Polymyositis.Care & specialists in Ireland

Polymyositis is a rare autoimmune disease in which the immune system attacks muscle fibers and causes symmetrical weakness in the hips, shoulders, and neck. It develops over weeks to months and is treated with steroids combined with steroid-sparing immunosuppressants.

The earliest sign is symmetrical proximal muscle weakness, usually difficulty rising from a chair, climbing stairs, or lifting objects overhead. Creatine kinase levels are often raised on routine blood tests. Skin rash, joint pain, and Raynaud's phenomenon may precede weakness in antisynthetase syndrome.

Both cause symmetrical proximal weakness with raised creatine kinase. Dermatomyositis additionally produces characteristic skin changes including a heliotrope rash, Gottron's papules, and the shawl sign, and is more strongly linked to cancer. Polymyositis has no skin involvement.

Diagnosis combines muscle enzyme testing (raised creatine kinase), electromyography, MRI of muscles, muscle biopsy showing CD8+ T-cell infiltration, and myositis-specific autoantibody panels. Cancer screening is also performed because of the association between inflammatory myopathy and malignancy.

First-line treatment is oral prednisone 1 mg/kg per day combined with a steroid-sparing immunosuppressant such as methotrexate or azathioprine. Mycophenolate mofetil, IVIG, and rituximab are added for refractory disease or lung involvement. Physiotherapy is started within 2 weeks of starting steroids.

Polymyositis is rarely cured outright but is controllable in most patients. About 20% achieve sustained drug-free remission, 50-60% require long-term maintenance immunosuppression with intermittent flares, and 15-20% have refractory disease needing advanced therapies such as rituximab or IVIG.

With modern treatment, 5-year survival is 75-90% and 10-year survival is 70-80% in specialist cohorts. The main causes of reduced life expectancy are interstitial lung disease, associated cancer, infections from immunosuppression, and cardiac involvement.

Mild muscle aching occurs in 25-50% of patients, but pain is usually less prominent than weakness. Severe pain raises the suspicion of necrotizing myopathy, rhabdomyolysis, or an alternative diagnosis such as polymyalgia rheumatica or fibromyalgia.

Polymyositis is not inherited in a Mendelian pattern but susceptibility is influenced by HLA-DRB1*03:01 and other immune-related genes. First-degree relatives have a small absolute increase in risk for autoimmune disease in general but rarely develop polymyositis themselves.

Yes. Interstitial lung disease develops in 20-40% of patients over the disease course, particularly in those with anti-Jo-1 or other antisynthetase antibodies. It is the leading cause of mortality. Screening with high-resolution chest CT and pulmonary function tests is performed at diagnosis.

Dysphagia affects 30-50% of patients because the upper-pharyngeal muscles are striated and susceptible. Patients describe food sticking in the throat, nasal regurgitation, and aspiration during meals. Speech and swallowing therapy and modified diets are part of standard management.

Polymyositis carries a 1.4-2 fold increased risk of malignancy within 3 years of diagnosis. Dermatomyositis carries a higher risk (3-5 fold). Age-appropriate cancer screening is performed at diagnosis and during the first 3 years; certain autoantibodies such as anti-TIF1-gamma identify higher-risk patients.

Antisynthetase syndrome is defined by autoantibodies against aminoacyl-tRNA synthetases (anti-Jo-1 most common) and features inflammatory myopathy, interstitial lung disease, non-erosive arthritis, mechanic's hands, Raynaud's phenomenon, and fever. Lung involvement determines prognosis.

Statins can cause anti-HMG-CoA reductase necrotizing myopathy, which resembles polymyositis with raised creatine kinase and weakness but shows necrosis rather than inflammation on biopsy. Symptoms persist after stopping the statin and usually require steroids plus IVIG.

Supervised aerobic and resistance exercise improves strength and aerobic capacity without raising creatine kinase or worsening inflammation. Trials show 15-25% gains in maximal voluntary contraction over 6 months. Exercise is started within 2 weeks of beginning immunosuppression.

Creatine kinase, aldolase, AST, ALT, and LDH track muscle enzyme activity. Improvement typically appears within 2-6 weeks of starting prednisone. Inflammatory markers (CRP, ESR) and KL-6 follow lung involvement. Antibody titres are not reliable activity markers.

Yes. Relapse occurs in 30-40% of patients annually, usually during steroid taper or after dose reduction of the steroid-sparing agent. Relapses typically respond to brief steroid pulses and increased immunosuppression. Long-term remission is more likely with combination therapy from the outset.

True polymyositis is rare in children. Most pediatric inflammatory myopathies are juvenile dermatomyositis, which has distinctive skin features and a different treatment approach. Reclassification of historical pediatric polymyositis as juvenile dermatomyositis or other myopathies is common.

Prednisone is started at 1 mg/kg daily and maintained for 4-8 weeks, then tapered over 9-12 months. Many patients remain on a low maintenance dose (5-10 mg daily) for 1-2 years. Steroid-sparing immunosuppressants are added to limit cumulative exposure and side effects.

Yes. The 2022 ProDERM trial showed Total Improvement Score response in 79% of patients treated with intravenous immunoglobulin versus 44% with placebo at 16 weeks. IVIG is now FDA- and EMA-approved for adult dermatomyositis and used in refractory polymyositis and dysphagia.

Cardiac involvement occurs in 10-30% of patients and includes myocarditis, conduction disease, arrhythmia, and rarely cardiomyopathy. Baseline ECG and echocardiogram are standard at diagnosis. Troponin monitoring is added when new chest pain, dyspnea, or palpitations develop.