(See also Overview of Vasculitis.)
Microscopic polyangiitis is inflammation of mainly small blood vessels throughout the body.
Microscopic polyangiitis is rare. It can occur at any age. The cause is unknown. People with this disorder usually have abnormal antibodies called antineutrophil cytoplasmic antibodies in their blood.
Most people have a fever, feel tired, and lose weight. Muscles and joints often ache.
Various organs may be affected:
Kidneys: The kidneys are affected in up to 90% of people. Blood, protein, and red blood cells appear in the urine, but often there is no sign of kidney malfunction until it is severe. Kidney failure may develop rapidly unless diagnosis and treatment are prompt.
Respiratory tract: If the lungs are affected, bleeding in the lungs may occur, causing people to cough up blood, feel short of breath, or both. The lungs may fill with fluid, and scar tissue may eventually develop. Fluid buildup and scar tissue cause difficulty breathing. Bleeding in the lungs, which may occur early in the disorder, requires immediate medical attention.
Skin: About one third of people have a rash of reddish purple spots and bumps, usually on the legs, feet, or buttocks. The nails may contain thin purplish lines, indicating bleeding (called splinter hemorrhages). Rarely, the blood supply to the fingers and toes is reduced.
Digestive tract: Abdominal pain, nausea, vomiting, and diarrhea may occur. Stools may contain blood.
Nerves: People may have tingling, numbness, or weakness in a limb.
Other organs are affected less often.
Doctors suspect the diagnosis based on symptoms. Blood and urine tests are done. These tests cannot specifically identify the disorder but can confirm that inflammation is present. Blood tests can also help doctors detect bleeding in the digestive tract. Blood is tested for abnormal antibodies, such as antineutrophil cytoplasmic antibodies, which attack certain white blood cells. Levels of erythrocyte sedimentation rate (ESR) and C-reactive protein, white blood cells, and platelets can be very high, indicating active inflammation. The level of red blood cells can be very low, indicating severe anemia due to bleeding in the lungs. A sample of urine is tested for red blood cells and protein. This information can help doctors determine whether the kidneys are affected.
Chest imaging is usually done to determine whether the lungs are affected. Computed tomography (CT) is much more likely than a chest x-ray to reveal small amounts of bleeding in the lungs. If there are signs of bleeding, a flexible viewing tube is inserted through the nose or mouth into the airways to directly view the lungs (bronchoscopy). This procedure can confirm the presence of bleeding (or infection, another possible cause of respiratory tract symptoms).
A biopsy of affected tissue (usually the skin, lungs, or kidneys) is done to confirm the diagnosis.
If symptoms are mild, a corticosteroid plus sometimes another drug that suppresses the immune system (immunosuppressant), such as azathioprine or methotrexate, are given. If vital organs are affected, cyclophosphamide, a stronger immunosuppressant, or rituximab and high doses of a corticosteroid are used. Sometimes plasma exchange (plasmapheresis) or methylprednisolone (given by vein [intravenously]) is used.