- Microscopic Polyangiitis is a vasculitis of small to medium-sized vessels including arterioles, capillaries, and venules. Although this disease was once thought to be a component of polyarteritis nodosa it is primarily distinguished by its distribution to smaller vasculature, especially glomerular capillaries and the pulmonary vasculature.
- This disease primarily affects the elderly and has a slight predilection for men.
- Microscopic Polyangiitis is a ANCA-associated vasculitis and p-ANCA auto-antibodies are observed. Because there is no evidence of immune complex deposition microscopic polyangiitis it is considered a "Pauci-immune" vasculitis.
- Microscopic Polyangiitis is characterized by a necrotizing vasculitis similar to that of polyarteritis nodosa. In addition to its distinct distribution compared to polyarteritis nodosa the lesions of Microscopic Polyangiitis are contrasted by their predilection to progress through morphological stages simultaneously. Microscopic Polyangiitis results in a pauci-immune glomerulonephritis highly similar to that seen in wegener granulomatosis; however, there is an absence of granulomatous inflammation as observed in wegener granulomatosis.
- There is significant overlap between the symptomology of microscopic polyangiitis and wegener granulomatosis; however, there are no upper respiratory tract symptoms in this disease. Involvement of the glomerular capillaries results in a pauci-immune glomerulonephritis which manifests with symptomology of nephritic syndrome such as hematuria. Involvement of the pulmonary circulation can lead to hemorrhage from the pulmonary capillaries, resulting in hemoptysis. More rarely, GI bleeding and skin rash can result from involvement of corresponding vasculature.