What is the management and treatment of Polymyositis (inflammatory muscle disease)?

Management and Treatment of Polymyositis

Initial Treatment Strategy

For adult patients with polymyositis, begin high-dose corticosteroids (prednisone 0.5-1 mg/kg/day, typically 60-80 mg daily) as a single morning dose, and concurrently initiate a steroid-sparing immunosuppressive agent such as methotrexate, azathioprine, or mycophenolate mofetil from day one. 1, 2

This dual-therapy approach from treatment onset is critical because:

• Starting both agents together allows for earlier corticosteroid tapering and reduces cumulative steroid exposure 2
• Steroid-sparing agents take 8-12 weeks to demonstrate therapeutic effect, making early initiation essential 3
• Monotherapy with corticosteroids alone is insufficient in most cases and increases risk of steroid-related complications 1, 3

Corticosteroid Tapering Protocol

After 2-4 weeks of high-dose therapy (depending on clinical response), implement the following structured taper 2:

• Weeks 2-8: Reduce by 10 mg every 2 weeks until reaching 30 mg/day 2
• 30-20 mg/day range: Reduce by 5 mg every 2 weeks 2
• Below 20 mg/day: Reduce by 2.5 mg every 2 weeks until discontinuation 2

The tapering schedule should be guided by muscle enzyme levels (particularly CK), muscle strength assessment, and inflammatory markers 1, 2. If relapse occurs during taper, return to the pre-relapse dose and maintain for 4-8 weeks before attempting gradual reduction again 1.

Steroid-Sparing Agent Selection

Choose among the following first-line agents 1, 2, 3:

• Methotrexate: 15-25 mg weekly (oral or subcutaneous), with therapeutic effect expected within 8 weeks 3
• Azathioprine: 2-3 mg/kg/day, divided into 1-2 doses 1, 4
• Mycophenolate mofetil: 2-3 g/day in divided doses 1, 4

All three agents have comparable efficacy based on available evidence, though methotrexate is most commonly used due to extensive clinical experience 3, 4.

Management of Severe or Refractory Disease

For patients with severe weakness, dysphagia, respiratory muscle involvement, or extensive extramuscular manifestations 1, 2:

• Add intravenous methylprednisolone pulse therapy: 10-20 mg/kg or 250-1000 mg daily for 3-5 consecutive days 2
• Consider intravenous immunoglobulin (IVIG): 2 g/kg divided over 2-5 days, repeated monthly if effective 3, 4
• For IVIG failures: Consider rituximab, cyclophosphamide, cyclosporine, or tacrolimus as third-line agents 3, 4

Critical warning: TNF-α antagonists are contraindicated in polymyositis as they can exacerbate muscle inflammation and increase infection risk 3.

Diagnostic Monitoring

Before initiating treatment, obtain baseline 1:

• Muscle enzymes (CK, aldolase, AST, ALT, LDH)
• Inflammatory markers (ESR, CRP)
• Myositis-specific autoantibodies (anti-Jo-1, anti-Mi-2, anti-SRP, anti-HMGCR)
• MRI of affected muscle groups using T1-weighted, T2-weighted, and STIR sequences 1, 2

During treatment, monitor muscle enzymes and clinical strength every 2-4 weeks initially, then monthly once stable 1, 2.

Special Populations and Considerations

Immune-Mediated Necrotizing Myopathy (IMNM)

This statin-triggered or paraneoplastic subtype requires similar treatment but often needs more aggressive immunosuppression due to severe weakness and minimal inflammatory infiltrate on biopsy 1, 2.

Checkpoint Inhibitor-Induced Myositis

For cancer patients on PD-1/PD-L1 or CTLA-4 inhibitors who develop myositis 1:

• Immediately hold checkpoint inhibitor therapy 1
• Administer high-dose corticosteroids (1-2 mg/kg/day prednisone equivalent) as bolus in severe cases 1
• Check for concomitant myocarditis (troponin, ECG, echocardiogram) as this combination has high mortality 1
• Consider plasmapheresis for corticosteroid-refractory cases or life-threatening presentations 1

Elderly Patients

Exercise particular caution with high-dose corticosteroids in patients over 65 years due to increased risk of steroid myopathy, which can paradoxically worsen weakness 5. Consider nutritional support with branched-chain amino acids if steroid myopathy develops 5.

Common Pitfalls to Avoid

• Delaying steroid-sparing agents: Starting immunosuppressants weeks after corticosteroids leads to prolonged steroid exposure and increased toxicity 2, 3
• Inadequate initial corticosteroid dosing: Doses below 0.5 mg/kg/day are associated with treatment failure 1, 6
• Too-rapid steroid taper: Tapering faster than recommended increases relapse risk; the average successful treatment duration is 27 months 6
• Missing malignancy screening: Adult polymyositis patients, especially those with dermatomyositis features, require age-appropriate cancer screening as paraneoplastic myositis occurs in 15-25% of cases 1, 2
• Misdiagnosis of inclusion body myositis: Patients over 50 with asymmetric weakness, finger flexor weakness, and quadriceps atrophy likely have IBM, which responds poorly to immunosuppression 7, 4

Adjunctive Therapies

• Physical therapy: Implement individualized exercise programs to maintain muscle mass and function, particularly important during prolonged corticosteroid therapy 1
• Bone protection: Prescribe calcium, vitamin D, and bisphosphonates for patients on long-term corticosteroids 1
• Dysphagia management: Evaluate swallowing function regularly; severe cases may require speech therapy or nutritional support 7