Continuous Positive Airway Pressure (CPAP) is the most common treatment for obstructive sleep apnea. It works by pressurizing the upper airway so that the soft palate cannot prolapse and cut off the breath while the patient is asleep. The pressurized air flows through a mask, and that mask must seal firmly with the nose or face. For side-sleepers, this can cause a problem, because the side-pressure changes the shape of the face, breaking the seal with the mask.
Nasal Pillow Masks
Nasal Pillow masks don't cover the nose, but deliver air through two cushioned ports (called pillows) directly into the nostrils. These masks work best for side-sleepers because shape of the nose does not distort from side-pressure the way that the cheeks do, so there's less likelihood of breaking the pressure seal.
Soft Cloth Masks
Soft cloth masks are made from leak-proof fabric rather than plastic, so they are less likely to break their seal or to be pushed aside when you lie with the side of your face on the pillow. As an added benefit, they are less likely to leave marks on your face in the morning. For patients who can't tolerate air directed into their nostrils with pillows, this may be a more comfortable option.
An additional type of mask for side-sleepers is a nasal-pillow mask that attaches to a bracket which is, in turn, attached to an upper mouth guard. This kind of mask uses no headgear at all, and because the top teeth never change position relative to the nose, the air seal is constant regardless of sleeping position. Some people may have an issue with wearing the dental appliance, however, and people with certain dental problems can't use this device.
Composite Full-Face Masks
Side sleeping is hardest for people who need full-face masks, which are for those who breathe through their mouth. Even for mouth-breathers, the nose must be kept pressurized or air will escape through it. The traditional full-face masks, then, cover both mouth and nose and are easy to push aside if you sleep on your side. The solution is a mask that covers the mouth but uses nasal pillows to keep the nostrils pressurized. A number of manufacturers now make such masks.