Microscopic polyangiitis (MPA): symptoms, diagnosis and treatment

Vasculitis is the name for a group of conditions in which the immune system attacks healthy blood vessels, causing them to become swollen and narrow. The NHS explains that it can range from a minor skin problem to a serious illness affecting organs such as the heart or kidneys.

Microscopic polyangiitis belongs to the ANCA-associated vasculitides, a group that Vasculitis UK says also includes granulomatosis with polyangiitis (GPA) and eosinophilic granulomatosis with polyangiitis (EGPA). ANCA stands for antineutrophil cytoplasmic antibody. In MPA the inflammation affects small blood vessels, and unlike GPA there is no granulomatous inflammation, meaning no clusters of cells that build nodules and cavities.

In MPA the antibodies are often directed against an enzyme called myeloperoxidase (MPO). Vasculitis UK explains that these antibodies activate white blood cells called neutrophils, which then attach to the small vessel walls and damage them.

The symptoms depend on which organs are involved and vary markedly between people. Vasculitis UK says that common general symptoms are tiredness, loss of appetite, and joint and muscle aches. The NHS list of possible symptoms is wider and includes the following.

• A rash, and red, sore eyes.

• Shortness of breath and coughing up blood.

• Pins and needles or numbness, and joint stiffness or muscle aches.

• Loss of appetite and losing weight without trying.

• Feeling tired, or flu-like symptoms such as a high temperature and an aching body.

• Kidney problems, which may not cause obvious symptoms in the early stages.

MPA can damage the kidneys and lungs quietly. Vasculitis UK notes that it can slowly and undetected scar kidney and lung tissue for a while. This is why a combination of unexplained tiredness, weight loss, a rash, new breathlessness or blood in the urine should be taken to a GP promptly rather than put down to a virus.

Coughing up blood, severe breathlessness, passing very little urine, or sudden weakness or numbness are urgent. Call 111 for advice, or 999 or go to A&E if symptoms are severe or rapidly worsening. People already diagnosed should follow the contact arrangements given by their vasculitis or renal team.

The cause of MPA is not yet known. Vasculitis UK says genetic studies have shown that some people carry gene subtypes that raise the risk of autoimmune disease in general, but no specific gene fault has been found to cause ANCA vasculitis. The charity says further triggers such as infections, toxins or trauma are thought to push at-risk people to lose tolerance to particles in their own bodies and develop ANCA.

The NHS says vasculitis may be triggered by an infection, another underlying condition or a medicine, although often the cause is unknown. MPA is not infectious and it is not caused by anything a person did.

Vasculitis UK adds that among the ANCA vasculitides, MPA is more often found in southern Europe and Asia, whereas GPA is more common in northern Europe.

Vasculitis UK says that it is important to recognise the pattern of symptoms. After clinical suspicion and a positive MPO ANCA blood test, the aim is to confirm the diagnosis with a tissue biopsy. When the kidneys are affected a kidney biopsy is often performed. Lung involvement is more likely to be assessed with high-resolution CT imaging, and a bronchoscopy with washing or biopsy may help to check that it is vasculitis and not an infection or cancer.

Alongside the ANCA test, doctors usually check blood tests of kidney function, inflammation markers and a urine sample for blood and protein. Healthcare professionals discuss the history and results in a multidisciplinary team meeting to decide how much testing is needed to be sure of the diagnosis before starting treatment.

Vasculitis UK describes treatment as having changed in recent years. Steroids (prednisolone) and cyclophosphamide used to be the main standard of therapy. Rituximab has been shown to be as effective as cyclophosphamide in most cases and may be used instead of, or alongside, it. The charity also describes avacopan, a complement-based treatment licensed for severe cases that may allow steroids to be reduced or replaced.

In overwhelming inflammation, large doses of steroids and plasma exchange may be added for a short while to speed up control of active vasculitis. Once the disease is under control a person stays on less intense immune-suppressing medication to stop vasculitis coming back. The charity names rituximab and azathioprine as the standard maintenance drugs, with methotrexate and mycophenolate mofetil sometimes used. The physician will try to withdraw steroids and later the maintenance treatment as well.

These drugs lower resistance to infection, so people are monitored with regular blood tests and are usually advised on vaccinations and on contacting the team early if they develop a fever. Side effects differ between drugs, and Vasculitis UK has separate pages on each.

Vasculitis UK says the prognosis depends on disease severity and the organ damage caused by the vasculitis. It says relapses are less likely than with some other ANCA vasculitides but that some people will relapse and others will not, which is why teams check whether a person really needs prolonged treatment to avoid overtreatment.

Kidney damage may leave a person with chronic kidney disease, and a small number need dialysis. Follow-up therefore includes regular kidney tests, blood pressure checks and attention to bone health when steroids have been used. Fatigue and low mood are common and worth raising with the team.

MPA is usually managed by NHS rheumatology, nephrology or respiratory specialists working together, often in a dedicated vasculitis clinic. Because drugs such as rituximab and avacopan are given under specialist supervision with regular monitoring, most people receive both diagnosis and treatment through NHS services. Private consultations can provide a second opinion, but the monitoring schedule and drug supply are normally arranged through the treating NHS team.