Biphasic Flow-Volume Loop in a Patient With Idiopathic Unilateral Mainstem Bronchus Obstruction

Unilateral mainstem obstruction is an uncommon cause of dyspnea in the clinic setting. However, it is identifiable on spirometry as the "two-compartment phenomenon," in which the expiratory and/or inspiratory flow is decreased, followed by a further rapid decrease, resulting in a flattened end-expiratory or end-inspiratory tail, respectively. This case report outlines a 48-year-old woman with prior subglottic stenosis who presented with recurrent dyspnea. On spirometry, she had the characteristic finding of a flattened end-expiratory tail and was confirmed on imaging to have a left-sided unilateral mainstem bronchial obstruction. Her symptoms improved following a bronchoscopic intervention, and her spirometry pattern returned to normal. Though there are numerous known causes of unilateral mainstem obstruction, the workup for this patient was unrevealing, raising the possibility of idiopathic causes of this disease process. This is a unique case of idiopathic subglottic stenosis and left-sided unilateral mainstem bronchial obstruction occurring in the same patient.