Microscopic polyangiitis

Microscopic polyangiitis

Microscopic polyangiitis

Microscopic polyangiitis is a pauci-immune nongranulo­matous necrotizing vasculitis that (1) affects small blood vessels (capillaries, venules, or arterioles), (2) often causes glomerulonephritis and pulmonary capillaritis, and (3) is often associated with ANCA on immunofluorescence test­ing (directed against MPO, a constituent of neutrophil granules). Because microscopic polyangiitis may involve medium-sized as well as small blood vessels and because it tends to affect capillaries within the lungs and kidneys, its spectrum overlaps those of both polyarteritis nodosa and granulomatosis with polyangiitis.

Microscopic polyangiitis (MPA) causes small blood vessels to be inflamed.

In rare instances, medications, particularly propylthioura­cil, hydralazine, allopurinol, penicillamine, minocycline, and sulfasalazine, induce a systemic vasculitis associated with high titers of p-ANCA and features of microscopic polyangiitis.

Symptoms and Signs

A wide variety of findings suggesting vasculitis of small blood vessels may develop in microscopic polyangiitis. These include “palpable” (or “raised”) purpura and other signs of cutaneous vasculitis (ulcers, splinter hemorrhages, vesiculobullous lesions).

Microscopic polyangiitis is the most common cause of pulmonary-renal syndromes, being several times more common than anti–glomerular basement membrane disease.

Interstitial lung fibrosis that mimics usual interstitial pneu­monitis is the presenting condition.

Pulmonary hemor­rhage may occur. The pathologic findings in the lung are typically those of capillaritis.

Vasculitic neuropathy (mononeuritis multiplex) is also common in microscopic polyangiitis.

Diagnosis

Three-fourths of patients with microscopic polyangiitis are ANCA-positive. Elevated acute-phase reactants are also typical of active disease. Microscopic hematuria, protein­uria, and red blood cell casts in the urine may occur. The kidney lesion is a segmental, necrotizing glomerulonephri­tis, often with localized intravascular coagulation and the observation of intraglomerular thrombi upon renal biopsy.

Distinguishing this disease from granulomatosis with poly­angiitis may be challenging in some cases. Microscopic polyangiitis is not associated with the chronic destructive upper respiratory tract disease often found in granuloma­tosis with polyangiitis. Moreover, as noted, a critical differ­ence between the two diseases is the absence of granulomatous inflammation in microscopic polyangiitis. Because their treatments may differ, microscopic polyangi­itis must also be differentiated from polyarteritis nodosa.

Treatment

Microscopic polyangiitis is usually treated in the same way as granulomatosis with polyangiitis: patients with severe disease, typically involving pulmonary hemor­rhage and glomerulonephritis, require urgent induction treatment with corticosteroids and either cyclophospha­mide or rituximab. If cyclophosphamide is chosen, it may be administered either in an oral daily regimen or via intermittent (usually monthly) intravenous pulses; fol­lowing induction of remission, cyclophosphamide may be replaced with azathioprine, rituximab, or methotrexate (provided the patient has normal kidney function). In cases of drug-induced MPO-ANCA–associated vasculitis, the offending medication should be discontinued; signifi­cant organ involvement (eg, pulmonary hemorrhage, glomerulonephritis) requires immunosuppressive therapy.

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