ABSTRACT
Microscopic polyangiitis is considered to be a microvascular variant of classical polyarteri tis nodosa (PAN). The major difference is that in classic PAN there is involvement of small and medium vessels as opposed to the predominantly capillary focus in microscopic polyangiitis. The renal involvement is different as well. The renal lesion in classic PAN results in hypertension and microaneurysms, since it involves larger vessels. In micro scopic poly angiitis, a focal segmental necrotizing glomerulonephritis occurs. Another im portant difference is that in approximately one-third of patients with microscopic polyangi itis, the lung is involved (pulmonary capillaritis and DAH); in classic PAN, lung disease is rare. Microscopic polyangiitis can be accompanied by leukocytoclastic vasculitis of the skin and by peripheral neuropathy. Glomerulonephritis is present in all cases. Serum antimyeloperoxidase antibody (p-ANCA) is present in over 80% of cases. The remainder are c-ANCA-positive. In the latter case, differentiation from Wegener’s granulomatosis is difficult unless a granulomatous vasculitis eventually develops. Patients with microscopic polyangiitis who are c-ANCA-positive have a poorer outcome than the group as a whole.
