Inflammatory myopathy

Inflammatory myopathy, also called myositis, is a subtype of myopathy where there is inflammation of muscle.

Etiology

• Polymyositis (PM)
• Dermatomyositis (DM)
• Necrotising myositis
• Inclusion body myositis (IBM)
• Overlap myositis
• Paraneoplastic myositis

Dermatomyositis and necrotising myositis can either be primary (idiopathic) or secondary to adenocarcinoma.

Clinical features

In most types of myositis there is proximal weakness, atrophy, and muscle pain. The only exception is inclusion body myositis, in which there is distal muscle weakness. IBM affects elderly.

In dermatomyositis there are also skin symptoms, namely heliotrope rash (erythematous/purple rash of periorbital area) and Gottron papules (erythematous papules on knuckles) and shawl erythema (erythema of neck and upper trunk). Polymyositis is like dermatomyositis but without skin symptoms. Necrotising myositis is similar to them but worse.

Diagnosis and evaluation

The creatine kinase (CK) is elevated in most cases. MRI can show the distribution and extent of inflammation and can guide the location for biopsy.

Muscle biopsy is usually required for diagnosis. On muscle biopsy, inflammation, and MHC-I expression (which is usually not expressed on muscle) can be seen. In necrotising myositis, necrosis can be seen, and in IBM, vacuoles are present.

EMG can differentiate between neurogenic and myopathic problems.

In dermatomyositis and necrotising myositis it’s important to rule out adenocarcinoma.

Treatment

The treatment is usually immunosuppressants, usually steroids + azathioprine or methotrexate. Immunosuppressants is not effective for IBM, however.