Myositis: A new classification system based on phenotypic, biological and immunological criteria

Affecting between 3,000 and 5,000 people in France, myositis (or inflammatory myopathies) is a group of rare autoimmune muscle diseases. Until now, three types of myositis (polymyositis, dermatomyositis, inclusion body myositis) had been identified according to a system based essentially on clinical and histological criteria. However, Professor Olivier Benveniste, head of the “Myopathies and Targeted Innovative Therapies” team at the Myology Centre for Research, Institute of Myology, who has been monitoring patients at the Pitié-Salpêtrière for 20 years, has identified important diagnostic errors related to this incomplete and non-homogeneous classification of these pathologies that sometimes lead to errors in the treatment given to patients.
 
With his team and in collaboration with clinicians, he launched a study on 260 patients from whom he collected and analysed all the clinical characteristics, and in particular, the presence of autoantibodies, sometimes the cause or consequence of the disease. The authors were able to establish a new classification comprising 4 major types of myositis:
 
Inclusion body myositis: This myositis affects more often men over 60 years of age. It is slowly progressive but ultimately induces a very disabling motor deficit. It particularly affects the quadriceps (thigh muscles used to climb stairs, get up from a chair, maintain stability when walking…), the muscles used to close and shake hands and muscles used for swallowing. This disease is resistant to classical immunosuppressive treatments such as corticosteroids. It is due to the presence of an inflammatory reaction (myositis) in the muscle and a neurodegenerative process related to Alzheimer’s disease (presence of inclusions).
 
Dermatomyositis: It affects women more often. Children can be affected. A risk of associated cancer appears in the oldest subjects (usually after 60 years). In addition to myositis, which causes predominant muscle weakness in the shoulders, this disease is characterised by the presence of typical dermatological lesions. Dermatomyositis is due to an imbalance of the immune system involving type 1 interferon that helps protect against viruses. New therapies specifically targeting this interferon pathway are under development. Dermatomyositis specific antibodies are anti-Mi2, anti-SAE, anti-NXP2, or anti-TIF1 gamma.
 
Immune-mediated necrotising myopathy: This is characterised by muscular weakness affecting patients of all ages. In the absence of treatment, this type of myositis leads to the most severe and disabling muscle atrophy. This disease is related to the presence of two specific anti-SRP or anti-HMGCR antibodies that directly attack and destroy the muscles. Anti-HMGCR may appear after taking statins. Treatment aims to remove these antibodies.
 
Anti-synthetase syndrome: This disease affects muscles but also joints (leading to rheumatism), and the lungs (sometimes leading to severe shortness of breath). Here too, certain antibodies appear to be responsible: anti-Jo1, anti-PL7 or anti-PL12.
 
These findings, published online in JAMA Neurology in September, pave the way for reliable diagnosis and personalised treatments.