AKT · Musculoskeletal/Connective tissue diseaselow yield

Polymyositis & dermatomyositis

Autoimmune inflammatory myopathy (dermatomyositis adds skin disease; paraneoplastic association)

Overview

Idiopathic inflammatory myopathies causing symmetrical proximal muscle weakness from immune-mediated muscle inflammation. Dermatomyositis adds characteristic skin signs (heliotrope rash, Gottron's papules). Both — dermatomyositis especially — carry a paraneoplastic association, so an age-appropriate malignancy screen is essential. Creatine kinase is markedly raised.

Recognise

Symmetrical PROXIMAL muscle weakness (difficulty rising from a chair, climbing stairs, combing hair); muscle pain may be mild
Dermatomyositis skin signs: heliotrope (lilac) periorbital rash, Gottron's papules over the knuckles, shawl/V sign, mechanic's hands
Markedly raised creatine kinase; associated interstitial lung disease (anti-Jo-1); dysphagia

Red flags

Underlying malignancy (especially with dermatomyositis) → age-appropriate cancer screen
Respiratory muscle weakness / ILD or severe dysphagia (aspiration) → urgent support

Differentials & how to tell them apart

Polymyalgia rheumaticastiffness rather than true weakness, normal CK, dramatic steroid response

Inclusion body myositisolder men, distal + proximal, asymmetrical, poor steroid response

Statin/endocrine myopathy / muscular dystrophydrug history, TFTs, genetic context

Gottron papules over the knuckles in dermatomyositis
Mohammad2018 / CC BY-SA 4.0 — Wikimedia Commons

Investigations

Creatine kinase (markedly raised); EMG (myopathic); muscle biopsy (inflammatory infiltrate — definitive); myositis-specific antibodies (anti-Jo-1 → ILD); MRI of muscle; MALIGNANCY screen (CT chest/abdomen/pelvis, age-appropriate) especially in dermatomyositis.

Management

High-dose corticosteroids + steroid-sparing immunosuppressant; screen for malignancy

1Confirm with raised CK, EMG, antibodies and muscle biopsy. Induce remission with high-dose corticosteroids and add a steroid-sparing immunosuppressant early.Gate: Always perform an age-appropriate MALIGNANCY screen — especially in dermatomyositis, which has a strong paraneoplastic association.
2Refractory disease, ILD or severe dysphagia → IVIg or rituximab; treat any underlying cancer (which may improve the myositis).

High-dose corticosteroids — first-line to induce remission

Steroid-sparing immunosuppressant (azathioprine, methotrexate, mycophenolate) — added early to reduce steroid burden

IVIg / rituximab for refractory disease — and for significant dysphagia/ILD

Treat any underlying malignancy — paraneoplastic disease may improve with cancer treatment

Key points

Symmetrical PROXIMAL weakness + markedly raised CK = inflammatory myopathy; add heliotrope rash + Gottron's papules = dermatomyositis. Steroids + steroid-sparer. CRITICAL: screen for underlying malignancy (paraneoplastic, esp. dermatomyositis). Anti-Jo-1 → ILD.

Monitor & prognosis

CK and muscle power, ILD (PFTs), steroid/immunosuppressant safety, malignancy surveillance.

Many respond to immunosuppression; ILD and underlying malignancy worsen prognosis.

Source: BSR; cross-ref endocrine (paraneoplastic panel)