Factors associated with improvements in subjective symptoms of obstructive sleep apnea syndrome after continuous positive airway pressure therapy.

Continuous positive airway pressure (CPAP) therapy improves subjective symptoms in obstructive sleep apnea syndrome (OSAS) patients; however, factors predicting symptom improvement post-CPAP therapy and the CPAP duration necessary for improving subjective symptoms are unclear. This study aimed to identify these factors and the appropriate nightly CPAP duration for improving subjective symptoms. We recruited 359 subjects who completed both overnight polysomnography and subjective symptom assessments using the Epworth Sleepiness Scale (ESS), Zung Self-Depression Scale (SDS), and Pittsburgh Sleep Quality Index (PSQI). Firstly, we analyzed subject characteristics, and the associations between each assessment score and the Apnea-Hypopnea Index. These assessments were then repeated for 138 subjects who could continue for 3 months after starting CPAP. Secondly, associations between changes in self-reported outcome measures and nightly CPAP duration were analyzed. We identified subjects with abnormal initial ESS, PSQI, and SDS scores and divided them into "improvement" and "non-improvement" groups to examine factors associated with a positive outcome after CPAP therapy. Subjective symptom scores and proportions of subjects exceeding the cutoff values of each symptom score were not significantly related to OSAS severity. ESS, SDS, and PSQI scores improved 3 months after CPAP treatment, and factors involved in each improvement were found. Remarkably, longer CPAP nightly duration resulted in improvements in all subjective symptom scores. Furthermore, minimum durations between 4.75 and 5.40 h were necessary for improvements in subjective symptoms based on ROC curve analysis. Longer nightly CPAP use significantly improved OSAS subjective symptoms.