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 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 of 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 fever, 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
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:
- Medical history to look for the presence of MPA symptoms
- Physical examination to detect sites of organ involvement and to exclude
other illnesses that may have a similar appearance
- Blood tests to look for sites of organ involvement and testing for
antineutrophil cytoplasmic antibodies (ANCA)
- Urinalysis to detect excessive protein or the presence of red blood cells
- 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 to treat 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. A recent study
found that a newer medication called rituximab was as effective as
cyclophosphamide for treating severe active MPA. Further studies are ongoing and
while these results were important and encouraging, the role of rituximab in the
overall treatment of MPA remains unclear at this time.
Each immunosuppressive drug 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.
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 to 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 1 to 2 years before consideration is given to possibly
slowly reduce the dose toward 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. 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 damaged 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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