Microscopic Polyangiitis

What is microscopic polyangiitis?

Microscopic polyangiitis (MPA) is an uncommon disease. It is the result of blood vessel inflammation (vasculitis ), which can damage 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 granulomatosis with polyangiitis (Wegener's), 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 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 of tissue injury that is seen in this disease.

What are the features of MPA?

Because many different organ systems may be involved, a wide range of symptoms and signs are possible in MPA. Patients who have MPA may feel generally ill and fatigued, have fever, or have loss of appetite and weight. They usually also have symptoms related to areas of involvement such as rashes, muscle and/or joint pain. When MPA affects the lungs they may have shortness of breath or coughing up of blood. 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 doctor, 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 causes blood vessel and tissue inflammation and damage.

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:

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 ®) or rituximab (Rituxan®). In patients who have less severe MPA, corticosteroids and methotrexate can be used initially. The goal of treatment is to stop all injury 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 only given until the time of remission (usually around 3 to 6 months), after which time it is switched to another immunosuppressive agent, such as methotrexate, azathioprine (Imuran®), or mycophenolate mofetil (Cellcept®) 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 1 to 2 years before consideration is given to possibly slowly reduce the dose toward discontinuation.

All of these medications are also used to treat other medical conditions. Azathioprine and mycophenolate mofetil are used to prevent organ transplant rejection. Methotrexate is used to treat rheumatoid arthritis and psoriasis. 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. Rituximab belongs to a class of medications called biologic agents that target a specific element of the immune system. Recent studies found that rituximab was as effective as cyclophosphamide for treating severe active MPA.

Because these medications suppress the immune system, there is an increased risk of developing serious infections. Each immunosuppressive drug also has a unique set of potential side effects. Monitoring for the side effects associated with each drug is critical to prevent or minimize their occurrence. Also, the fact that a patient may initially tolerate treatment does not guarantee that tolerance will remain the same over time. This makes ongoing monitoring essential, and in some instances, monitoring for long-term side effects may be important even after the drug is stopped.

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. Outcome is strongly related to the severity of illness. Although MPA can be a progressive and serious illness, many people with MPA do extremely well.

Organ damage can best be minimized by prompt initiation of treatment followed by careful monitoring by a doctor 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 doctor of any new symptoms, regular doctor 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.



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