A Diagnostic Uncertainty in an Immunocompromised Patient: Rapidly Progressive Interstitial Lung Disease vs. Pneumocystis Pneumonia.

Idiopathic inflammatory myopathies (IIM) are a diverse group of autoimmune disorders characterized by muscle weakness and involvement of extra-muscular organs, including the skin and lungs. The occurrence of interstitial lung disease (ILD) is considered a poor prognostic factor. While antibodies such as anti-Jo-1 and anti-MDA5 are associated with an increased risk of developing ILD, the presence of anti-Mi2 antibodies typically confers a favorable prognosis, with rare lung involvement. This case report presents a diagnostically challenging instance of a patient with positive anti-Mi2 antibodies who developed rapidly progressive ILD (RP-ILD) versus possible Pneumocystis jiroveci pneumonia (PJP).​ We present a 74-year-old Caucasian woman with a history of Stage IV adenocarcinoma with brain metastasis, complicated by vasogenic edema, treated with 12 mg of dexamethasone daily. Two weeks later, she developed proximal muscle weakness and was diagnosed with steroid-induced myopathy. Four weeks afterward, she presented with progressive shortness of breath and hypoxia, requiring high-flow nasal cannula and ICU admission. CT imaging revealed new multifocal opacities with perihilar ground-glass opacities. Review of previous investigations showed linear ground-glass opacities in the left lung and a positive anti-Mi2 autoantibody. The patient was started on intravenous steroids and intravenous immunoglobulin (IVIG) with subsequent improvement. On day five of hospitalization, serum beta-D-glucan returned elevated. Due to overlapping features with PJP, trimethoprim/sulfamethoxazole was initiated. The patient improved significantly and was later discharged on room air. Follow-up imaging six months later showed near-complete resolution. This case highlights the diagnostic complexity in critically ill, immunosuppressed patients with acute respiratory failure, where both autoimmune and infectious etiologies, such as PJP, should be considered.