Behçet's syndrome within the spectrum of systemic vasculitides

Behçet's syndrome (BS) is a complex, relapsing inflammatory disorder of unknown etiology, distinguished by its unique geographic distribution and severe disease course, particularly among young males. It would be difficult to include BS in a specific disease classification system. Unlike classical vasculitides, BS predominantly affects veins, with a pronounced tendency for thrombosis, whereas arterial involvement is less common. Histopathological evaluations of many major organs highlight a neutrophil-dominant inflammatory response, particularly around the venules, without the granulomas, fibrinoid necrosis, or immune complex deposition characteristic of other vasculitides. Pathogenesis involves heightened innate immunity, with neutrophils and neutrophil extracellular trap formation contributing to vascular inflammation and thrombosis. Further complicating its classification, BS shares overlapping features with autoinflammatory conditions, spondyloarthropathies, and more recently, MHC-I-opathies. The term "syndrome" is often preferred by experts to reflect its diverse clinical phenotypes, suggesting again the heterogeneous nature of the disease. This review aims to discuss the problems with these classifications, particularly within the spectrum of systemic vasculitides.