Microscopic Polyangiitis Primarily Affects Small Vessels, But Medium Vessels Can Occasionally Be Involved
Microscopic polyangiitis (MPA) is definitively classified as a small-vessel vasculitis that predominantly affects capillaries, venules, and arterioles—however, medium-sized arteries may occasionally also be involved. 1
Vessel Size Classification
According to the most authoritative and recent guidelines:
Primary target: MPA characteristically causes necrotizing inflammation of small vessels (arterioles, capillaries, and venules) with little or no immune complex deposition (pauci-immune pattern) 1
Secondary involvement: Medium or large vessels may occasionally also be involved 1
The 2012 Chapel Hill Consensus Conference definition explicitly states that "necrotizing arteritis involving small and medium arteries may be present" in MPA 2
Clinical Evidence of Medium Vessel Involvement
The occasional medium vessel involvement in MPA is not merely theoretical—it has real clinical consequences:
Aneurysm formation: Case reports document ruptured aneurysms in medium-sized abdominal arteries (hepatic, renal, gastric, splenic, gastroepiploic, and mesenteric arteries) in MPA patients 3
These aneurysms typically rupture 45 days after symptom onset and 15 days after immunosuppressive treatment initiation, with a 50% mortality rate 3
Medium vessel involvement can manifest as cardiovascular complications, cerebral infarction, and subarachnoid hemorrhage 4
Key Distinguishing Feature from Polyarteritis Nodosa
The critical distinction is one of predominance, not exclusivity:
MPA: Predominantly small vessels, occasionally medium vessels, no granulomatous inflammation 1, 2
Polyarteritis Nodosa (PAN): Predominantly medium-sized arteries, spares small vessels (capillaries, venules, arterioles)
Clinical Implications
When evaluating a patient with suspected MPA:
Expect typical small-vessel manifestations: rapidly progressive glomerulonephritis (100% in rupture cases), pulmonary capillaritis, purpura 3, 2
Remain vigilant for medium vessel complications: visceral aneurysms, particularly in abdominal arteries, which can develop asymptomatically and rupture acutely 3
Consider imaging of abdominal vessels in patients with visceral pain or hemodynamic instability, especially within the first 2 months of disease or early after treatment initiation 3
The answer to your question: MPA does not primarily affect medium-sized vessels, but medium vessel involvement can and does occur as a secondary feature of this predominantly small-vessel vasculitis.