What Does Microscopic Polyangiitis Mean?
Simply inflammation ("itis") of many ("poly") small blood vessels including small (microscopic) arteries, arterioles, capillaries and venules.
Microscopic Polyangiitis (abbreviated' to MPA) should not be confused with classical Polyarteritis Nodosa (abbreviated to PAN)
MPA is more common than PAN in Britflin today. Both disorders can affect men and women, and are seen occasionally in children.
MPA tends to attack very small blood vessels (including tiny capillaries in the kidneys) while PAN tends to attack larger arteries, often causi ng blockages.
Blockages in larger blood vessels can prevent blood reaching whole organs or parts of organs. Inflammation and blockages of arterioles and capillaries also prevents oxygen reaching important tissues.
MPA has many similarities to Wegener's Granulomatosis, another form of vasculitis. However, granuloma which are collections of inflammatory cells, develop in Wegener's but not in MPA.
Anti-neutrophil cytoplasm antibodies (ANCA) are present in the blood in MPA, as they are in Wegener's Granulomatosis. The ANCA blood test is a useful guide to doctors when they make a diagnosis of MPA or Wegener's.
CAPILLARY
PAN
Symptoms associated with MPA
Sometimes MPA just affects the kidneys so an individual may only begin to feel unwell when the kidney function has deteriorated, allowing poisonous substances to build up in the blood stream. Kidney failure can be detected by blood tests that show the kidneys are no longer getting rid of urea and creatinine.
Other patients with MPA have feelings of lethargy, aches and pains in the joints or muscles, red eyes, skin rashes or may have lung involvement. Lung involvement in MPA may cause shortness of breath and blood may be coughed up. These features are similar to those experienced by patients with Wegener's.
Making a diagnosis
The ANCA blood test and a kidney biopsy test, when a small piece of kidney is taken to look at under a microscope, are very helpful tests for MPA.
Avoiding confusion with PAN
In the past doctors have often confused MPA with PAN. PAN is very rare but is seen occasionally in association with infection by Hepatitis B virus. PAN can cause high blood pressure, abdominal pains due to poor blood supply to the bowel, skin rashes and joint pains.
The ANCA test is negative in PAN. An X-ray of the blood vessels themselves (an angiogram) which is done by injecting a special dye, is helpful for PAN.
Treatment
In MPA, cortisone (prednisolone) and cyclophosphamide are usually used in the early stages. If the disease is severe, large doses of cortisone (Methylprednisolone) or plasma exchange may be given. When the disease becomes quiet, azathioprine is sometimes used instead of cyclophosphamide. A newer drug called mycophenolate mofetil is being used more often now, particularly if azathioprine does not suit someone. However, a number of newer treatments, often with fewer side-effects, are under evaluation, particularly in specialist centres. These include Rituximab and Infliximab.
Does the treatment have side effects?
Yes. Cortisone and cyclophosphamide reduce the inflammation present in MPA but they also reduce the body's ability to fight infection. The white blood cell count has to be carefully monitored.
Cortisone can also cause sugar diabetes, high blood pressure, thinning and bruising of the skin and weight gain. Nowadays most doctors try to reduce the dose of prednisolone as quickly as possible to prevent side effects.
Cyclophosphamide can cause inflammation of the bladder and infertility if used over long periods of time.
Can MPA relapse?
Yes. Unfortunately about half of all patients will relapse sometime in the months or years following initial illness. That is why most doctors continue to observe patients in the out-patients clinic so that early signs of active disease can be treated quickly.