HSS is a rare disease characterized by a large-vessel vasculitis, typically involving one or more pulmonary and bronchial arteries, leading to in situ thrombosis and pulmonary artery and/or bronchial artery aneurysm formation, as well as systemic venous thrombosis [1]. To date, there are no established diagnostic criteria, but a combination of pulmonary and/or bronchial artery aneurysms with in situ thrombosis and recurrent thrombophlebitis and/or deep vein thrombosis in different regions in the absence of other etiological factors and normal coagulation profile is considered the main feature of HSS [1]. HSS is considered a variant of Behçet’s disease, which shows oral and genital ulcers, and may be associated with thrombophlebitis, as well as pulmonary artery aneurysms [1].

The pathogenesis of aneurysm and thrombus formation in the pulmonary arteries in HSS remains controversial. The inflammation of vessel walls is most likely leading to the formation of aneurysms, which is also seen in Behçet disease, to which HSS shows several similarities [2]. It is still debated whether the thrombotic masses in the pulmonary arteries are formed in situ or are the result of emboli, or both. It has been primarily argued, however, that the thrombotic occlusions of the pulmonary arteries in HSS are due to an underlying arterial vasculitis rather than thromboembolism.

Especially in early stages of the disease, thrombotic occlusion of the pulmonary arteries without dilatation of the vessel lumen or formation of an aneurysm may be mistaken for pulmonary embolisms. The most important radiological evidence of vasculitis as the cause of thrombotic occlusion is contrast enhancement of the vessel wall adjacent to the thrombotic mass, which reflects inflammation of the vessel wall and is a typical finding for vasculitis due to HSS but is not observed in CTEPH [3, 4]. An additional approach to visualizing vessel wall inflammation in patients with suspected pulmonary arterial vasculitis is through 18F-fluorodeoxyglucose positron emission tomography computed tomography (FDG-PET-CT). The FDG-PET-CT has been demonstrated to yield positive results in anecdotal cases of Hughes-Stovin syndrome and Behcet’s disease [5, 6]. Consequently, it can be regarded as a supplementary modality for investigating patients with suspected HSS.

Because of the different treatment strategies, it is important to differentiate the nature of pulmonary occlusion: for patients with PE or CTEPD, the main treatment option is anticoagulation, while for patients with HSS, the main treatment option is immunosuppressive therapy, and anticoagulants should be administered with great caution [1, 7].