Microscopic polyangiitis


Microscopic polyangiitis is an ill-defined autoimmune disease characterized by a systemic, pauci-immune, necrotizing, small-vessel vasculitis without clinical or pathological evidence of necrotizing granulomatous inflammation.

Signs and symptoms

Clinical features may include constitutional symptoms like fever, loss of appetite, weight loss, fatigue, and kidney failure. A majority of patients may have blood in the urine and protein in the urine. Rapidly progressive glomerulonephritis may occur. Because many different organ systems may be involved, a wide range of symptoms are possible in MPA.
Purpura and livedo racemosa may be present.

Cause

While the mechanism of disease has yet to be fully elucidated, the leading hypothesis is that the process is begun with an autoimmune process of unknown cause that triggers production of p-ANCA. These antibodies will circulate at low levels until a pro-inflammatory trigger — such as infection, malignancy, or drug therapy. The trigger upregulates production of p-ANCA. Then, the large number of antibodies make it more likely that they will bind a neutrophil. Once bound, the neutrophil degranulates. The degranulation releases toxins that cause endothelial injury. Most recently, two different groups of investigators have demonstrated that anti-MPO antibodies alone can cause necrotizing and crescentic glomerulonephritis.

Diagnosis

Laboratory tests may reveal an increased sedimentation rate, elevated CRP, anemia and elevated creatinine due to kidney impairment. An important diagnostic test is the presence of perinuclear antineutrophil cytoplasmic antibodies with myeloperoxidase specificity, and protein and red blood cells in the urine.
In patients with neuropathy, electromyography may reveal a sensorimotor peripheral neuropathy.

Differential diagnosis

The signs and symptoms of microscopic polyangiitis may resemble those of granulomatosis with polyangiitis but typically lacks the significant upper respiratory tract involvement frequently seen in people affected by GPA.

Treatment

The customary treatment involves long term dosage of prednisone, alternated or combined with cytotoxic drugs, such as cyclophosphamide or azathioprine.
Plasmapheresis may also be indicated in the acute setting to remove ANCA antibodies.
Rituximab has been investigated, and in April 2011 approved by the FDA when used in combination with glucocorticoids in adult patients.