What is MAS?

Macrophage activation syndrome (MAS) is an extremely rare but potentially life-threatening complication associated with systemic JIA and other rheumatological conditions.

In MAS, for reasons we don’t fully understand, there is over-activation and unregulated production of certain cells of the immune system that promote inflammation.

Two types of cells are affected:

  • Macrophges: These cells normally help the body to detect, engulf, and destroy harmful cells and substances. In MAS, these macrophages begin to consume blood-producing cells in the bone marrow.
  • T cells: In cases of MAS, T cells begin to produce excess inflammatory chemical messengers (cytokines). This causes uncontrolled inflammation that can affect many different parts of the body.


How common is it?

It is extremely rare. The vast majority of children with JIA will never develop MAS.


Who is affected by it?

MAS most commonly affects children with systemic JIA. Some children with polyarticular JIA may also be affected.

MAS is associated with a number of other rheumatic conditions, including:

  • systemic lupus erythematosus
  • juvenile dermatomyositis
  • systemic vasculitis
  • mixed connective tissue disease

What causes it?

We don’t fully understand what causes MAS or why some children with JIA develop MAS while others never do. Research continues into the factors (genetic mutation, infection, underlying inflammation) that may trigger MAS.



What problems can MAS cause?

MAS can create significant problems in several bodily organs and systems. Effects of MAS include:

  • Rash
  • Anaemia
  • Persistent high temperature
  • Low blood counts
  • Enlarged liver and abnormal liver function
  • Enlarged lymph nodes
  • Kidney problems
  • Headache, confusion, irritability, seizure, coma

In many children, MAS responds very well to treatment. In a minority of cases, however, the condition can—despite treatment—prove fatal.



How is MAS diagnosed?

There is no single diagnostic test for MAS, and it can be very difficult to distinguish signs of MAS from symptoms of other, more common conditions. To identify MAS, doctors look for abnormal patterns in the results of blood tests.

It is important to recognise cases of MAS as early as possible. If you have a child with a condition that is known to put them at risk of MAS (e.g., some children with systemic JIA) and they seem less well than you would expect with a ‘normal’ virus, you should contact your local rheumatology team for advice. An at-risk child who seems particularly unwell should be taken to a local A+E department, where staff can contact the rheumatology team for advice.

What treatments are available?

Patients who develop MAS require admission to hospital, where medical staff will attempt both to address the causes of the uncontrolled inflammation and to treat any complications of the condition. Patients with MAS may receive:

  • A course of high-dose steroids, administered intravenously;
  • Further medications (e.g. ciclosporin, anakinra, immunoglobulin, etoposide);
  • Antibiotics, blood and platelet transfusions, and treatment to support the kidneys.

Your rheumatology team would be happy to discuss any worries or anxieties that you may have about MAS at any time. Please remember this is an extremely rare complication of JIA.