Introduction
Polymyositis is an inflammatory disorder causing symmetrical, proximal muscle weakness.
Epidemiology
- Incidence: 1.00 cases per 100,000 person-years
- Peak incidence: 50-60 years
- Sex ratio: more common in females 2:1
| <1 | 1-5 | 6+ | 16+ | 30+ | 40+ | 50+ | 60+ | 70+ | 80+ |
Pathophysiology
- thought to be a T-cell mediated cytotoxic process directed against muscle fibres
- may be idiopathic or associated with connective tissue disorders
- associated with malignancy
- dermatomyositis is a variant of the disease where skin manifestations are prominent, for example a purple (heliotrope) rash on the cheeks and eyelids
- typically affects middle-aged, female:male 3:1
Clinical features
Features
- proximal muscle weakness +/- tenderness
- Raynaud's
- respiratory muscle weakness
- interstitial lung disease: e.g. fibrosing alveolitis or organising pneumonia
- dysphagia, dysphonia
Investigations
Investigations
- elevated creatine kinase
- other muscle enzymes (lactate dehydrogenase (LD), aldolase, AST and ALT) are also elevated in 85-95% of patients
- EMG
- muscle biopsy
- anti-Jo-1 antibodies are seen in pattern of disease associated with lung involvement, Raynaud's and fever