Acute Hypoxemic Respiratory Failure Caused by Nonspecific Interstitial Pneumonia in Mixed Connective Tissue Disease: A Case Report.

Mixed connective tissue disease (MCTD) is an autoimmune overlap condition characterized by features of systemic lupus erythematosus, systemic sclerosis, and polymyositis, frequently involving pulmonary manifestations. Interstitial lung disease (ILD) is one of its most serious complications and may present acutely with hypoxemic respiratory failure. Among the ILD patterns associated with MCTD, nonspecific interstitial pneumonia (NSIP) predominates and is characterized by diffuse ground-glass opacities, reticulation, and potential responsiveness to early immunosuppressive therapy. We describe a 42-year-old male active-duty service member who presented with acute hypoxemic respiratory failure and was ultimately diagnosed with MCTD-associated NSIP following bronchoscopy and confirmatory serologies. He demonstrated marked clinical and radiographic improvement after receiving high-dose corticosteroids and mycophenolate mofetil. This case highlights the importance of considering autoimmune ILD in patients with diffuse pulmonary opacities and unexplained hypoxemia.