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Stem
A 52-year-old woman presents with a three-month history of progressive difficulty combing her hair, lifting shopping bags and climbing stairs. She has noticed a lilac-coloured rash around her eyes that worsens in sunlight, and scaly red bumps over her knuckles. She has no raynaud, no dry cough and no joint swelling. On examination there is symmetric weakness of shoulder abduction and hip flexion (Medical Research Council grade 4), a violaceous heliotrope rash on the upper eyelids and Gottron papules over the metacarpophalangeal and proximal interphalangeal joints. Investigations: creatine kinase 3200 U/L (reference under 170), aldolase raised, alanine aminotransferase mildly raised; anti-nuclear antibody positive (1:320); anti-Mi-2 positive. Chest radiograph and pulmonary function tests are normal.
Questions
a) What is the most likely diagnosis and on what basis? (2 marks)
The most likely diagnosis is dermatomyositis. The basis is the combination of symmetric proximal muscle weakness (shoulder and hip girdle) with a markedly raised creatine kinase, plus the pathognomonic skin manifestations — the heliotrope rash (violaceous periorbital erythema) and Gottron papules (over the knuckles). The positive anti-Mi-2 antibody supports a classic dermatomyositis phenotype. Weakness rather than pain, a subacute course, and a normal chest radiograph and pulmonary function tests further support isolated dermatomyositis without interstitial lung disease.
b) Outline the pathophysiology and the key muscle-biopsy finding you would expect. (2 marks)
Dermatomyositis is driven by a type-I-interferon-driven, complement-mediated microangiopathy: immune-complex and complement activation generates the membrane attack complex (C5b-9) on endomysial capillaries, causing capillary destruction and ischaemia. The perifascicular region (the watershed zone) is most vulnerable and atrophies. The expected biopsy finding is therefore perifascicular atrophy with perifascicular membrane attack complex deposition — distinguishing dermatomyositis from polymyositis (which shows endomysial CD8-positive T-cell infiltration of MHC-class-I-upregulated fibres).
c) What investigations would you arrange to stage the disease and screen for complications? (3 marks)
- Myositis-specific antibody panel — already anti-Mi-2 positive; specifically test anti-TIF1-gamma and anti-NXP2 (cancer-associated) and anti-MDA5 and anti-Jo-1 (interstitial lung disease risk) to risk-stratify.
- Malignancy screen (mandatory in adult dermatomyositis) — CT chest, abdomen and pelvis plus age-appropriate tests (mammography, cervical screening, colonoscopy as appropriate); intensified and extended to up to three years if anti-TIF1-gamma or anti-NXP2 is positive.
- Interstitial lung disease screen — high-resolution CT chest and pulmonary function tests (forced vital capacity and diffusing capacity), with electrocardiogram and echocardiogram to assess the myocardium and conduction. (Magnetic resonance imaging with short tau inversion recovery can guide the muscle-biopsy site.)
d) Outline the management. (3 marks)
- Induction — high-dose corticosteroids (prednisolone 0.5 to 1 mg/kg/day, with an intravenous methylprednisolone pulse if severe), tracking the creatine kinase and strength to guide the taper.
- Early steroid-sparing agent (because steroid monotherapy is inadequate and toxic) — methotrexate, azathioprine or mycophenolate mofetil; intravenous immunoglobulin for refractory disease or severe dysphagia; rituximab or a JAK inhibitor if refractory.
- Skin and supportive care — rigorous photoprotection, hydroxychloroquine and topical corticosteroids for the rash; physiotherapy; and prophylaxis against corticosteroid toxicity — Pneumocystis jirovecii prophylaxis, bone protection and gastric protection. Multidisciplinary follow-up with serial creatine kinase, pulmonary function tests and malignancy surveillance.