Most of my patients with obstructive sleep apnea (OSA) report they are "mouth breathers" at night. Because of that, they often think they should use a CPAP mask that covers their nose AND mouth...the full face mask. Full face masks often leak excessively, especially for side sleepers. I find that most of the time, my patients do well with a mask that just covers their nose or goes right in their nose (nasal pillows). But I have a small segment of patients who really struggle to breathe just through their nose. Almost all of these patients have daytime nasal stuffiness as well. Often, airflow through their nose is not improved with by-mouth medications or nasal sprays. In these cases, patients can opt to use the full face mask, or consider surgical therapy to improve nasal airflow. If the Ear, Nose and Throat (ENT) physician determines there is significant anatomical obstruction, then surgical therapy may improve nasal airflow enough to allow the patient to utilize an over-the-nose mask, instead of the full face mask.
Here is a study supporting my clinical experience. It's a meta-analysis of current literature. After examining past studies, the authors concluded that nasal surgery to relieve obstruction resulted in an average lowering of CPAP pressure by 2.66 cwp. Regardless of nasal surgery type, CPAP pressures were lower on average 1.9 cwp between preoperative and postoperative periods. The greatest difference was in those patients undergoing combined septoplasty with turbinoplasty. Finally, overall CPAP use increased after nasal surgery, possibly due to overall lowered CPAP pressures. The study did not mention changes in mask styles after surgery, such as from full face mask to nasal mask. This change alone, even without reduction in CPAP pressure may improve CPAP adherence.