**8.3. Polysomnography (PSG)**

Nocturnal, laboratory-based polysomnography is the gold standard diagnostic tool for obstructive sleep apnea syndrome. It is used for diagnosing, determining the severity of disease, and evaluating various other sleep disorders that can exist with or without OSAS. It is also used after OSAS treatment for the evaluation of the treatment effectiveness. A classical PSG consists of electroencephalogram (EEG), electromyogram (EMG), and electrooculogram (EOG) records. PSG measures sleep cycles and stages by recording airflow in and out of lungs during breathing, the level of oxygen in blood, breathing effort and rate, brain waves, electrical activity of muscles, eye movements, and heart rate. The most important parameters measured by PSG are apnea index (AI), hypopnea index (HI), apnea-hypopnea index (AHI), and minimum oxygen desaturation. PSG can be done either at a sleep center or in the patients at home. But home-based PSG records do not have the same reliability as sleep center-based PSG records.
