Obstructive sleep apnea, or OSA, is a condition where breathing repeatedly stops and starts during sleep. This occurs because the muscles in the back of the throat relax and block the upper airway, either partially or completely. To diagnose this condition, doctors use a comprehensive test called polysomnography (PSG). Commonly known as a sleep study, polysomnography is designed to monitor a variety of bodily functions overnight to identify sleep-related breathing disorders.
The Polysomnography Procedure
An in-lab polysomnography is conducted overnight in a sleep center or hospital, which is set up to be comfortable and private, much like a hotel room. Before you go to sleep, a technologist will attach a number of sensors to your body. These sensors are non-invasive and are applied using mild adhesives. The wires connecting the sensors to the monitoring equipment are long enough to allow you to move around and turn over in bed during the night.
Electrodes are placed on your scalp to record brain wave activity (electroencephalogram or EEG), which helps determine when you are in light, deep, or REM sleep. Other sensors are placed near your eyes to record eye movements (electrooculography or EOG) and on your chin to monitor muscle activity (electromyography or EMG). These also help identify different sleep stages.
To monitor your breathing, elastic belts are placed around your chest and abdomen to measure respiratory effort. A small tube called a nasal cannula is positioned under your nose to measure airflow, and a microphone records snoring. Additionally, a small clip called a pulse oximeter is placed on your finger to continuously monitor the oxygen level in your blood. If you need to use the restroom, you can notify the technologist, who will disconnect the wires as needed.
Key Metrics Measured for Obstructive Sleep Apnea
The data provides metrics to identify the presence and severity of obstructive sleep apnea. The primary events monitored are apneas and hypopneas. An apnea is a near-complete or complete pause in breathing that lasts for at least 10 seconds. A hypopnea is a partial blockage of the airway that results in a significant decrease in airflow, also for at least 10 seconds, leading to shallow breathing.
Technicians count every apnea and hypopnea event, analyzing their duration and frequency. The test documents how often breathing is disrupted per hour of sleep. These respiratory events are directly linked to blood oxygen saturation.
When an apnea or hypopnea occurs, it often causes a drop in blood oxygen, known as a desaturation. The American Academy of Sleep Medicine recommends scoring a hypopnea when there is at least a 30% drop in airflow accompanied by a 3% or greater oxygen desaturation or an arousal from sleep.
Interpreting the Results and Diagnosis
After the sleep study is complete, a sleep specialist interprets the collected data to make a diagnosis. The most significant metric is the Apnea-Hypopnea Index (AHI). The AHI is calculated by adding the total number of apneas and hypopneas and dividing that number by the total hours of sleep to get an average number of respiratory events per hour.
The AHI score is used to classify the severity of obstructive sleep apnea in adults. A score of fewer than five events per hour is considered normal. An AHI between 5 and 14 indicates mild sleep apnea. A score of 15 to 29 is classified as moderate sleep apnea, while an AHI of 30 or more signifies severe sleep apnea.
A diagnosis is not based solely on the AHI number. The physician will also consider your reported symptoms, such as excessive daytime sleepiness, loud snoring, or witnessed breathing pauses.
In-Lab Study Versus Home Sleep Apnea Test
While in-lab polysomnography is the standard for diagnosing sleep disorders, a Home Sleep Apnea Test (HSAT) is another option in certain situations. An HSAT is a simplified version of a sleep study that you can do in your own home. These tests use fewer sensors, monitoring breathing, respiratory effort, and blood oxygen levels.
HSATs are prescribed for individuals who have a high probability of having moderate to severe obstructive sleep apnea and do not have other significant medical conditions. The convenience and lower cost make it an accessible first step for many. However, because an HSAT does not measure brain waves, it cannot determine sleep stages or detect arousals from sleep.
An in-lab PSG is used when an HSAT result is negative or inconclusive, but the clinical suspicion for OSA remains high. It is also the preferred method for diagnosing mild OSA or for patients with coexisting medical conditions or other suspected sleep disorders that require more detailed monitoring.