Classification of myositides

1. Big picture

Myositides are inflammatory diseases of skeletal muscle. They belong to the broader group of skeletal muscle diseases, together with muscular dystrophies, non-inflammatory myopathies and ion-channel disorders.

The key clinical pattern is:

Subacute or chronic proximal muscle weakness, often with muscle pain, raised CK and inflammatory markers, without sensory loss.

In neurology exams, myositis must be distinguished from:

• myasthenia gravis;
• muscular dystrophy;
• motor neuron disease;
• polyneuropathy;
• radiculopathy;
• endocrine/toxic/drug-induced myopathy.

The most important myositis types are:

1. Polymyositis
2. Dermatomyositis
3. Inclusion body myositis
4. Immune-mediated necrotizing myopathy
5. Overlap/connective-tissue disease-associated myositis
6. Antisynthetase syndrome
7. Juvenile myositis
8. Infectious myositis
9. Paraneoplastic myositis

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2. Definition

Myositis means inflammation of skeletal muscle causing muscle fiber injury, weakness, pain or tenderness, and usually increased muscle enzymes.

Typical laboratory signs:

• increased creatine kinase;
• increased lactate dehydrogenase;
• increased AST/GOT and ALT/GPT;
• inflammatory markers may be increased;
• autoantibodies may be present.

CK = creatine kinase. LDH = lactate dehydrogenase. AST/GOT and ALT/GPT are transaminases that can rise due to muscle damage, not only liver disease.

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3. Classification of myositides