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Microscopic polyangiitis (MPA) is an autoimmune disorder. It is a form of vasculitis (inflammation of the blood vessels) that affects only the smallest blood vessels and causes cell death in the affected vessels. The disease is systemic, meaning it may affect any organ with microscopic blood vessels, but the kidneys and lungs are especially susceptible. It is associated with glomerulonephritis (inflammation of the kidneys' small vessels) and alveolar hemorrhage (bleeding from the alveoli, small round tissues in the lungs that are the site of oxygen exchange).
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What are the Symptoms?
The most common and serious symptoms of the disease are the following:
- Kidney involvement (in up to 90% of patients)
- Alveolar hemorrhage, leading to buildup of scar tissue in the lungs
Other signs and symptoms that may be present include the following:
- Nonspecific gastrointestinal symptoms
- Damage to the peripheral nerves (mononeuritis multiplex), which may cause numbness, muscle weakness, or pain
- Inflammation of the whites of the eyes (sclera)
- Red or purple rash (about 1/3 of patients)
- Nail bed damage and/or tiny vertical lines under the nails (splinter hemorrhages).
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How is Microscopic Polyangiitis Diagnosed?
Diagnosis is based on clinical signs and symptoms and the results of laboratory tests, including tests for C-reactive protein and ANCA. Once microscopic polyangiitis is suspected, confirming the diagnosis is relatively simple, requiring only a biopsy. However, getting to the point of testing for the disease may involve a long process of differential diagnosis because it is a rare condition and therefore not one of the first diseases suspected in a patient with symptoms. Lung and kidney biopsies are serious procedures that may lead to complications, so doctors only order them if the need is clear.
The combination of symptoms is the best clue for diagnosis of microscopic polyangiitis. Neuropathy, scleritis, and fingernail problems combined with kidney and lung dysfunction are strong indicators of microscopic polyangiitis.
Tests that can aid in diagnosis include chest x-rays and bronchoscopy to check for alveolar hemorrhages, routine blood tests that can indicate increased activity of the immune system and kidney dysfunction, and urinalysis to check for blood or protein in the urine, which are common signs of kidney problems. A type of test called an ELISA test for the antibody ANCA provides important clues because ANCA is present in at least 60% of cases.
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MPA is called a pauci-immune disorder because hypersensitivity of the immune system is not present; instead, it is associated with an IgG antibody called anti-neutrophil cytoplasmic antibody (ANCA). This disorder is rare but serious. Other forms of vasculitis, particularly polyarteritis nodosa, can have similar symptoms and must be excluded during differential diagnosis. Only MPA causes glomerulonephritis and alveolar hemorrhage, and it affects only the tiniest blood vessels, unlike polyarteritis nodosa, which affects medium-sized vessels. Also, the type of immune system involvement differs between the two diseases.
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"Microscopic Polyangiitis." Merck Manual of Diagnosis and Therapy.