Microscopic polyangiitis (MPA) is an inflammatory, necrotizing, systemic vasculitis that affects predominantly small vessels (i.e. small arteries, arterioles, capillaries, venules) in multiple organs.
Prevalence is unknown. MPA has an annual incidence of approximately 1/100,000 and a mean age of onset of 50-60 years old. Pediatric-onset MPA (<10 years of age) is uncommon.
MPA affects small vessels (more rarely medium-sized arteries) in any organ, resulting in a wide variety of non-specific symptoms. The early clinical manifestations are indicative of systemic inflammation: fever, arthralgias, myalgias, fatigue, and/or loss of appetite. As the disease progresses, 90% of patients show renal involvement with pauci-immune necrotizing and crescentic glomerulonephritis that can have a rapidly progressive course if not treated promptly. Pulmonary involvement (alveolar hemorrhage, pleural effusion) is frequent and manifests with symptoms such as dyspnea, cough or hemoptysis. The most severe manifestation is combined pulmonary and renal disease (pulmonary-renal syndrome). Gastro-intestinal involvement can present with abdominal pain, nausea, or vomiting, and can be life-threatening in case of peritonitis, ischemia or perforation. Neurological symptoms and signs (mainly mononeuritis multiplex), dermatologic (mainly leukocytoclastic angiitis; see this term) and musculoskeletal (arthralgias, myalgias) involvement, and ocular signs (e.g. episcleritis, retinal vasculitis, uveitis) are also observed. Cardiovascular involvement (e.g. pericarditis, cardiac insufficiency) is rare.
MPA is an antineutrophil cytoplasmic autoantibodies (ANCA)-associated auto-immune disease with little or no immune complex deposition. Evidence indicates that ANCA can activate neutrophils and monocytes, and cause them to attack vessel walls. Infectious agents could trigger the disease.
Diagnosis can be difficult and is essentially made by exclusion of other diseases. Detection of ANCA is a useful serologic marker. Generally, 50% of patients have myeloperoxidase (MPO)-ANCA, 40% proteinase 3 (PR3)-ANCA and 10% are ANCA-negative. Blood tests can reveal elevated C-reactive protein, leukocytosis, and anemia. Urine tests can show proteinuria, hematuria, and leukocyturia. Renal, pulmonary and skin biopsy can support the diagnosis. Pathologically, MPA is characterized by segmental vascular necrosis with infiltration of neutrophils and monocytes, with, often, leukocytoclasia and accumulation of fibrin.
The differential diagnosis includes Wegener's granulomatosis and Churg-Strauss syndrome (distinguished from MPA by the presence of necrotizing granulomatous inflammation in the absence or presence of asthma, respectively), and Henoch-Schönlein purpura and cryoglobulinemic vasculitis (distinguished from MPA by IgA-dominant and cryoglobulin immune deposits, respectively) (see these terms).
Treatment consists of three phases: induction of remission, maintenance of remission, and treatment of relapse. The first-line induction therapy consists of oral or intravenous administration of high dose corticosteroids with immunosuppressive treatment (e.g. cyclophosphamide, or rituxan). In case of severe renal disease and major alveolar hemorrhage, plasma exchange is beneficial. Maintenance treatment consists in continuation of cyclophosphamide or rituxan, or substitution with azathioprine. Relapses are treated with increased or reinstituted immunosuppression. In case of renal failure, dialysis and/or renal transplantation are appropriate.
With early diagnosis and treatment, prognosis is improved. Induction of remission occurs in 80% of patients. Optimum treatment reduces relapses (20-30% of patients have a relapse within 2 years of remission). Five-year patient survival ranges from 75-85%.
Last update: June 2011