**2. Upper airway diseases**

Upper airway diseases decrease sleep time and/or quality, which leads to excessive daytime sleepiness, fatigue, and lack of concentration. Upper airway diseases can be classified into two groups: upper airway resistance syndrome and sleep apnea syndrome.

Snoring is the most evident symptom of these syndromes, but it is not a disease. Snoring is just a sound resulting from soft tissues in the upper airway vibrating during inspiration, due to the increased velocity of air, caused by the decrease in size of airway space. Almost all patients with upper airway resistance syndrome and sleep apnea syndrome snore. However, snoring patients may or may not also have sleep apnea or upper airway resistance syndromes. Prevalence of snoring is estimated in 35–40% of the population. Snoring is a sign that some type of resistance is occurring in the upper respiratory system.

Upper airway resistance syndrome (UARS) occurs when resistance of upper respiratory system increases and breathing effort crosses over from harmless snoring. The transformation of snoring to UARS can be caused by aging (muscle tone in the throat decreases in time) and weight gain. In upper airway resistance syndrome, breathing needs much effort due to the narrow upper airway, and this extra effort leads to sleep arousal, frequent nocturnal awakenings, chronic insomnia, and excessive daytime sleepiness. Apnea and hypopnea are not symptoms of this disease.

UARS and obstructive sleep apnea syndrome (OSAS) have similar etiologies and symptoms, so differential diagnosis is very important. The key differences between UARS and OSAS consist of apnea-hypopnea existence, gender, and weight differences. In UARS apnea and hypopnea are either absent or very low. UARS patients are often in average weight, but OSAS patients are generally overweight or obese. UARS affects two genders equally, but OSAS affects men more than women.

UARS treatment consists of behavior and lifestyle changes, oral-dental appliances, and continuous positive airway pressure (CPAP) therapy. When UARS is untreated or treatment is unsuccessful, it can end up developing sleep apnea syndrome.

Sleep apnea syndrome is a chronic, progressive, and life-threatening disorder which occurs during sleeping and characterized by apneic and hypopneic events, hypoxia, blood oxygen desaturation, sleep arousal, and/or awakening. Sleep apnea has three types: central, obstructive, and mixed. In central apnea respiratory muscles need extra effort because breathing is limited or completely blocked due to the obstruction in the upper airway. Mixed apnea is the combination of central and obstructive apnea. Central and mixed apnea can be treated only by medical practitioners. But in the treatment of obstructive sleep apnea, dentists have an important role [1–4].
