A feature of microscopic polyangiitis is -
Microscopic polyangitis (MPA)

It is a necrotizing vasculitis.
It is considered as a variant of polyarteritis nodosa (PAN), but in contrast to classical PAN, microscopic polyangiitis involves smaller vessels, i.e. arterioles, venules and capillaries.
It is also differentiated from classical PAN by -


It involves lung (in classical PAN lungs are not involved).
In kidney it causes glomerulonephritis (in classical PAN, glomerulonephritis is absent).
In skin it causes manifestation due to small vessel involvement, i.e. palpable purpura, while classical PAN has manifestation of medium vessel involvement i.e digital ischemia and gangrene, livido reticulars, nodules, papules.
It is positive for p-ANCA, while classical PAN is not associated with ANCA.
Presentation

Constitutional symptoms

Fever, weight loss, malaise and fatigue.
Kidney (a most common organ involved).
There is rapidly progressive glomerulonephritis.
The glomerulonephritis is pauci-immune, i.e. there is no immunoglobulin or complement deposition.
Renal hypertension does not occur, in contrast to classical PAN, where hypertension is the prominant feature of kidney involvement.


Lung

Lung involvement can be dramatic and life threathing.
Capillaritis causes alveolar hemorrhage.
Hemoptysis, pleuritis and interstitial fibrosis.

Skin

Features of small vessel involvement, i.e. purpura, papule.
Involves the dependent area of the body, i.e. feet & leg.
Peripheral nervous system → Mononeuritis multiplex
Upper respiratory tract  → Rhinitis, sinusitis, otitis media
Musculoskeletal system → Myalgia, arthralgia, Non-destructive migratory arthritis
Eye → Conjunctivitis, episcleritis
Note
Microscopic polyangitis has the same spectrum of manifestations as wegener's granulomatosis, but can be differentiated it by following two features -


In wegener's granulomatosis there is granulometous vasculitis i.e. granuloma formation, granulomas are absent in MPA.
Wegener's granulomatosis causes nodules and cavity in the lung.
IgG deposits in kidney
Bronchospasm
Renal involvement
All of the above