What is Microscopic polyangiitis?
Microscopic polyangiitis (MPA) is an uncommon disease that results from blood vessel inflammation (“vasculitis”) that can result in damage to organ systems. The areas most commonly affected by MPA include the kidneys, lung, nerves, skin, and joints. MPA shares many common features with another form of vasculitis called Wegener’s granulomatosis, and treatment approaches for these illnesses are similar.
What is vasculitis?
Vasculitis is a general term that refers to inflammation of the blood vessels. When blood vessels become inflamed, they can only react in limited ways. When inflamed, the blood vessel may become weakened and stretch forming an aneurysm or become so thin that it ruptures resulting in bleeding into the tissue. Vasculitis can also cause blood vessel narrowing to the point of closing off the vessel entirely. This can cause organs to become damaged from loss of oxygen and nutrients that were being supplied by the blood.
MPA affects small to medium sized blood vessels, which directly reflects on the type if tissue injury that is seen in this disease.
What are the symptoms of MPA?
Because many different organ systems may be involved, a wide range of symptoms are possible in MPA.
Patients who have MPA may feel generally ill and fatigued, have fevers, or have loss of appetite and weight. They usually also have symptoms related to areas of involvement such as shortness of breath or coughing up of blood from disease affecting the lungs, rashes, muscle and/or joint pain. MPA affecting the nerves may cause an abnormal sensation followed by numbness or loss of strength. Any combination of these symptoms may be present.
Kidney disease caused by MPA often does not produce symptoms. Inflammation of the kidney may not be apparent to the patient until the kidneys begin to stop working. Therefore, it is very important for the physician, in dealing with any form of vasculitis, to always examine the urine.
What causes MPA?
The cause of MPA is unknown. MPA is not a form of cancer, it is not contagious, and it does not usually occur within families. Evidence from research laboratories strongly supports the idea that the immune system plays a critical role in MPA such that the immune system becomes overactive causing blood vessel and tissue inflammation and damage. What causes the immune system to become overactive is not currently known.
Who is affected by MPA?
MPA can occur in people of all ages, from children to the elderly and appears to affect men and women equally.
How is MPA diagnosed?
Suspicion for MPA is based on information gathered from a variety of sources, including:
Medical history to look for the presence of MPA symptoms
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Physical examination to detect sites of organ involvement and to exclude other illnesses that may have a similar appearance.
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Blood tests to look for sites of organ involvement and testing for antineutrophil cytoplasmic antibodies (ANCA)
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Urinalysis to detect excessive protein or the presence of red blood cells.
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Imaging tests such as x-rays, computed tomography (CT) or magnetic resonance (MR) scans, which can show abnormalities in affected areas such as the lungs.
A positive blood test for ANCA can support a suspected diagnosis of MPA. However, the blood test does not by itself prove the diagnosis of MPA or determine disease activity.
Once the diagnosis of MPA is suspected, a biopsy (tissue sample) of an affected area is often performed to try to confirm the presence of vasculitis. Biopsies are only recommended for organ sites in which there are abnormal findings present by examination, laboratory tests, or imaging.
How is MPA treated?
Medications that suppress the immune system form the foundation of treatment for MPA. There are a variety of immunosuppressive medications that are used in MPA, each of which has individual side effects.
People with MPA who have critical organ system involvement are generally treated with corticosteroids combined with another immunosuppressive medication such as cyclophosphamide (cytoxan), methotrexate, or azathioprine (imuran). All of these medications are also used to treat other medical conditions. Azathioprine is used to prevent organ transplant rejection and has been applied to the treatment of rheumatoid arthritis and systemic lupus erythematosus. Both cyclophosphamide and methotrexate are given at high doses as a treatment for certain types of cancer and therefore are sometimes referred to as “chemotherapy.” In cancer treatment, these medications work by killing or slowing the growth of rapidly multiplying cancer cells. In vasculitis, these medications are given at doses that are 10 to 100 times lower than those used to treat cancer, and their primary effect is to influence the behavior of the immune system in a manner that results in immunosuppression.
All immunosuppressive medications can have side effects for which monitoring plays a critical role in preventing or minimizing their occurrence. Each medication has a unique side effect profile which forms the basis for the monitoring plan. The fact that someone with MPA may initially tolerate treatment does not guarantee that the person will have the same degree of tolerance over time. This makes ongoing monitoring essential, and in some instances, monitoring for long term effects may be important even after the medication is stopped.
The goal of treatment is to stop all damage that is occurring as a result of MPA. If disease activity can be completely "turned off," this is called “remission.” Once it is apparent that the disease is improving, doctors slowly reduce the corticosteroid dose and eventually hope to discontinue it completely. When cyclophosphamide is used, it is often only given until the time of remission (usually around 3-6 months), after which time it is switched to another immunosuppressive agent such as methotrexate or azathioprine to maintain remission. The treatment duration of the maintenance immunosuppressive medication may vary between individuals. In most instances, it is given for a minimum of one to two years before consideration is given for whether it would be appropriate to slowly reduce the dosage to discontinuation.
What is the outlook for patients with MPA?
Because MPA is an uncommon disease, accurate statistics on overall outcome are only approximate. On average, after 5 years of illness, over 80% of people have survived the effects of MPA. How people with MPA do is strongly related to the severity of their illness. Although MPA can be a progressive and serious illness, many people with MPA do extremely well.
The best opportunity for minimal organ damage to occur comes when treatment has been promptly initiated and is carefully monitored by a physician who is knowledgeable about MPA. Even patients who have the most severe MPA can achieve remission when treated promptly and followed closely.
After achieving remission, it is possible for MPA to recur (often referred to as a “relapse”). Relapses occur in about 50% of people with MPA. Such relapses may be similar to what the patient experienced at the time of their diagnosis or they may be different. The likelihood of experiencing a severe relapse can be minimized by prompt reporting to the physician of any new symptoms, regular physician follow-up, and ongoing monitoring with laboratory tests and imaging. The treatment approach for relapses is similar to that of newly diagnosed disease. Achieving remission is again possible for most people with MPA.
