Myositis

Overview – Myositis

Myositis refers to inflammation of skeletal muscle, most often due to autoimmune processes. It encompasses a group of idiopathic inflammatory myopathies including polymyositis, dermatomyositis, and inclusion body myositis. These conditions are characterised by muscle weakness and, in some cases, cutaneous or systemic involvement. Recognition of the distinct clinical patterns and histological findings is essential for diagnosis and treatment, particularly in final-year exams and OSCE stations.

Definition

Myositis is a chronic inflammatory condition affecting skeletal muscle, leading to muscle weakness. It may occur as a primary autoimmune disease or secondary to other systemic disorders.

Aetiology

• Autoimmune inflammation and immune-mediated skeletal muscle injury
• Can be triggered or associated with:
  • Viral infections (e.g. HTLV-1, HIV)
  • Autoimmune diseases (e.g. SLE, systemic sclerosis)
  • Malignancies (especially with dermatomyositis)

Risk Factors

• Increasing age
• Female sex (Polymyositis and Dermatomyositis)
• Male sex (Inclusion Body Myositis)
• Chronic viral infections
• Co-existing autoimmune disease
• Underlying malignancy (especially in dermatomyositis)

Pathophysiology

• Dermatomyositis: Humoral (antibody-mediated) immune response → Perivascular inflammation
• Polymyositis: T-cell mediated cytotoxic damage to myocytes (cellular immunity)
• Inclusion Body Myositis (IBM): Cellular immune response plus degenerative features (e.g. cytoplasmic inclusion bodies)

Clinical Features

• Proximal muscle weakness (classic finding in all types except IBM)
• Distal muscle weakness (predominantly seen in Inclusion Body Myositis)
• Skin manifestations in Dermatomyositis:
  • Heliotrope rash (eyelids)
  • Gottron’s papules (over knuckles)
• Systemic involvement:
  • Dysphagia (oesophageal muscle involvement)
  • Pulmonary fibrosis
  • Cardiomyopathy

Investigations

• Serum Creatine Kinase (CK): Elevated in polymyositis and dermatomyositis; normal or mildly elevated in IBM
• Electromyography (EMG): Myopathic changes
• Muscle biopsy:
  • Dermatomyositis: Perimysial & perivascular inflammation
  • Polymyositis: Endomysial inflammation with T-cell infiltration
  • IBM: Endomysial inflammation + intracytoplasmic inclusion bodies
• Autoantibodies (e.g. anti-Mi-2 in dermatomyositis, anti-SRP in polymyositis)
• MRI of muscles may assist in detecting oedema or inflammation

Management

• Corticosteroids (e.g. prednisolone): First-line for polymyositis and dermatomyositis
• Immunosuppressants:
  • Methotrexate
  • Azathioprine
  • IVIG (refractory cases or malignancy-associated)
• Inclusion Body Myositis is often treatment-resistant
• Supportive care:
  • Physiotherapy
  • Swallowing assessment (if dysphagia)
  • Malignancy screening (especially in dermatomyositis)

Comparison Table

Complications

• Dysphagia and aspiration pneumonia
• Interstitial lung disease (especially dermatomyositis)
• Cardiac complications (arrhythmias, cardiomyopathy)
• Increased risk of malignancy (notably in dermatomyositis)

Summary – Myositis

Myositis encompasses a group of immune-mediated myopathies presenting with progressive muscle weakness, with or without systemic features. Dermatomyositis, polymyositis, and inclusion body myositis differ in pathophysiology, histology, and treatment response. Timely diagnosis and immunosuppressive therapy can improve outcomes. For a broader context, see our Musculoskeletal Overview page.