Continuous positive airways pressure (CPAP) is one of the most common treatments of sleep apnea, and dry mouth is a common and bothersome side effect. Dry mouth can be the reason patients are nonadherent to CPAP therapy. The absence of saliva in the mouth causes an unpleasant and scratchy sensation, difficulty swallowing, bad breath, and the growth of fungi and bacteria in the mouth with mucosal lesions and perlèche because the antimycotic and antibacterial action of saliva is lacking. It is thought that dry mouth is caused by an influx of air that dries up the oral mucosa. This would seem to be the case when using a full-face CPAP appliance, or if a patient's mouth remained open when using a nasal-only CPAP appliance. However, dry mouth also occurs when the mouth is closed with a chin strap, taping, adhesive, or bandages to force breathing through the nose. If the absence of saliva is not caused by the flow of air through the mouth, then what is the cause of dry mouth?
Saliva is produced by the activity of salivary glands, which send secretions into the oral cavity through ducts at a determined pressure. If the pressure in the oral cavity is higher than that of the secretion pressure at the duct outlet, the flow of saliva is blocked. In resting conditions, the secretion pressure is approximately 6.20 +1.48 Pa (equal to 0.06 cmH2O). Under stimulation, the secretion pressure can reach 537 +118.28 Pa (equal to 5.47 cmH2O).1 Because the pressure of CPAP devices in the oral cavity or in the nasopharynx that is in communication with the oral cavity typically ranges from 6 to 14 cmH2O, it is likely that this high pressure is impeding the flow of saliva.
With oronasal masks there is nothing that can be done to reestablish a normal flow of saliva, even with saliva secretory stimulants, because the increase in pressure comes from the mouth. With nasal devices, the only remedy for decreasing the pressure against the outlets of the salivary ducts is to close the opening between the oral cavity and pharynx, where there is increased pressure. The isthmus faucium can be closed by contracting the base of the tongue against the pillars and opening the mouth, so that the oral cavity regains normal atmospheric pressure. Decreased oral pressure can also be achieved with suction in the closed mouth. However, these treatments require that the patient be awake, and thus would interrupt the patient's sleep. One might try (although it is often found insufficient) heated humidification to soothe the discomfort caused by dry mouth, but this is not an option when using a nasal mask. The only solution that remains is consumption of sips of water at intervals during the night, but this is counter to the purpose of CPAP therapy—to provide patients with undisturbed sleep.
Prof. Bortolotti has indicated no financial conflicts of interest.
Bortolotti M. The cause of dry mouth during CPAP application. J Clin Sleep Med. 2017;13(4):647.
Xu Y, Zhou W, Zhu P, et al. Computational fluid dynamics for investigation of saliva pressure in parotid obstruction. Laryngoscope. 2012;122(5):1051–1056. [PubMed]