[Clinicopathological Features of Myositis and Necrotizing Myopathy: How to Distinguish between Myositis and Muscular Dystrophy on Muscle Pathology].

In the field of neurology, idiopathic inflammatory myopathy has been classified into four sub-categories, namely, dermatomyositis, anti-synthetase syndrome, inclusion body myositis, and immune-mediated necrotizing myopathy (IMNM), based upon histological and serological findings. Among them, IMNM has features similar to muscular dystrophy, and it may thus be difficult to differentiate between these two conditions, not only clinically but also pathologically, especially in chronic cases and pediatric patients. This is partly because the main pathological feature of both IMNM and muscular dystrophy is myofiber necrosis and regeneration. Furthermore, IMNM patients with anti-SRP antibodies tend to have more prominent muscle atrophy, especially in the shoulder girdle, which mimics the muscle involvement pattern in facioscapulohumeral muscular dystrophy. Some IMNM patients with anti-HMGCR antibodies have onset in their childhood or even in infancy, and may be misdiagnosed with muscular dystrophy. On the other hand, some muscular dystrophies have been reported to show more prominent lymphocyte infiltration than others, which may also mislead muscle pathologists. Nevertheless, these conditions can be distinguished using the relevant histological evaluations, including muscle immunohistochemistry for the MHC and C5b-9 antigens, in addition to appropriate clinical and laboratory examinations including muscle MRI and autoantibody testing.