Granulomatosis with polyangiitis (GPA) and microscopic polyangiitis (MPA) are both autoimmune vasculitides associated with antineutrophil cytoplasmic antibodies (ANCA), but they differ in clinical features, pathology, and organ involvement. Below is a concise comparison:

1. Definition

• GPA: A necrotizing granulomatous vasculitis affecting small- and medium-sized vessels, often involving the respiratory tract and kidneys.
• MPA: A necrotizing vasculitis primarily affecting small vessels (capillaries, venules, arterioles), without granulomas, commonly targeting kidneys and lungs.

2. ANCA Association

• GPA: Typically associated with c-ANCA (PR3-ANCA) (anti-proteinase 3).
• MPA: Typically associated with p-ANCA (MPO-ANCA) (anti-myeloperoxidase).
• Note: ANCA patterns may overlap, and some cases are ANCA-negative.

3. Clinical Features

• GPA:
  • Upper respiratory tract: Sinusitis, nasal crusting, epistaxis, saddle-nose deformity.
  • Lower respiratory tract: Pulmonary nodules, cavities, or infiltrates; hemoptysis.
  • Kidneys: Rapidly progressive glomerulonephritis (RPGN) with crescentic glomeruli.
  • Other: Skin (purpura), joints, eyes (scleritis), nervous system.
  • Classic triad: Upper airway, lower airway, and renal involvement.
• MPA:
  • Kidneys: RPGN, hematuria, proteinuria (most common feature).
  • Lungs: Pulmonary hemorrhage, interstitial lung disease.
  • Other: Skin (purpura), peripheral neuropathy, gastrointestinal involvement.
  • No upper respiratory tract involvement (key differentiator from GPA).

4. Pathology

• GPA: Necrotizing vasculitis with granulomatous inflammation in affected tissues (e.g., lungs, sinuses).
• MPA: Necrotizing vasculitis without granulomas; pauci-immune glomerulonephritis on kidney biopsy.

5. Epidemiology

• GPA: More common in older adults (40–60 years), slight male predominance, higher prevalence in Caucasian populations.
• MPA: Similar age group, no strong gender or ethnic predilection.

6. Diagnosis

• GPA:
  • Clinical features (respiratory + renal involvement).
  • Positive c-ANCA/PR3-ANCA (high specificity).
  • Biopsy showing granulomatous vasculitis.
  • Imaging: CT chest for nodules/cavities.
• MPA:
  • Clinical features (renal + pulmonary hemorrhage).
  • Positive p-ANCA/MPO-ANCA.
  • Biopsy showing pauci-immune vasculitis without granulomas.
  • Rule out other causes of RPGN or pulmonary-renal syndrome.

7. Treatment

• Both are treated similarly:
  • Induction: Glucocorticoids (e.g., prednisone) + cyclophosphamide or rituximab for severe disease; methotrexate for milder GPA.
  • Maintenance: Azathioprine, rituximab, or methotrexate to prevent relapse.
  • Supportive care: Dialysis for severe renal failure, plasma exchange for pulmonary hemorrhage or severe RPGN.
• GPA may require longer maintenance due to higher relapse risk.

8. Prognosis

• GPA: Relapses are more common; untreated, high mortality due to renal or respiratory failure.
• MPA: High risk of renal failure; pulmonary hemorrhage carries poor prognosis if untreated.
• Both have improved outcomes with early diagnosis and aggressive treatment.

Key Differentiators

• Granulomas: Present in GPA, absent in MPA.
• Upper airway involvement: Common in GPA, rare/absent in MPA.
• ANCA type: c-ANCA/PR3 (GPA) vs. p-ANCA/MPO (MPA).
• Pulmonary findings: Nodules/cavities in GPA vs. hemorrhage/fibrosis in MPA.

Summary Table

Feature	GPA	MPA
ANCA Type	c-ANCA (PR3)	p-ANCA (MPO)
Granulomas	Yes	No
Upper Airway	Common (sinusitis, epistaxis)	Rare/absent
Lung Involvement	Nodules, cavities	Hemorrhage, fibrosis
Kidney Involvement	RPGN	RPGN
Biopsy	Granulomatous vasculitis	Pauci-immune vasculitis
Treatment	Steroids + rituximab/cyclophosphamide	Similar
Relapse Risk	Higher	Lower

Disclaimer: owerl is not a doctor; please consult one.