A 68-year-old woman was hospitalized for evaluation of progressive dyspnea, cough, and hemoptysis. She had a 2-month history of malaise, anorexia, arthralgias, and a 30-lb weight loss. Symptoms did not improve with outpatient antibiotic therapy. Laboratory studies were significant for profound anemia, elevated erythrocyte sedimentation rate (ESR), C-reactive protein (CRP), and decreased renal function. Perinuclear-pattern antineutrophil cytoplasmic antibody (p-ANCA) and myeloperoxidase (MPO) antibody were positive. Urinalysis showed microscopic hematuria with granular and red blood cell casts. Chest radiograph revealed bilateral alveolar infiltrates (Figure 79-1). Bronchoscopy with bronchoalveolar lavage was consistent with alveolar hemorrhage, and transbronchial lung biopsy showed changes of pulmonary capillaritis (Figure 79-2). An extensive infectious workup was negative. A diagnosis of microscopic polyangiitis (MPA) was made. The patient was treated with high-dose corticosteroids and oral cyclophosphamide, resulting in gradual clinical improvement. She was subsequently switched to azathioprine for maintenance of remission.

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  MPA is a rare, multisystem autoimmune disorder.

  
  
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  It is characterized by inflammation of small blood vessels such as arterioles, venules, and capillaries.

  
  
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  It most commonly occurs in the 65- to 74-year age group.

  
  
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  Males are affected slightly more frequently than females.

  
  
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  Incidence of MPA is 3 to 6 per million per year; prevalence is about 60 per million population.^1,2

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  The exact etiology of MPA remains unknown.

  
  
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  Genetic and environmental factors, including infections, are thought to play a role in disease pathogenesis.

  
  
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  Antineutrophil cytoplasmic antibodies (ANCAs) are present in most patients with MPA and are specific for MPO, an antigen in neutrophil granules and monocyte lysosomes.

  
  
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  In vitro and experimental animal observations suggest that ANCAs have an important pathogenic role in the pathogenesis of MPA. However, other proinflammatory stimuli are required for the development of clinical disease.

  
  
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  Antiendothelial cell antibodies (AECAs) may also play a role in pathogenesis.

  
  
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  Histologically, MPA is characterized by necrotizing small vessel vasculitis (SVV) without evidence of granulomatous inflammation.^2,3

Clinical Features

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  MPA mainly involves the kidney along with the upper and lower respiratory tract.

  
  
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  Symptoms are variable and related to pattern of involvement of the internal organs.

  
  
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  Constitutional symptoms including fever and weight loss are common.

  
  
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  Almost all patients have renal involvement, characterized by rapidly progressive glomerulonephritis, which may progress to end-stage kidney disease.

  
  
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  Pulmonary manifestations occur in about 50% of patients. Pulmonary capillaritis from MPA causes alveolar hemorrhage with hemoptysis, dyspnea, cough, and pleuritic chest pain. Other pulmonary features may include pulmonary nodules, infiltrates, or pleural effusion.

  
  
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  Skin involvement occurs in 30% to 60% of patients and typically presents with palpable purpura (Figure 79-3), livedo reticularis, nodules, or ulcerations.

  
  
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  Upper respiratory involvement may present as sinusitis (Figure 79-4) and/or otitis media and occurs in about a third of cases.

  
  
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  Other manifestations of MPA may include ocular inflammation such as scleritis or episcleritis (Figure 79-5) or retinal vasculitis, peripheral neuropathy or mononeuritis multiplex, arthritis, and gastrointestinal vasculitis.^{2,3, and 4}