皮膚筋炎による肺炎を装った呼吸筋力低下の診断遅延

Dermatomyositis is an idiopathic inflammatory myopathy with characteristic involvement of skin and muscle. While interstitial lung disease (ILD) is a well-recognized pulmonary manifestation, hypoventilation due to respiratory muscle weakness remains underrecognized and may masquerade as primary lung pathology. We present the case of a 40-year-old woman with a history of polycystic ovarian syndrome who initially presented with dyspnea and was diagnosed with presumed pneumonia based on computed tomography findings. Despite antibiotic therapy, her symptoms progressively worsened over the next three weeks, during which evolving proximal muscle weakness and a diffuse pruritic rash developed, yet the diagnosis remained elusive. Repeat imaging ultimately suggested atelectasis rather than pneumonia; infectious workup was negative, and marked elevation of muscle enzymes with characteristic cutaneous findings, including the V sign and Gottron's papules, raised suspicion for dermatomyositis, later confirmed by biopsy. With high-resolution computed tomography of the chest showing no ILD and cardiopulmonary evaluation otherwise unremarkable, her dyspnea was attributed to extrapulmonary restrictive disease secondary to respiratory muscle weakness. She showed significant clinical improvement following immunosuppressive therapy. This case underscores how respiratory muscle weakness in dermatomyositis can mimic primary pulmonary disease and contribute to diagnostic delay. Recognizing evolving weakness and pathognomonic rashes, even in the absence of ILD, can shorten time to diagnosis and improve outcomes.