Table 1 Comparison of idiopathic inflammatory myopathies and muscular dystrophies

Clinical characteristics

Systemic symptoms and arthritis are common; rash. Systemic symptoms and arthritis are common. Onset often acute or subacute.

Selective patterns of muscle hypertrophy or atrophy, with longstanding insidious onset of motor functional difficulties

Pattern of weakness

Proximal, but neck flexor and abdominal weakness is a common early feature

Usually limb girdle or rarely axial or distal weakness depending on muscular dystrophy subtype

Serum CK levels

Typically moderately raised CK; can be normal or very high

Extremely high CK, often above 5,000 units

Typical muscle MRI features

Proximal muscle inflammation with high signal on T2 weighted STIR images [10]

Typically specific patterns of muscle involvement with abnormal signal on T1 weighted images in different muscular dystrophies

Typical muscle biopsy features

MHC – class 1 upregulation, inflammatory infiltrate (frequently peri-vascular), peri-fascicular muscle fibre atrophy and endothelial cell abnormalities [11]

Dystrophic features (fibre necrosis, regeneration, fibrosis and fat infiltration) with absence or reduction of specific protein/enzyme, depending on specific muscular dystrophy subtype.

Genetics

Polygenic trait

Autosomal dominant, recessive or X-linked inheritance