Overview
Polymyositis
1. Clinical Overview
Summary
Polymyositis is an idiopathic inflammatory myopathy characterised by symmetric proximal muscle weakness. Unlike dermatomyositis, there are no skin manifestations. It has a strong association with malignancy and interstitial lung disease, making screening essential.
Key Facts
| Aspect | Detail |
|---|---|
| Classification | Idiopathic inflammatory myopathy |
| Key Feature | Proximal weakness WITHOUT skin rash |
| CK Levels | Markedly elevated (10-50x normal) |
| Key Antibody | Anti-Jo-1 (anti-synthetase syndrome) |
| Malignancy Risk | Screen all patients |
Clinical Pearls
- Proximal > Distal: Difficulty rising from chair, climbing stairs, reaching overhead
- Painless weakness: Unlike other myopathies which are often painful
- No rash = Polymyositis: Dermatomyositis has heliotrope rash, Gottron papules
- Anti-Jo-1 = Lungs: Associated with interstitial lung disease
2. Epidemiology
Demographics
| Factor | Detail |
|---|---|
| Peak age | 40-60 years |
| Gender | Female > Male (2:1) |
| Prevalence | 5-10 per 100,000 |
Associations
| Association | Notes |
|---|---|
| Malignancy | 10-25% (ovarian, lung, GI) |
| ILD | Especially with anti-Jo-1 |
| Other autoimmune | Overlap syndromes |
3. Pathophysiology
Mechanism
T-cell Mediated Attack (CD8+)
↓
Direct Muscle Fibre Cytotoxicity
↓
Endomysial Inflammation
↓
Muscle Fibre Necrosis + Regeneration
↓
PROXIMAL MUSCLE WEAKNESS
Antibody Associations
| Antibody | Association |
|---|---|
| Anti-Jo-1 | Anti-synthetase syndrome (ILD, arthritis, mechanic's hands) |
| Anti-SRP | Severe, treatment-resistant disease |
| Anti-Mi-2 | Dermatomyositis, good prognosis |
4. Clinical Presentation
Classic Features
| Feature | Description |
|---|---|
| Proximal weakness | Shoulders, hips, neck flexors |
| Insidious onset | Weeks to months |
| Difficulty | Rising from chair, climbing stairs, lifting arms above head |
| Muscle bulk | Initially preserved |
| Pain | Minimal or absent |
Systemic Features
| System | Manifestations |
|---|---|
| Respiratory | ILD, respiratory muscle weakness |
| GI | Dysphagia (pharyngeal muscles) |
| Cardiac | Cardiomyopathy, arrhythmias (rare) |
| Joints | Arthralgia (especially with anti-Jo-1) |
5. Clinical Examination
Motor Examination
| Finding | Details |
|---|---|
| Neck flexor weakness | Ask to lift head off bed |
| Shoulder girdle | Test deltoids, cannot abduct |
| Hip girdle | Cannot rise from squat |
| Gower's sign | May be positive |
| Reflexes | Usually normal |
What to Look For
| Examination | Purpose |
|---|---|
| Skin | Exclude dermatomyositis (no rash in pure PM) |
| Chest | Respiratory muscle function, ILD signs |
| Swallowing | Ask about choking, nasal regurgitation |
| Hands | Mechanic's hands (anti-synthetase) |
6. Investigations
Blood Tests
| Test | Finding |
|---|---|
| CK | Markedly elevated (10-50x normal) |
| LDH, AST, ALT | Elevated (from muscle) |
| ESR/CRP | May be elevated |
| ANA | Positive in ~50% |
| Myositis-specific antibodies | Anti-Jo-1, anti-SRP, anti-Mi-2 |
Additional Tests
| Test | Purpose |
|---|---|
| EMG | Myopathic pattern (fibrillations, small motor units) |
| MRI muscles | Oedema in affected muscles |
| Muscle biopsy | CD8+ T-cell infiltration, endomysial inflammation |
| HRCT chest | ILD screening |
| Malignancy screen | CT CAP, age-appropriate cancer screening |
7. Management
First-Line Treatment
| Treatment | Details |
|---|---|
| High-dose corticosteroids | Prednisolone 1mg/kg/day (max 60mg) |
| Duration | Maintain for 4-6 weeks, then slow taper |
Second-Line / Steroid-Sparing
| Drug | Notes |
|---|---|
| Methotrexate | Common first choice |
| Azathioprine | Alternative |
| Mycophenolate | For ILD-associated disease |
| Rituximab | Refractory disease |
| IVIG | Severe/refractory cases |
Adjunctive
| Intervention | Purpose |
|---|---|
| Physiotherapy | Maintain strength, prevent contractures |
| Bone protection | If long-term steroids |
| PCP prophylaxis | If immunosuppressed |
Malignancy Screening
- CT CAP
- Age-appropriate screening (mammogram, colonoscopy)
- Repeat in 3 years if initial negative
8. Complications
| Complication | Notes |
|---|---|
| Respiratory failure | ILD or respiratory muscle weakness |
| Aspiration | Dysphagia |
| Malignancy | Screen essential |
| Steroid side effects | Osteoporosis, diabetes, infections |
| Falls | Due to weakness |
9. Prognosis & Outcomes
| Factor | Outcome |
|---|---|
| Treatment response | 70-80% improve with corticosteroids |
| Anti-Jo-1 positive | Associated ILD, moderate prognosis |
| Anti-SRP positive | Severe, often resistant |
| Malignancy | Prognosis tied to cancer outcome |
| 5-year survival | ~80% |
10. Evidence & Guidelines
| Organisation | Key Points |
|---|---|
| BSR | Myositis guidelines |
| ACR/EULAR | Classification criteria |
| Myositis UK | Patient information |
11. Patient / Layperson Explanation
What is polymyositis? It's a condition where your immune system mistakenly attacks your muscles, causing weakness. The muscles closest to the centre of your body (shoulders, hips, neck) are most affected.
What are the symptoms?
- Difficulty getting up from a chair
- Trouble climbing stairs
- Struggling to lift your arms above your head
- Sometimes difficulty swallowing
How is it treated?
- Steroids: The main treatment to reduce inflammation
- Other immune-suppressing drugs: To control the condition long-term
- Physiotherapy: To maintain strength
Why do I need cancer screening? There's a small link between polymyositis and some cancers. We routinely screen to make sure there's nothing else going on.
What's the outlook? Most people respond well to treatment and can have a good quality of life, though you'll need ongoing monitoring and treatment.
12. References
- BSR Myositis Guidelines. 2022.
- Lundberg IE, et al. EULAR/ACR Classification Criteria for Myositis. Ann Rheum Dis. 2017.
- Dalakas MC. Inflammatory Muscle Diseases. NEJM. 2015.