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Q1: Presentation & diagnosis (2 min)
A 52-year-old woman presents with a three-month history of progressive difficulty combing her hair and climbing stairs, a lilac rash around her eyes and scaly bumps over her knuckles. Take us through your diagnostic approach. Cover: the gateway recognition — symmetric proximal weakness with a raised creatine kinase (weakness, NOT pain — the contrast with polymyalgia rheumatica); the pathognomonic dermatomyositis skin signs (heliotrope rash — periorbital; Gottron papules/sign — over knuckles; shawl sign, V-sign, nailfold telangiectasia, mechanic's hands, poikiloderma); the subacute tempo (weeks to months, unlike acute rhabdomyolysis or chronic dystrophies); the diagnostic pathway (proximal weakness + high CK + myopathic EMG + muscle biopsy + myositis-specific antibodies, with MRI short tau inversion recovery to guide biopsy); and the 2017 EULAR/ACR classification that supersedes Bohan and Peter.
Q2: The subtypes & antibody phenotypes (3 min)
How do the idiopathic inflammatory myopathy subtypes differ, and why does it matter? Cover the antibody-phenotype keys: anti-Mi-2 (classic DM, good prognosis, steroid-responsive); anti-TIF1-gamma and anti-NXP2 (cancer-associated DM — intensify malignancy screening, rescreen for up to three years); anti-MDA5 (rapidly progressive ILD with amyopathic/mild DM and cutaneous ulcers — high mortality); anti-Jo-1 and the antisynthetases (antisynthetase syndrome — ILD, fever, mechanic's hands, arthritis, Raynaud); anti-SRP and anti-HMGCR (immune-mediated necrotising myopathy — very high CK, necrosis with minimal inflammation, statin-exposed for HMGCR). Then the biopsy hallmarks: DM = perifascicular atrophy (complement microangiopathy); PM = endomysial CD8 T cells; IBM = rimmed vacuoles; IMNM = necrosis with minimal inflammation. Emphasise that the subtype decides who to screen for cancer and ILD and whether steroids will work.
Q3: Inclusion body myositis — the trap (2 min)
Why is inclusion body myositis the trick question? Cover: older men (over 50, the commonest inflammatory myopathy in this age group); asymmetric, mixed distal + proximal weakness with quadriceps (early falls, knee buckling) and finger flexor (forearm) involvement plus dysphagia; only mildly raised CK; biopsy shows endomysial inflammation with rimmed vacuoles (TDP-43, p62, beta-amyloid) reflecting a degenerative layer on top of the immune process; and crucially a poor response to corticosteroids — it is largely refractory, so management is supportive (physiotherapy, fall prevention, swallowing therapy) and escalating immunosuppression is futile and harmful. It is frequently misdiagnosed as refractory polymyositis.
Q4: Screening & complications (2 min)
What are the dangerous complications, and who must you screen? Cover: interstitial lung disease is the leading cause of morbidity and mortality (especially anti-MDA5 and antisynthetase) — screen every patient with high-resolution CT and pulmonary function tests; occult malignancy (strongest with anti-TIF1-gamma and anti-NXP2 in adult DM) — age-appropriate screening with CT chest/abdomen/pelvis, mammography, colonoscopy and cervical screening, rescreen for up to three years; aspiration from pharyngeal weakness (assess swallow); respiratory muscle weakness (monitor forced vital capacity); calcinosis (juvenile DM); and corticosteroid toxicity (always co-prescribe Pneumocystis prophylaxis, bone and gastric protection).
Q5: Management (2 min)
How do you manage dermatomyositis and polymyositis? Cover the phenotype-stratified approach: induction with high-dose corticosteroids (prednisolone 0.5 to 1 mg/kg/day, intravenous methylprednisolone for severe disease); early steroid-sparing agents (methotrexate, azathioprine, mycophenolate mofetil — the latter two favoured when ILD coexists); IVIG for refractory DM and severe dysphagia; rituximab and calcineurin inhibitors/cyclophosphamide for refractory disease and severe ILD; JAK inhibitors for type-I-interferon-driven DM and anti-MDA5 ILD; rigorous photoprotection, hydroxychloroquine and topical steroids for the skin; supportive physiotherapy, speech and language therapy, and prophylaxis (Pneumocystis, bone, gastric, vaccination). Emphasise finding and treating any underlying malignancy (cancer-associated myositis) and the supportive, non-immunosuppressive approach to inclusion body myositis.