Polymyositis-Like Myopathy With Anti-PL-12 Antibody Positivity and Coexisting Antiphospholipid Syndrome: Diagnostic and Management Challenges in the Absence of Systemic Features.

Polymyositis-like inflammatory myopathies are a rare subset of idiopathic inflammatory myopathies (IIMs) characterized by proximal muscle weakness, elevated muscle enzymes, and immune-mediated skeletal muscle damage. These conditions are often associated with myositis-specific autoantibodies (MSAs), such as anti-alanyl-tRNA synthetase antibody (anti-PL-12), which define subtypes like anti-synthetase syndrome. We present the case of a 50-year-old female patient with progressive proximal muscle weakness, elevated creatine kinase and aldolase levels, and serological evidence of anti-PL-12 antibodies. Muscle biopsy revealed hallmark findings of immune-mediated myopathy, including increased sarcolemmal major histocompatibility complex class I (MHC1) expression, vascular membrane attack complex (C5b-9) deposition, and focal myofasciitis. Despite the absence of hallmark systemic features of the anti-synthetase syndrome, such as interstitial lung disease (ILD) or arthritis, these findings confirmed a diagnosis of polymyositis-like myopathy within the anti-synthetase syndrome spectrum. The patient's concurrent antiphospholipid syndrome (APS) necessitated careful anticoagulation management while initiating immunosuppressive therapy with prednisone and mycophenolate mofetil, which led to clinical and biochemical improvement. This case underscores the diagnostic challenges posed by incomplete phenotypes of anti-synthetase syndrome and the critical role of muscle biopsy in confirming autoimmune myopathies. By illustrating the variability of presentations and the necessity of a multidisciplinary approach, this report contributes to the understanding and management of complex autoimmune myopathies.