Diagnosis of Obstructive Sleep Apnea (OSA)
The primary method for diagnosing OSA at present is to have the patient undergo a sleep study, known as polysomnography.
A sleep technician administers and attends the study. To prepare the patient for 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 the stages of sleep to the appropriate depth and if the patient is being aroused excessively 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 where 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 associated with the 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.
Obstructive sleep apnea is diagnosed if the patient has an apnea index greater than 5, that is, has more than five apneic episodes per hour, or a respiratory disturbance index (RDI), the combination of apneas and hypopneas, greater than 10 per hour. In the appropriate clinical setting, sleep apnea can be diagnosed by an RDI between 5 and 10. Experts disagree somewhat on precisely where the diagnostic threshold lies, so a reliable diagnosis needs to be made in the context of the individual. Furthermore, the criteria are even less precise in children, making an individual approach to diagnosis even more important.
Clinically speaking, an obstructive apnea is defined as a complete cessation of airflow for more than 10 seconds with persistent respiratory effort. An obstructive hypopnea is defined as a partial reduction in air flow of approximately 30% to 50% with persistent respiratory effort and a reduction in oxygen saturation by at least 3% to 4% and/or an arousal from sleep.
The many physiological measurements taken usually enable the physician to diagnose or reasonably exclude OSA. Sometimes, however, a patient does not sleep long enough to obtain all the data needed. Polysomnography can not provide data from patients who have mild OSA only at home or only after using certain medications or alcohol but who do not experience any episodes during the sleep study. Therefore, a polysomnogram must be interpreted with the entire clinical picture in mind.
Another condition, called upper airway resistance syndrome, cannot be seen on polysomnography. This syndrome is characterized by repetitive arousals from sleep that probably result from increasing respiratory effort during narrowing of the upper airway. These patients suffer the same sleep disruption and deprivation as other sleep apnea patients. In such cases, the only abnormality that appears on the polysomnogram is recurrent arousal. It is possible to measure an increase in the negative pressure exerted by the patient in an effort to breathe against increasing resistance as the airway narrows. This measurement of inspiratory pressure is not usually done during a standard polysomnogram. Patients with upper airway resistance syndrome, therefore, constitute a group whose OSA could be easily missed by the polysomnogram.
Because polysomnography is expensive and labor intensive, efforts are underway to find a better method of diagnosing or screening for OSA. The only alternative at present is a procedure called overnight oximetry, which measures a patient's oxygen saturations throughout the night. Overnight oximetry is not considered completely adequate as a screening test, however, as the oxygen levels in the blood of many patients with OSA do not provide the information needed to understand their condition.