Pulmonary Dysfunction Contributes to Nocturnal Hypoxemia Beyond Obstructive Sleep Apnea Severity in Primary Sjögren Syndrome

Background/Objectives: Pulmonary involvement is a common extraglandular manifestation of primary Sjögren syndrome (pSS) and may impair pulmonary gas exchange. Although obstructive sleep apnea (OSA) has been reported in pSS, the relationship between nocturnal hypoxemia and pulmonary dysfunction remains unclear. We investigated whether nocturnal oxygenation abnormalities in pSS are associated with pulmonary functional impairment beyond OSA severity. Methods: In this controlled cross-sectional study, 44 patients with pSS and 88 controls underwent overnight polysomnography. Pulmonary function testing, including diffusing capacity of the lung for carbon monoxide (DLCO), and thoracic computed tomography were available for all pSS patients. Associations between nocturnal hypoxemia parameters and pulmonary function indices were assessed using correlation analyses. Predictors of minimum nocturnal oxygen saturation were evaluated using multivariable linear regression adjusted for age, body mass index, and apnea–hypopnea index. Results: Sleep-disordered breathing severity and nocturnal oxygenation profiles were comparable between patients with pSS and controls, with the exception of a lower proportion of REM sleep in the pSS group. Within the pSS cohort, minimum nocturnal oxygen saturation correlated positively with DLCO, DLCO% predicted, DLCO/VA, and FVC% predicted. DLCO% predicted remained independently associated with minimum nocturnal oxygen saturation after adjustment for age, BMI, and AHI. However, this association was attenuated after additional adjustment for FVC, suggesting overlapping contributions of diffusion impairment and restrictive ventilatory dysfunction to nocturnal oxygenation. Conclusions: Although overall nocturnal oxygenation did not differ from that of matched controls, pulmonary functional impairment was associated with worse nocturnal oxygenation within the pSS cohort. These findings suggest that nocturnal hypoxemia in pSS reflects multiple interrelated aspects of pulmonary dysfunction rather than OSA severity alone. Assessment of pulmonary function, including both DLCO and FVC, may therefore provide clinically relevant information when evaluating nocturnal oxygenation abnormalities in patients with pSS.