Isolated necrotizing arteritis (localized polyarteritis nodosa): examination of the histological process and disease entity based on the histological classification of stage and histological differences from polyarteritis nodosa.

Although isolated necrotizing arteritis (INA) has been thought to be an isolated form of polyarteritis nodosa (PAN), a detailed histological comparison between INA and PAN has not been performed. Therefore, we examined the disease entity of INA based on the histological comparison of both diseases. In addition, a histological classification of INA, in which the histological process of INA is included, was described.A histological study, including CD3, CD20, and CD68 immunostains, was performed in seven operated patients with INA. Five untreated patients with PAN were also examined.In INA, arteritis with fibrinoid necrosis occurred in small and medium-sized arteries in a single organ. INA was divided histologically into acute (five cases) and healed stage (two cases). Endothelial injury and medial degeneration, followed by fibrinoid necrosis, occurred in the acute stage, and regression of fibrinoid necrosis and fibrosis were present in the healed stage. Infiltration of predominant T lymphocytes and macrophages was also observed in the affected arteries. Histological comparison between INA and PAN led to the finding that the extension of fibrinoid necrosis in the entire arterial wall, which indicates severe wall destruction, intense proliferation of fibroblasts and aneurysm formation occurred in PAN alone.We demonstrated some histological differences between INA and PAN. Based on the histological similarities and differences between INA and PAN, it was concluded that INA shall be classified as a mildly wall destructive form of PAN-type arteritis located in a single organ.