In the lung, MPA manifests as diffuse pulmonary hemorrhage and capillaritis. Microscopically, alveolar walls are distended and involved by neutrophils -- there is a background of hemorrhage (blood extravasated in the airspaces). Fibrin, hyaline membranes and plugs of fibroblastic tissue resembling organizing pneumonia can also be seen.
Hemosiderin laden macrophages are prominent. Note also the hyperplasia of type II pneumocytes.
Pulmonary hemorrhage is characteristic, seen as red blood cells filling the alveolar spaces.
Microscopic polyangiitis (MPA), also known as microscopic polyarteritis, is a systemic necrotizing vasculitis that affects small vessels such as arterioles, venules and capillaries. Most patients (up to 80%) have antineutrophilic cytoplasmic antibody (ANCA), which is usually P-ANCA in type.
Entities on the differential:
Polyarteritis nodosa (PAN) affects small to medium sized muscular arteries; PAN does not affect arterioles, capillaries or venules which is the site of pathology in MPA. Thus, glomerulonephritis is not seen in PAN. Furthermore, PAN usually spares the lung and is not associated with ANCA.
Wegeners granulomatosis, also an ANCA-associated vasculitis, is characterizied by granulomatous inflammation, which is absent in MPA; furthermore, Wegeners is c-ANCA positive whereas MPA is largely p-ANCA positive.
Goodpasture syndrome is also a cause of pulmonary-renal syndrome, but the process is caused by anti-GBM antibodies as demonstrated on immunofluorescence.
Affects middle to older individuals, most frequently in the 4th or 5th decade with a slight female predominance (F:M of 1.5:1). Usually, the onset of symptoms is rapid and patients commonly present with glomerulonephritis (90% of cases), pulmonary lesions (50% of cases) as well as nonspecific systemic symptoms of fever, weight loss, myalgias and arthralgis. The skin can also be involved and would manifest as palpable purpura, which can be biopsied (Kumar).
Corticosteroids and cyclophosphamide leads to complete recovery in 70% of patients, but relapse occurs in 40% of patients (Zander).
Skin, lung, and kidneys are the organs usually most affected.
80% of patients have circulating p-ANCA antibodies.
Expansion of alveoli by neutrophils and hemorrhage in the airspaces are key histologic features.
Kumar V, Abbas AK, Fausto N. Robbins and Cotran Pathologic Basis of Disease. 7th Ed. Philadelphia, PA: Elsevier; 2005: 539-541.
Zander DS, Farver CF. Pulmonary Pathology: Foundations in Diagnostic Pathology. Philadelphia, PA: Elvesier; 2008: 141-4.