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.
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 capillaritis
- 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 localization 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 hemorrhage:
- diffuse pulmonary hemorrhage: seen in 30-40% of cases 2,3
- hemorrhages 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
- features of interstitial pneumonitis 12 / pulmonary fibrosis 11
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 9.
With treatment, there is often complete remission of the disease in a majority of cases.
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
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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- 11. Tzelepis GE, Kokosi M, Tzioufas A et-al. Prevalence and outcome of pulmonary fibrosis in microscopic polyangiitis. Eur. Respir. J. 2010;36 (1): 116-21. doi:10.1183/09031936.00110109 - Pubmed citation
- 12. Aika Suzuki, Susumu Sakamoto, Atsuko Kurosaki, Yasuyuki Kurihara, Keita Satoh, Yusuke Usui, Toshihiro Nanki, Yoshihiro Arimura, Hirofumi Makino, Yasunori Okada, Masayoshi Harigai, Kunihiro Yamagata, Hitoshi Sugiyama, Hiroaki Dobashi, Akihiro Ishizu, Naotake Tsuboi, Joichi Usui, Ken-ei Sada, Sakae Homma, for Japan Research Committee of the Ministry of Health, Labour, and Welfare for Intractable Vasculitis and Research Committee of Intractable Renal Disease of the Ministry of Health, Labour, and Welfare of Japan. Chest High-Resolution CT Findings of Microscopic Polyangiitis: A Japanese First Nationwide Prospective Cohort Study. (2019) American Journal of Roentgenology. doi:10.2214/AJR.18.20967 - Pubmed