Idiopathic inflammatory myopathies and genetic muscular dystrophies are distinct and well-recognised clinical entities. However, it is increasingly evident that there is overlap in both the clinical presentation¹ and the pathophysiology² of the two groups.

The preferred management of polymyositis and dermatomyositis has traditionally been with corticosteroids and, although controlled clinical trial data to support this are lacking, the efficacy of these drugs has long been established in clinical practice. However, not all patients respond to steroids.

Non-response rates of up to 40% have been reported, but it has become increasingly clear in recent years that polymyositis in particular has been overdiagnosed, and many of these patients actually had inclusion-body myositis (IBM), for which there is no strong evidence of a response to steroids, or a form of muscular dystrophy. The classical Bohan and Peter criteria for the diagnosis of polymyositis have been modified to include other criteria, such as the presence of CD8+ T cell infiltration along with expression of major histocompatibility class I molecules …