Microscopic Polyangiitis

Diagnosis

Indications for Testing

• Multiorgan system involvement in patient

Laboratory Testing

• Nonspecific testing
  • BUN/creatinine – usually normal unless significant renal involvement
  • CBC – mild anemia; rule out infectious process
  • Urinalysis – hematuria, proteinuria, granular or red cell casts
  • Erythrocyte sedimentation rate/C-reactive protein – elevated >50% of the time
  • ANCA – antimyeloperoxidase predominant
    • Most effective test to demonstrate vasculitis
    • Positive in 50-75% of patients
• Antiglomerular basement membrane antibodies – small number (5-10%) positive; use to rule out Goodpasture syndrome

Histology

• Gold standard for diagnosis
• Small and mid-size artery, venule, and capillary necrotizing vasculitis (no granulomas visualized)
• Renal biopsy – focal segmental necrotizing, glomerulonephritis with crescents (identical to biopsy in Wegener granulomatosis and Churg-Strauss syndrome)

Imaging Studies

• Chest x-ray – bilateral, patchy opacities suggest hemorrhage
• CT – ground glass attenuation; eventual honeycombing when fibrosis occurs

Differential Diagnosis

• Vasculitis
  • Churg-Strauss syndrome
  • Polyarteritis nodosa
  • Wegener granulomatosis
  • Cryoglobulinemia
  • Connective tissue disease
• Autoimmune disease
  • Systemic lupus erythematosus
  • Mixed connective tissue disease
  • Goodpasture syndrome
  • Endocarditis
• Glomerulonephritis
  • Goodpasture syndrome
  • Crescenteric glomerulonephritis
  • Endocarditis
  • Acute nephritis
  • Post-streptococcal glomerulonephritis
• Pulmonary – renal hemorrhage
  • Wegener granulomatosis
  • Goodpasture syndrome
  • Systemic lupus erythematosus

Clinical Background

Microscopic polyangiitis (MPA) is a necrotizing vasculitis of the small vessels without granulomatous inflammation.

Epidemiology

• Incidence – 16-25/1,000,000 for systemic vasculitis as a group (includes Churg-Strauss, MPA, polyarteritis nodosa and Wegener granulomatosis)
• Age – peak onset is 50-60 years
• Sex – M>F, 1.8:1
• Ethnicity – higher prevalence in Europeans (Southern >Northern descent)

Pathophysiology

• Necrotizing vasculitis of the microscopic vessels (small arteries, arterioles, capillaries or venules) in sites other than kidney
• Necrotizing crescenteric glomerulonephritis on renal biopsy

Clinical Presentation

• Constitutional – weight loss, fever, myalgias
• Otorhinolaryngological – oral ulcers, epistaxis, sinusitis
• Renal – rapidly progressing glomerulonephritis
• Cardiovascular – pericarditis, endocarditis
• Neurological – mononeuritis multiplex, sensorimotor polyneuropathy
• Dermatological – palpable purpura, papules, ulcers
• Ophthalmological – scleritis, uveitis, episcleritis
• Pulmonary – hemoptysis, chest pain (pleuritis), dyspnea, cough
• Gastrointestinal – abdominal pain

Indications for Laboratory Testing

• Tests generally appear in the order most useful for common clinical situations
• Click on number for test-specific information in the ARUP Laboratory Test Directory

Click the plus sign to expand the table of additional tests.

Test Name and Number	Comments
Anti-Neutrophil Cytoplasmic Antibody, IgG 0050811 Method: Semi-Quantitative Indirect Fluorescent Antibody	If ANCA screen detects antibodies ≥1:20 dilution, titer to end point is added
Myeloperoxidase Antibody 0050526 Method: Semi-Quantitative Multiplex Bead Assay

Diagnostic Algorithm

  Vasculitis in Adults Testing Algorithm