Anti-PM/Scl Antibody-Positive Dermatomyositis With Rapidly Progressive Interstitial Lung Disease in a 19-Year-Old Male: Clinical Implications of a Presumptive Diagnosis Based on Line Immunoassay Positivity.

Anti-PM/Scl antibodies are classically linked to systemic sclerosis-polymyositis overlap, but a dermatomyositis phenotype with interstitial lung disease has also been described. However, the clinical course and therapeutic approach for short-term progressive interstitial lung disease in anti-PM/Scl-positive dermatomyositis remain incompletely defined. A 19-year-old Japanese male developed cough and exertional dyspnea after several weeks of progressive myalgia and proximal muscle weakness. Marked creatine kinase elevation, a high-titer nucleolar-pattern antinuclear antibody, Gottron papules, and mechanic's hands were noted, and dermatomyositis was diagnosed based on muscle magnetic resonance imaging and skin histopathology. High-resolution chest computed tomography showed bilateral, lower-lobe-predominant ground-glass opacities with infiltrative changes, consistent with a subacute-to-rapidly progressive course of interstitial lung disease. Anti-PM/Scl-75 and PM/Scl-100 antibodies were positive by a line immunoassay. As immunoprecipitation was unavailable, orthogonal testing on a different platform (wet protein array) also demonstrated strong positivity, increasing diagnostic confidence. After methylprednisolone pulse therapy, followed by high-dose glucocorticoids, interstitial lung disease findings persisted, prompting initiation and up-titration of mycophenolate mofetil. Imaging abnormalities and serum Krebs von den Lungen-6 improved over time after mycophenolate mofetil initiation, although the independent effect could not be determined because multiple interventions overlapped. Intravenous immunoglobulin was added for residual myositis activity, leading to improvement in muscle enzymes and discharge. This case highlights that anti-PM/Scl-positive dermatomyositis in young adults can present with short-term progressive interstitial lung disease and illustrates a pragmatic diagnostic and therapeutic strategy when immunoprecipitation is not feasible.