Myositis or dystrophy? Traps and pitfalls

Some hereditary myopathies can mimic acquired myositis especially when they first present in adulthood with a limb-girdle distribution of weakness. Although inherited myopathies are generally painless and progress very slowly there are exceptions, which can further add to diagnostic confusion. The diagnosis is made even more difficult when inflammatory infiltrates are present on muscle biopsy. This is common in certain dystrophies in particular e.g. dysferlinopathies or facioscapulohumeral dystrophy. On the other hand, acquired (and treatable with immunosuppressants) necrotizing myopathies with anti-SRP antibodies can be very slowly progressive, with clinical and pathological features compatible with limb girdle dystrophies. These two situations can lead to either inappropriate immunosuppressant treatment in a patient with dystrophy, having mistaken it for an acquired inflammatory myopathy, or to therapeutic abstention in a patient with a treatable acquired myopathy thinking that it was a dystrophy. Pointers helping to distinguish between these two traps are here reviewed.