Anti-PM/Scl antibody-positive myositis with muscle pathology resembling anti-synthetase syndrome

Anti-polymyositis/scleroderma (PM/Scl) antibodies were originally thought to be autoantibodies associated with overlap syndromes involving idiopathic inflammatory myopathies and systemic sclerosis, but some patients with these antibodies present with clinical features similar to those of anti-synthetase syndrome (ASyS). Herein, we report the case of a 63-year-old Japanese woman with anti-PM/Scl antibody-positive, but anti-aminoacyl tRNA synthetases antibody-negative myositis with interstitial lung disease and dermatomyositis-like skin manifestations. Her muscle pathological findings resembled those observed in ASyS, including perifascicular fibre necrosis and increased alkaline phosphatase activity in the perimysium. Owing to the potential risk of systemic sclerosis overlap and scleroderma renal crisis, calcineurin inhibitor treatment was avoided, and mycophenolate mofetil was administered in addition to glucocorticoids. This approach enabled rapid tapering of glucocorticoids, resulting in a favourable clinical course. To our knowledge, this report provides novel insights into the muscle pathology of anti-PM/Scl antibody-positive myositis, highlighting pathological similarities with ASyS. Our findings also suggest that mycophenolate mofetil may be a reasonable immunosuppressive therapy, especially when there is a risk of scleroderma renal crisis.