Dermatomyositis
Dermatomyositis (DM) is an autoimmune idiopathic inflammatory myopathy (IIM) characterised by pathognomonic cutaneous signs (Gottron papules, heliotrope rash, Gottron sign, shawl sign, V-sign, holster sign, mechanic's hands, nailfold capillary changes) and a spectrum of muscle involvement ranging from fulminant proximal weakness to clinically amyopathic disease (ADM/CADM). Pathogenesis centres on two interlocking pathways — a type I interferon signature (plasmacytoid dendritic cell release of IFN-alpha/beta, MHC class I upregulation, sustained innate/adaptive activation) and complement-mediated microangiopathy (C1q → C3 → C5b-9 membrane attack complex on endomysial capillaries → capillary dropout → perifascicular atrophy, the histological hallmark). Myositis-specific autoantibodies (anti-Mi-2, anti-MDA5/CADM-140, anti-TIF1-gamma/p155-p140, anti-NXP2, anti-SAE, anti-Jo-1/anti-synthetase, anti-SRP, anti-CN1A, anti-Mi-2-alpha/beta) define discrete clinical phenotypes; anti-MDA5 carries the highest 6-month mortality because of rapidly progressive interstitial lung disease (RP-ILD). Approximately 15 to 30 percent of adult DM is paraneoplastic, mandating age-appropriate cancer screening (CT chest/abdomen/pelvis, mammography, colonoscopy, ovarian CA-125 + transvaginal ultrasound, PSA, nasopharyngoscopy in Asian populations) at diagnosis and annually for 3 to 5 years. Management combines high-dose corticosteroids (prednisolone 1 mg/kg/day, or IV methylprednisolone 500–1000 mg/day × 3–5 days for severe disease) with steroid-sparing immunosuppression (methotrexate 7.5–25 mg/week, azathioprine 2 mg/kg/day, mycophenolate mofetil 2–3 g/day, ciclosporin or tacrolimus), IVIG (2 g/kg/cycle over 2–5 days, every 4 weeks) for refractory skin and severe oesophageal/diaphragmatic involvement, rituximab 1 g on days 0 + 14 for refractory myositis, JAK inhibitors (tofacitinib 5 mg BD) in selected cases, hydroxychloroquine 200–400 mg/day for cutaneous disease (with annual OCT maculopathy surveillance), strict photoprotection (SPF 50+ daily), and aggressive combination immunosuppression for anti-MDA5 RP-ILD (high-dose IV methylprednisolone + calcineurin inhibitor + cyclophosphamide +/− rituximab, with early lung transplant evaluation for refractory disease). Fellowship candidates must master the pathognomonic cutaneous signs, autoantibody stratification, the paraneoplastic work-up, the EULAR/ACR 2017 classification criteria, the juvenile DM complication set (calcinosis cutis, GI and CNS vasculopathy), and the operative distinction from polymyositis (PM), inclusion body myositis (IBM), immune-mediated necrotising myopathy (IMNM), and cutaneous lupus erythematosus (CLE).
