The primary method of diagnosing OSA at present is to have the patient undergo a sleep study, known as a polysomnography.
A sleep technician administers and attends the study. To prepare the patient for a sleep study, numerous physiological monitors are attached to the patient to record nighttime breathing, brain activity, and physical activity. Several electrodes are pasted to the patient’s head to measure brain electrical activity with an electroencephalogram, or EEG. Electrical activity in the brain during the different stages of sleep is distinctly different from that while awake. The EEG allows the physician to see if the patient is reaching all stages of sleep to the appropriate depth and if the patient is being aroused from these stages.
Electrodes are also taped to the skin near the outer edges of the eyes to record data for an electrocculogram (EOG). This tells the examiner when the patient is in rapid eye movement sleep (REM). A device is placed near the patient’s nose and mouth to measure airflow. Electrodes are connected to an electromyogram (EMG) and taped or pasted on the patient’s chin to detect activity in the jaw muscles. The EMG detects the presence of REM sleep when the jaw muscles relax.
Special belts are placed around the patient’s chest and abdomen to detect and record the rising and falling movements of respiration. A pulse oximeter, a noninvasive device for measuring oxygen content in the blood, is attached to the finger, and electrodes to provide an electrocardiogram (ECG) are attached to the chest to measure heart rate. Various types of instruments, either straps around the feet or electrodes pasted to the lower legs, measure leg movements, which may indicate another sleep disorder called periodic limb movement disorder.
OSA is diagnosed if the patient has an apnea index greater than 5, that is, has more than five apnea episodes per hour, or a respiratory disturbance index (RDI), the combination of apneas and hypopneas, greater than 10 per hour.
Clinically speaking an obstructive apnea is defined as a complete cessation of airflow for more than 10 seconds with persistent respiratory effect. An obstructive hypopnea is defined as a partial reduction in airflow of approximately 30% to 50% with persistent respiratory effect and a reduction in oxygen saturation by at least 3% to 4% and/or arousal from sleep.