RESUMEN
INTRODUCTION: Systemic lupus erythematosus (SLE) is associated with an increased risk of pulmonary infections, as well as a rare condition known as shrinking lung syndrome (SLS). The diaphragm has an important role to play in lung physiology and might also play a role in these adverse events. Here, we aimed to investigate whether SLE patients have impairment in their diaphragmatic muscle thickness and function with respect to another connective-tissue disease: primary Sjögren's syndrome (pSS). METHOD: Patients diagnosed with SLE who were in remission or who had minimal disease activity and had at least one year of follow-up were included in this study. Patients with known lung pathology and smokers were excluded. Patients with pSS constituted the second experimental group. Ultrasonographic evaluation of the diaphragmatic muscle was conducted by an experienced independent sonographer at three time points, diaphragmatic thickness during deep and quiet inspiration and maximum expiration being measured. Diaphragmatic muscle function was evaluated with maximum expiratory pressure (MEP) and maximum inspiratory pressure (MIP). RESULTS: A total of 115 patients were studied (n = 39 SLE; n = 76 pSS). The mean ± standard deviation (SD) thickness of the diaphragmatic muscles during quiet inspiration was significantly reduced in patients with SLE compared to patients with pSS (2.32 mm vs. 2.81 mm; p < 0.05). Similarly, the thickness during deep inspiration and at maximum deep expiration were significantly lower in SLE patients (2.88 mm vs. 3.29 mm and 1.92 mm vs. 2.33 mm, respectively; p < 0.01). MIPs and MEPs, defined as the percentages of expected values, were significantly lower in patients with SLE compared to those with pSS (80% vs. 92% and 76% vs. 120%, respectively; p < 0.05). Diaphragmatic muscle thickness during deep inspiration demonstrated a moderate correlation with MIP (r = 0.434; p = 0.001). CONCLUSION: SLE patients had reduced diaphragmatic muscle thickness compared to those with pSS, which was associated with impaired functional tests. Further prospective studies are needed to investigate whether structural and functional impairments in diaphragmatic muscle play a role in an increased risk of pulmonary infections and SLS in patients with SLE.