Oral devices have been shown to have beneficial effects relating to several areas. These include the polysomnographic indexes of OSA, subjective and objective measures of sleepiness, blood pressure, aspects of neuropsychological functioning, and quality of life. Elucidation of the mechanism of action of oral devices has provided insight into the factors that predict treatment response and may improve the selection of patients for this treatment modality. A 2008 study published in "Sleep" on the influence of nasal resistance (NAR) on oral device treatment outcome in OSA demonstrates the need for an interdisciplinary approach between ENT surgeons and sleep physicians to treating OSA. The study suggests that higher levels of NAR may negatively affect outcome with MAS and subsequently methods to lower nasal resistance may improve the outcome of oral device treatment. They are generally more successful at treating mild and moderate sleep apnea and less effective at treating severe sleep apnea, even though good success was measures even in severe sleep apnea. They may bring the level of apnea a patient experiences down significantly but fail to completely eliminate it. A meta-analysis of 51 randomized controlled trials investigating the effects of CPAP and oral devices on blood pressure found that oral devices were equally effective as continuous positive airway pressure (CPAP) devices in lowering the blood pressure of patients suffering from OSA. The medical dental sleep appliance, or MDSA, was clinically proven to conclusively show in a large and complex randomized controlled study that CPAP and MAS are effective in treating sleep-disordered breathing in subjects with AHI 5–30. CPAP was thought to be more effective, but randomised control evidence (such as that reviewed in 2013) suggests splints may be as effective in patients with a range of severities of obstructive sleep apnoea. Both methods appear effective in alleviating symptoms, improving daytime sleepiness, quality of life and some aspects of neurobehavioral function, with CPAP usage being less than self-reported MAS usage. More test subjects and their domestic partners felt that CPAP was the most effective treatment, although MAS was easier to use. Nocturnal systemic hypertension was shown to improve with MAS but not CPAP, although the changes are small. In a survey study of patients, many discontinued use because of discomfort, lack of efficacy or switching to CPAP and frequent side-effects included dry mouth, tooth pain, dental discomfort and jaw pain. Long term use is not associated with temporamandibular disorders but was associated with permanent changes in how the upper and lower teeth meet and the appliances needed about 0.8 repairs/relines per year.