ABSTRACT
Microscopic polyangiitis (MPA) has only recently been accepted as a distinct disease, separate from polyarteritis nodosa (PAN) (1). Polyarteritis nodosa was described a century ago as a rare (probably new) disease. Further case descriptions expanded the concept of vasculitis, lumping virtually all idiopathic vasculitides under the same umbrella. However, the last 50 years has seen a process ofpeeling off one entity after another, again making classic PAN a rare disease. Wegener's granulomatosis (WO) and Churg-Strauss syndrome (CSS), two other illnesses with similar renal involvement to MPA, were also split off from PAN. Better understanding of clinical, histopathological, and pathogenetic aspects of MPA has justified its unique and separate identity among the vasculitides. Renal involvement was included in the early descriptions of PAN, and subsequent widespread use of the microscope lead to recognition of small-vessel disease. "Polyarteritis nodosa" was studied extensively in the United Kingdom, where patients were frequently recognized to have glomerular inflammation (2). Further work by the Hammersmith group (3) defined the clinical features of "microscopic polyarteritis" as a syndrome that was recognized by nephrologists as a common, distinct form of small-vessel vasculitis and a major cause of rapidly progressive glomerulonephritis (ON). Other specialties were slower to classify MPA separately from PAN. The proceedings of the Chapel Hill Consensus Conference (CHCC) have led to widespread acceptance of the term "microscopic polyangiitis," which emphasizes small-vessel vasculitis in MPA.
