Diaphragm Ultrasound Identifies Respiratory Pump Dysfunction in Bronchiolitis Obliterans Syndrome After Hematopoietic Stem Cell Transplantation

<title>Abstract</title> <p>Background. Bronchiolitis obliterans syndrome (BOS) is a life-threatening noninfectious pulmonary complication following hematopoietic stem cell transplantation (HSCT), characterized by progressive airflow obstruction. This study investigated diaphragmatic structure and function using ultrasound and examined their relationship with BOS severity. Methods. In this cross-sectional study, 41 patients diagnosed with BOS after HSCT and 30 age- and sex-matched healthy controls were enrolled. Diaphragm ultrasound was performed to measure diaphragm thickness at end-expiration and end-inspiration, diaphragm thickening fraction (DTF), and diaphragm excursion during quiet breathing and deep breathing. Pulmonary function tests were collected for BOS patients. Correlation analyses were performed between diaphragm ultrasound parameters and pulmonary function indices. Ordinal logistic regression was used to assess the ability of diaphragm parameters to predict BOS severity. Results. M-mode ultrasound showed distinct diaphragmatic motion patterns in BOS patients compared with healthy controls. BOS patients had significantly greater diaphragm thickness at both end-expiration and end-inspiration on both sides (all p &lt; 0.05), together with higher DTF. In contrast, right diaphragmatic excursion during deep breathing was significantly reduced in BOS patients (p = 0.010), and the difference between deep and quiet breathing excursion was markedly decreased (p &lt; 0.001). Patients with severe BOS showed greater diaphragm thickening but lower diaphragm excursion than those with mild or moderate BOS. Inspiratory diaphragm thickness and left DTF were negatively correlated with FEV1% predicted, whereas diaphragm excursion during deep breathing was positively correlated with forced vital capacity and negatively correlated with the residual volume/total lung capacity ratio. An ordinal logistic regression model incorporating left DTF and right diaphragm excursion showed potential in predicting severe BOS. Conclusions. Diaphragm ultrasound reveals distinct alterations in diaphragmatic structure and function in BOS patients. These findings suggest that respiratory pump dysfunction may contribute to disease severity in BOS and highlight diaphragm ultrasound as a potential non-invasive tool for disease assessment.</p>