Definitions

– Polymyositis = muscle weakness without cutaneous signs
– Dermatomyositis = muscle weakness and cutaneous signs
– Amyopathic DM = cutaneous signs without muscle weakness

Epidemiology

• F:M = 2:1
• Peak incidence is age 40-50 but any age affected

Clinical Manifestations

• Muscle weakness in 90% of PM patients and 50-60% in DM – in DM, rashes usually come before or at the same time
• Symmetric and proximal muscle weakness
• If +distal muscle weakness, usually mild
• Insidious onset usually for months before they go to doctors
• Skin findings
  • Usually on the joints involved whereas lupus shows cutaneous things between the joints
  • Gottron’s papules and heliotrope rash
  • Facial erythema – includes nasolabial folds
  • Shawl and v-signs
  • Holster sign – lateral aspect of the thigh
  • Inflammation of the nail fold
• Lung disease – 10% of cases of DM and PM
• Malignancy – generally parallels that of the general population distribution
• Esophageal disease – weakness of the muscles à dysphagia/regurgitation
• Cardiac disease – conduction abnormalities and arrhythmias, increased risk of MI

Labs

• Elevated muscle enzymes (CK, LDH, AST, ALT, aldolase)
• ANA + in 80% of patients and myositis ab in 30-40% of patients (Jo, Mi2, SRP)
• Elevated serum and urine myoglobin
• ESR is normal or midly elevated

EMG

• Normal in 10% of the patients