Polymyositis: Clinical Features, Diagnosis and Associations

Clinical Characteristics of Polymyositis

Myopathic Weakness

Myopathic weakness evolves over weeks to months, typically sparing the facial and eye muscles. It presents as difficulty in:

• climbing steps
• rising from a chair
• lifting objects
• combing hair

Disease Onset

Typically occurs above the age of 18 years.

Features the Patient Does Not Have

The following features are not characteristic of polymyositis in the typical presentation:

• Rash (characteristic of dermatomyositis)
• Family history of neuromuscular diseases
• Exposure to myotoxic drugs, especially penicillamine²⁶, zidovudine²⁷, and (rarely) statins^28,29
• Endocrine disease (hypothyroidism, hyperthyroidism, hypoparathyroidism, hypercortisolism)

Excluded Conditions

• Neurogenic disease (excluded by electromyography and neurological examination)
• Dystrophies and metabolic myopathies (excluded by history and muscle biopsy)
• Inclusion-body myositis (excluded by clinical examination and muscle biopsy)

Possible Associations

Polymyositis can be associated with other autoimmune or viral conditions, including:

• Systemic lupus erythematosus
• Rheumatoid arthritis
• Sjögren's syndrome
• Crohn's disease
• Vasculitis
• Sarcoidosis
• Primary biliary cirrhosis
• Adult celiac disease
• Chronic graft-versus-host disease
• Discoid lupus
• Ankylosing spondylitis
• Behçet's syndrome
• Myasthenia gravis
• Acne fulminans
• Dermatitis herpetiformis
• Psoriasis
• Hashimoto's disease
• Granulomatous diseases
• Agammaglobulinaemia
• Hypereosinophilic syndrome
• Lyme disease
• Kawasaki disease
• Autoimmune thrombocytopenia
• Hypergammaglobulinaemic purpura
• Hereditary complement deficiency
• HIV and HTLV-1 infection

When to Reconsider Polymyositis

If the diagnosis was based on Bohan and Peter's criteria, reconsider polymyositis in patients with any of the following features:

• Disease onset before the age of 18 years
• Slow-onset myopathy that evolved over months to years (in such cases consider inclusion-body myositis or dystrophy)
• Fatigue and myalgia, and muscle weakness without clear objective signs, even if a transient rise in creatine kinase activity is seen (such patients may have fibromyalgia or fasciitis; their muscle biopsy sample may be normal or show very few endomysial inflammatory cells in the septae)
• No typical histological features of polymyositis