Juvenile dermatomyositis — formative SAQs

SAQ 1 (10 marks)

A seven-year-old girl presents with a three-week history of progressive difficulty climbing stairs and lifting her arms above her head, a violaceous rash around her upper eyelids with periorbital oedema, and scaly red papules over her knuckles and elbows. Her creatine kinase is 2 400 U per litre and her aldolase is raised. Her alanine aminotransferase is mildly elevated but her synthetic liver function is normal. ^[1]

1. Give the diagnostic criteria for juvenile dermatomyositis under both the Bohan and Peter and the 2017 EULAR and ACR frameworks, and explain why the 2017 criteria are preferred for classification. (3) ^{[1] [2]}
2. Outline your investigation plan, including the role of imaging, biopsy and the myositis-specific antibody panel, and what the antibody result might predict. (4) ^{[1] [5]}
3. Give the first-line treatment, the evidence that underpins it, and the surveillance you would arrange. (3) ^[3]

Model answer — SAQ 1

(1) Diagnostic criteria (3). The Bohan and Peter criteria define definite juvenile dermatomyositis as the characteristic rash (heliotrope, Gottron papules or sign) plus any three of four muscle features: symmetric proximal weakness, raised muscle enzymes, a myopathic electromyogram, and a characteristic muscle biopsy; probable disease needs two muscle features and possible disease needs one. This girl's heliotrope rash, Gottron papules, proximal weakness and raised creatine kinase and aldolase meet definite criteria. The 2017 EULAR and ACR criteria are preferred for classification because they weight muscle biopsy, magnetic resonance imaging and myositis-specific antibodies into a probability score, avoiding the need for every old test such as electromyography and better reflecting modern practice. ^{[1] [2]}

(2) Investigation plan (4). I confirm muscle inflammation with creatine kinase, aldolase, lactate dehydrogenase and the transaminases, recalling that the alanine and aspartate aminotransferase here are myogenic rather than hepatic because the synthetic liver function is normal. I request magnetic resonance imaging of the proximal thigh muscles with short-tau inversion recovery sequences, which shows oedema in actively inflamed muscle, guides the site of biopsy if one is needed, and monitors response. A muscle biopsy is reserved for inconclusive cases and shows perivascular and perimysial inflammation, perifascicular atrophy and membrane attack complex deposition on capillaries. I send a myositis-specific antibody panel: anti-Mi-2 predicts the classic rash and a steroid-responsive good prognosis, anti-TIF1-gamma is the commonest in children and links with calcinosis, anti-NXP-2 marks severe muscle and calcinosis, and anti-MDA5 defines the amyopathic child at risk of rapidly progressive interstitial lung disease and should redirect me to lung function and a high-resolution chest scan. ^{[1] [5]}

(3) First-line treatment and surveillance (3). I give intravenous pulse methylprednisolone at 10 to 30 mg per kilogram per day to a maximum of 1 g, usually for three days, then oral prednisolone at 1 to 2 mg per kilogram per day to a maximum of 60 to 80 mg per day, and add methotrexate at 15 mg per square metre per week with folic acid as the steroid-sparing backbone. The evidence is the PRINTO randomised trial showing that adding methotrexate to prednisone was as effective as adding ciclosporin with a better safety and tolerability profile and faster steroid tapering than prednisone alone. Surveillance includes gastroprotection, calcium and vitamin D with a baseline bone density, regular disease activity scoring with the Childhood Myositis Assessment Scale, physiotherapy and occupational therapy from the outset, and screening for calcinosis, lipodystrophy and treatment toxicity. ^[3]