B35-15 Unraveling the Mystery: Myositis-associated Anti-pm-scl-100 Antibody as a Rare Trigger of Anti-synthetase Syndrome - A Case of Dramatic Recovery

Abstract Anti-synthetase syndrome (ASS) is a rare autoimmune disorder within the spectrum of idiopathic inflammatory myopathies (IIMs), characterized by the presence of aminoacyl-tRNA synthetase autoantibodies. ASS typically manifests with interstitial lung disease (ILD), myositis, arthritis, and “mechanic’s hands.” ILD is the most significant contributor to morbidity and mortality. We present a case of ASS associated with anti-PM/Scl-100 antibodies, an uncommon serologic profile that overlaps with systemic sclerosis features. Case Presentation A 56-year-old man with hypertension, obstructive sleep apnea, and obesity presented with 3 months of exertional dyspnea, nonproductive cough, weight loss, fatigue, arthralgia, and proximal muscle weakness. Physical exam revealed bilateral basal rales, proximal muscle weakness, and hyperkeratotic “mechanic’s hands.” High-resolution CT of the chest showed diffuse interstitial thickening with ground-glass opacities and traction bronchiectasis. Pulmonary function tests demonstrated a severe restrictive defect (FVC 47%) and reduced DLCO (43%). Laboratory results revealed elevated CK (3224), aldolase (40), ESR (28 mm/hr), and CRP (16). ANA, anti-PM/Scl-100 (200), and anti-SS-A 52 (187) antibodies were positive, while anti-Jo-1, PL-7, PL-12, OJ, and EJ was negative. MRI of the thighs showed muscular edema. The patient was diagnosed with anti-synthetase syndrome with ILD and initiated on prednisone 1 mg/kg daily and mycophenolate mofetil 500 mg twice daily, later titrated. Within two months, dyspnea, myalgia, and arthralgia improved markedly, skin lesions resolved, and follow-up HRCT showed significant regression of interstitial changes. Discussion ASS is a rare and heterogeneous autoimmune condition with ILD present in up to 95% of cases. Anti-PM/Scl-100 positive patients may exhibit overlapping features of myositis and systemic sclerosis, often with ILD as a dominant manifestation. HRCT typically reveals NSIP or organizing pneumonia patterns. Management is guided by expert consensus, as no controlled trials exist. Corticosteroids remain first-line, and early initiation of a steroid-sparing agent such as mycophenolate, azathioprine, or cyclophosphamide improves outcomes. Rituximab has shown promise in refractory cases. Conclusion Our case highlights the importance of early recognition and multidisciplinary management, which led to rapid clinical and radiologic improvement. Anti-synthetase syndrome with anti-PM/Scl 100 antibodies can present primarily with ILD and mimic other connective tissue diseases. Prompt diagnosis and combined immunosuppressive therapy can significantly improve pulmonary and systemic outcomes. This abstract is funded by: None