Anti-mda5 amyopathic dermatomyositis : A dermatologic mimicker not to be missed in Asian population

Background: Anti-melanoma differentiation-associated gene 5 (MDA5) amyopathic dermatomyositis (ADM) is a rare form of DM that has the hallmark cutaneous features of DM but lacks clinical and laboratory evidence of muscle involvement. The prevalence of anti-MDA5 ADM ranges from 1.6 to 36.6%, more commonly affecting Asian women in their 50s. The diagnosis is often challenging with as many as 41% of ADM misdiagnosed during initial evaluations. Early and accurate diagnosis of ADM is vital, given its association with rapid-progressive interstitial lung disease in 39–100% of patients and high mortality rates (80%). Case Report: A 33-year-old Iban lady with underlying hypertension presented in February 2024 with a three-month history of rashes on her face, bilateral hand knuckles, elbows and neck area. Additionally, she experienced excessive hair loss and joint pains distributed bilaterally over her hands, elbows, knees and ankles. The systematic review was otherwise unremarkable. She had no positive family history of connective tissue disease or malignancy. Physical examination revealed non scarring alopecia with an erythematous rash on her forehead, bilateral periorbital region as well as the malar area. There were erythematous papules on the dorsal part of metacarpophalangeal joints bilaterally (Gottron's papules) along with erythematous patches overlying the extensor aspect of bilateral elbows (Gottron's sign). There was no arthritis or Raynaud's phenomenon. Auscultations revealed bibasal end-inspiratory fine crackles with no loud P2. There were no signs of proximal muscle weakness. Vital signs: Blood pressure: 113/81 mmHg; heart rate: 82/min; SPO2: 96% under room air. Chest X ray showed bilateral lower zones pulmonary infiltrates. Laboratory investigations showed haemoglobin: 12.8 g/L, total white cell: 7.59 x 109/L, platelet: 254 x 109/L, urea: 4.3 mmol/L, creatinine: 62 μmol/L, total bilirubin: 8 μmol/L, AST: 55 U/L, ALT: 53 U/L, albumin: 36 g/L, creatinine kinase: 128 U/L. She tested negative for ANA, dsDNA and ENA, with normal C3 and C4. Muscle-specific antibodies (MSAs) showed positivity for anti-EJ (Glycyl-tRNA synthetase), anti-MDA5 and anti-PM- Scl- 100. A high-resolution CT scan of the thorax showed irregular reticular opacities at bilateral basal and subpleural distribution with patchy ground glass opacities, consistent with non-specific interstitial pneumonia (NSIP). She was treated with intravenous methylprednisolone followed by oral prednisolone and intravenous cyclophosphamide infusions. No neoplasms were detected. Conclusion: Potential misdiagnosis of ADM due to shared clinical phenotype with other autoimmune disorders might delay proper treatment initiation and lead to poor prognosis. Detection of MSAs can help in identifying DM subtypes accurately as well as prognosticate the patient.