An Uncommon Cause of Cystic Lung Disease.

A 59-year-old female with Sjögren's syndrome presented with a three-year history of dyspnea and cough beginning after COVID-19 infection. Serial chest computed tomography scans over a course of three years revealed multiple pulmonary cysts and nodules, including one that enlarged over time. Pulmonary function was largely preserved, aside from a mildly reduced diffusing capacity, while laboratory studies showed hypergammaglobulinemia and elevated kappa light chains. After a nondiagnostic bronchoscopic transbronchial biopsy, she underwent thoracoscopic wedge resection. Pathology revealed eosinophilic deposits without Congo red staining, and mass spectrometry confirmed light chain deposition disease. Bone marrow biopsy excluded malignancy, and treatment with mycophenolate led to symptomatic improvement. This case highlights the diagnostic challenges often encountered with an uncommon cause of cystic lung disease.