Classification criteria for idiopathic inflammatory myopathy

Dr Yves Allenbach is an associate professor in the Department of Internal Medicine and Clinical Immunology and Myology Research Center at Pitié-Salpetriere University Hospital (France).

What has changed in the classification criteria of idiopathic inflammatory myopathies? What subgroups of patients can be distinguished and what are the differences between them?

Yves Allenbach, MD, PhD: A lot of things have finally changed. Forty years ago we saw that basically there were 2 big groups of myositis: patients with skin rash—dermatomyositis (DM) patients; and patients without skin rash—polymyositis (PM) patients. This definition was made by Peter and Bohan 40 years ago. Recently the American College of Rheumatology (ACR) and European League Against Rheumatism (EULAR) group organized a meeting and reached a consensus to try to define how to diagnose myopathy.

One of the biggest changes is the selection of a restricted numbers of items, which allow you to establish a score and tell if the diagnosis of myositis is probable, definite, or if it can be excluded. The novelty in these items is that it is possible to reach enough points to make a diagnosis without using muscle biopsy. So things have changed because now it is possible to make the diagnosis of myopathy and myositis without making a muscle biopsy.

The other novelty is that myositis-specific antibodies play a huge role in diagnosing myopathy. The new ACR/EULAR criteria include only one myositis-specific antibody, anti-Jo-1, but we know that probably other myositis-specific antibodies could be included to make a diagnosis.

To answer the second part of the question—you asked me about the classification—the ACR and EULAR also make an attempt to classify patients once the diagnosis of myositis is made. They say that there are patients with DM, sporadic inclusion body myositis (sIBM), and PM, but PM probably encompasses 2 diseases: antisynthetase syndrome and immune-mediated necrotizing myopathy (IMNM). We know now that specific antibodies are really helpful in defining both of these entities. For example, IMNM is now defined based on the presence of either the anti–signal recognition particle (SRP) antibody or the anti–3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMGCR) antibody. So we know that myositis-specific antibodies are now crucial to not only diagnose myositis but also to classify it.

We believe that there are 4 main groups of myositis. There is a group of myositis involving only the muscles and not the skin and lungs; these are the sIBM patients. There is another group involving only the muscles but with a very acute and severe onset; this is the group of IMNM patients. And then there are the groups with muscle involvement plus something else: Muscle plus skin involvement are DM patients, the third group, and the last group are patients with muscle, skin, and also joint and lung involvement, which is antisynthetase syndrome. So we think that there are probably 4 main classification categories.