Assessment Tools for Pulmonary Involvement in Patients with Ankylosing Spondylitis: Is Diaphragmatic Ultrasonography Correlated to Spirometry?

Objective: Spondyloarthritis (SpA) is a chronic inflammatory rheumatic disease that can lead to spinal ankylosis and consequently, restrictive pulmonary dysfunction. Thus, the present study aimed to assess the accuracy of diaphragm ultrasound compared to spirometry in the screening of restrictive pulmonary disorders in radiographic SpA patients.Methods: We conducted a cross-sectional study of 50 patients with radiographic SpA, over six months. Sociodemographic data, clinical characteristics of the disease, as well as biological, radiological, and therapeutic data, were collected. Spirometry and diaphragm ultrasound were performed.Results: The mean age of the study participants (N= 50) was 42.7 +/- 11 years [range: 25-66] with male predominance (N= 41). Spirometry showed a restrictive disorder in 32% of cases. The mean chest expansion (CE) value was 3.9 +/- 1.81cm [range: 1-9] with a median of 4 cm. A pathological value (<5cm) was observed in 72% of cases. A significant positive correlation was found between the right inspiratory diaphragmatic thickness and forced vital capacity (FVC) (r= 0.36; p = 0.02) and the supine FVC (r=0.29; p = 0.04). The left inspiratory diaphragmatic thickness was correlated with the percentage of the FVC decrease (r= 0.35; p = 0.01) defined as the percentage of difference between FVC and supine FVC. The right expiratory diaphragmatic thickness was associated with the FVC (r=0.32; p = 0.02). A significant positive correlation was found between the CE and the presence of B lines (r=0.32; p = 0.02), but not between the CE and the FVC.Conclusion: The present study showed that diaphragm ultrasonography is correlated with spirometric findings in radiographic SpA patients. Further studies are required to assess its reliability, specificity, and sensitivity in this pathology.