microscopic polyangiitis

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Microscopic polyangiitis (MPA) is small vessel non-granulomatous necrotizing vasculitis. It most often affects venules, capillaries, arterioles, and small arteries, although it occasionally involves medium-sized arteries.

Epidemiology

It typically affects middle-aged individuals.

Clinical presentation

This condition can affect multiple organ systems. Common sites of involvement are:

kidneys: necrotizing glomerulonephritis (present in ~90% of cases )
lungs: pulmonary capillaritis
skin: purpura
gastrointestinal tract

Pathology

It is histologically very similar to polyarteritis nodosa except for involvement of vessels smaller than arteries (e.g. arterioles, venules, capillaries)

There is an absence or paucity of immunoglobulin localization in vessel walls which distinguishes MPA from immune complex-mediated small vessel vasculitis (e.g. Henoch-Schonlein purpura and cryoglobulinaemic vasculitis)

Markers

pANCA-positive in a significant (~70-90%) proportion of cases

Subtypes

Some classify two conditions as organ-specific subsets of this condition :

lung-isolated idiopathic pauci-immune pulmonary capillaritis
kidney-idiopathic pauci-immune rapidly progressive glomerulonephritis (RPGN)

Radiographic features

Pulmonary involvement

There can be a spectrum of findings which typically include pulmonary hemorrhage:

diffuse pulmonary hemorrhage: seen in 30-40% of cases
hemorrhages can be relapsing

Other described manifestations (which are non-specific as individual features) include :

regions of ground-glass attenuation
pulmonary consolidation
thickening of bronchovascular bundles
features of interstitial pneumonitis / pulmonary fibrosis
- honeycombing
- traction bronchiectasis

Treatment and prognosis

Treatment options in severe forms usually include methylprednisolone +/- cyclophosphamide. Other management options include tumor necrosis factor-alpha blockers rituximab, and nonpharmacologic modalities such as plasmapheresis and ventilatory management .

With treatment, there is often complete remission of the disease in a majority of cases.

Differential diagnosis

The differential can be broad dependent on the type of manifestations and feature.

For lung involvement consider:

eosinophilic granulomatosis with polyangiitis:
- has asthma and eosinophilia
granulomatosis with polyangiitis:
- granulomatous vasculitis
- can cavitate
Goodpasture syndrome
- circulating antiglomerular basement membrane (anti-GBM)
- pulmonary hemorrhage (ground glass, airspace opacities and reticular "crazy paving")

History and etymology

It is thought to have been initially described by Friedrich Wohlwill (1881-1958), a German physician, in 1923 .