OBJECTIVES Recent proposals for the nomenclature of systemic vasculitis have focused on a distinction between (classic) polyarteritis nodosa (PAN) and microscopic polyangiitis or polyarteritis (MPA). Thus, MPA may cause necrotising vasculitis of medium sized or small arteries but, unlike PAN, involvement of “microscopic” vessels must always be present in the former. This study aimed to show that the term “MPA” may represent a source of misinterpretation and to help illustrate difficulties of applying diagnostic criteria/definitions for conditions of unknown aetiology or variable clinical presentation and course.

RESULTS Six of the 64 patients who met definition criteria for MPA at the time of initial diagnoses had entered into complete clinical remission upon appropriate corticosteroid and cyclophosphamide treatment between two weeks and three months, though subsequently (20 to 72 months; mean time: 42.3 months) developed characteristic clinical and histological features of overt WG.

CONCLUSIONS Microscopic polyangiitis/polyarteritis may be a dynamic condition with clinical and histopathological features evolving over time to other forms of small vessel vasculitis, mainly WG, thereby meaning that follow up would be necessary not only to control a given patient but also to make a final diagnosis. This follow up should be for a long time as there may be a long interval between initial presentation and subsequent development of WG lesions.