DEFINITION OF MICROSCOPIC POLYANGIITIS
The term microscopic polyarteritis was introduced into the literature by Davson in 1948 in recognition of the presence of glomerulonephritis in patients with polyarteritis nodosa. In 1992, the Chapel Hill Consensus Conference on the Nomenclature of Systemic Vasculitis adopted the term microscopic polyangiitis to connote a necrotizing vasculitis with few or no immune complexes affecting small vessels (capillaries, venules, or arterioles). Glomerulonephritis is very common in microscopic polyangiitis, and pulmonary capillaritis often occurs. The absence of granulomatous inflammation in microscopic polyangiitis is said to differentiate it from granulomatosis with polyangiitis (Wegener’s).
INCIDENCE AND PREVALENCE OF MICROSCOPIC POLYANGIITIS
The incidence of microscopic polyangiitis is estimated to be 3–5/100,000. The mean age of onset is ~57 years, and males are slightly more frequently affected than females.
PATHOLOGY AND PATHOGENESIS OF MICROSCOPIC POLYANGIITIS
The vasculitis seen in microscopic polyangiitis has a predilection to involve capillaries and venules in addition to small- and medium-sized arteries. Immunohistochemical staining reveals a paucity of immunoglobulin deposition in the vascular lesion of microscopic polyangiitis, suggesting that immune-complex formation does not play a role in the pathogenesis of this syndrome. The renal lesion seen in microscopic polyangiitis is identical to that of granulomatosis with polyangiitis (Wegener’s). Like granulomatosis with polyangiitis (Wegener’s), microscopic polyangiitis is highly associated with the presence of ANCA, which may play a role in pathogenesis of this syndrome (see above).
CLINICAL MANIFESATIONS OF MICROSCOPIC POLYANGIITIS
Because of its predilection to involve the small vessels, microscopic polyangiitis and granulomatosis with polyangiitis (Wegener’s) share similar clinical features. Disease onset may be gradual, with initial symptoms of fever, weight loss, and musculoskeletal pain; however, it is often acute. Glomerulonephritis occurs in at least 79% of patients and can be rapidly progressive, leading to renal failure. Hemoptysis may be the first symptom of alveolar hemorrhage, which occurs in 12% of patients. Other manifestations include mononeuritis multiplex and gastrointestinal tract and cutaneous vasculitis. Upper airway disease and pulmonary nodules are not typically found in microscopic polyangiitis and, if present, suggest granulomatosis with polyangiitis (Wegener’s).
Features of inflammation may be seen, including an elevated ESR, anemia, leukocytosis, and thrombocytosis. ANCA are present in 75% of patients with microscopic polyangiitis, with antimyeloperoxidase antibodies being the predominant ANCA associated with this disease.
DIAGNOSIS OF MICROSCOPIC POLYANGIITIS
The diagnosis is based on histologic evidence of vasculitis or pauci-immune glomerulonephritis in a patient with compatible clinical features of multisystem disease. Although microscopic polyangiitis is strongly ANCA-associated, no studies have as yet established the sensitivity and specificity of ANCA in this disease.
TREATMENT OF MICROSCOPIC POLYANGIITIS
TREATMENT Microscopic Polyangiitis
The 5-year survival rate for patients with treated microscopic polyangiitis is 74%, with disease-related mortality occurring from alveolar hemorrhage or gastrointestinal, cardiac, or ...