Microscopic polyangiitis (MPA) is small vessel non-granulomatous necrotising vasculitis. It most often affects venules, capillaries, arterioles, and small arteries, although it occasionally involves medium-sized arteries 9.
It typically affects middle aged individuals.
This condition can affect multiple organ systems. Common sites of involvement are
- kidneys: necrotizing glomerulonephritis (present in ~90% of cases 1)
- lungs: pulmonary capillitis
- skin: purpura
- gastrointestinal tract
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 localisation in vessel walls which distinguishes MPA from immune complex-mediated small vessel vasculitis (e.g. Henoch-Schonlein purpura and cryoglobulinaemic vasculitis)
- pANCA -positive in a significant (~70-90%) proportion of cases
Some classify two conditions as organ specific subsets of this condition 7:
- lung-isolated idiopathic pauci-immune pulmonary capillaritis
- kidney-idiopathic pauci-immune rapidly progressive glomerulonephritis (RPGN)
There can be a spectrum of findings which typically include pulmonary haemorrhage:
- diffuse pulmonary haemorrhage: seen in ~30-40% of cases 2,3
- haemorrhages can be relapsing
Other described manifestations (which are non-specific as individual features) include 9:
- regions of ground-glass attenuation
- pulmonary consolidation
- thickening of bronchovascular bundles
- honeycombing: features of pulmonary fibrosis 11
Treatment and prognosis
Treatment options in severe forms usually include methylprednisolone +/- cyclophosphamide. Other management options include tumour necrosis factor-alpha blockers ituximab, and nonpharmacologic modalities such as plasmapheresis and ventilatory management 9.
With treatment there is often complete remission of the disease in a majority of cases.
The differential can be broad dependant 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
History and etymology
It is thought to have been initially described by Friedrich Wohlwill (1881-1958), a German physician, in 1923 10.
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