonary arteries. Di ffuse PAVM is a rare subtype of complex PAVM characterized by involvement of an entire segment, or sometimes an entire lung, by tangles of malformed vasculature [3]. PAVMs often have a lower lobe predominance [11].

PAVMs are most frequently asymptomatic, but may be associated with a wide spectrum of clinical manifestations, and if left untreated, they can result in serious complications. Physiologic consequences correlate with the size of PAVM and degree of right-to-left shunting, which can limit oxygenation and natural filtration in the lung. Patients may present with varying degrees of dyspnea, cyanosis, clubbing, or chest pain [2,3]. Due to decreased filtration of vasoactive substances into the systemic circulation, migraines are a common neurologic manifestation of PAVM [1]. More serious complications include brain abscess, paradoxical embolism resulting in stroke or transient ischemic attack (TIA), and less frequently, hemoptysis or intrapulmonary hemorrhage [10,12]. HHT patients with PAVMs are several hundred-fold more likely to develop brain abscess compared to the general population [13,14]. The risk of brain abscess is significantly correlated with number of PAVMs, and larger feeding artery size is significantly correlated with risk of ischemic stroke [15]. Current indications for treatment of PAVM include any (solitary or multiple) PAVM with feeding artery diameter ≥2–3 mm, measurable increase in PAVM size, paradoxical emboli, symptomatic hypoxemia, or any of the other aforementioned serious complications [16].

Embolization is the standard of care for treatment of PAVMs, with surgery reserved for refractory cases which have repeatedly failed embolotherapy [17]. This review aims to present updated recommendations for the diagnostic workup and managemen<sup>t</sup> of PAVMs, with an emphasis on screening protocol and aspects of care within particular subsets of patients, based on the most recent literature and the experiences at our institution, which has been an HHT Center of Excellence since 2010.
