Polymyositis

Polymyositis is an idiopathic inflammatory myopathy with symmetric proximal weakness and CD8 T cell mediated invasion of non-necrotic muscle fibres.

• True polymyositis is rare; many cases previously labelled are immune mediated necrotising myopathy, inclusion body myositis, antisynthetase syndrome or dystrophy.

• Adult onset (over 20 years), women more than men.

• Associations: connective tissue disease, interstitial lung disease, myocarditis, malignancy (modest risk), drugs, viral infection.

• Complications: dysphagia, respiratory muscle weakness, cardiac involvement, interstitial lung disease.

• Subacute progressive symmetric proximal weakness (difficulty rising from chair, climbing stairs, lifting arms), neck flexor weakness over weeks to months.

• Myalgia mild, dysphagia, dyspnoea.

• No skin rash (otherwise dermatomyositis).

• Cardiac arrhythmia or failure, ILD.

• CK elevated (5 to 50 times normal), aldolase, AST/ALT, LDH; myositis specific antibodies (anti-Jo-1, SRP, HMGCR to exclude necrotising myopathy); ANA, anti-Ro52.

• EMG: myopathic units with fibrillation; MRI muscle: oedema.

• Muscle biopsy: endomysial inflammation with CD8 T cells invading MHC-I positive non-necrotic fibres.

• HRCT, pulmonary function, ECG, echocardiography, troponin.

• Age appropriate malignancy screen. Exclude statin myopathy, thyroid disease, muscular dystrophy, inclusion body myositis, drug myopathy.

• Prednisolone 1 mg/kg/day (maximum 60 to 80 mg) for 4 to 8 weeks, then taper by 10 to 25% every 4 weeks; methylprednisolone 1 g IV for 3 days in severe disease.

• Steroid-sparing agent early: methotrexate 15 to 25 mg weekly with folic acid, azathioprine 2 to 3 mg/kg/day, or mycophenolate mofetil 1 to 1.5 g twice daily.

• Refractory: IVIG 2 g/kg over 2 to 5 days monthly, rituximab 1 g on days 1 and 15, tacrolimus, ciclosporin, cyclophosphamide.

• Supportive: physiotherapy (start after inflammation improves), swallowing assessment, ventilatory monitoring, bone protection, Pneumocystis prophylaxis, vaccination.

• Monitor strength and CK every 4 to 8 weeks; relapse on taper is common.

• Treat ILD, cardiac involvement, malignancy.