Nocturnal polysomnography, commonly known as a sleep study, is a comprehensive overnight test used to evaluate sleep patterns and diagnose various sleep disorders. It monitors several physiological functions while an individual sleeps in a controlled environment, providing detailed information about sleep architecture and any disturbances.
Purpose of a Sleep Study
A physician may recommend an overnight sleep study when an individual experiences symptoms suggestive of a sleep disorder. Loud, disruptive snoring is a common indicator, especially when accompanied by observed pauses in breathing or gasping for air during sleep, reported by a bed partner or family member. These breathing interruptions can lead to fragmented sleep.
Individuals often report persistent excessive daytime sleepiness, despite believing they have slept sufficiently. This fatigue can manifest as difficulty concentrating, irritability, or even falling asleep during quiet activities. Waking up with morning headaches or experiencing difficulty staying asleep through the night are additional symptoms that prompt a sleep study referral. The study also helps investigate unusual movements or behaviors that occur during sleep, known as parasomnias, to determine their origin.
The Polysomnography Procedure
Preparation for a sleep study involves a few simple steps to ensure accurate results. Patients are advised to avoid caffeine and alcohol on the day of the study and to skip naps. It is also recommended to wash hair thoroughly without using gels, oils, or sprays, as these can interfere with sensor adhesion.
Patients arrive at the sleep center in the early evening, around two hours before their usual bedtime. After checking in, a sleep technologist guides them to a private room designed to resemble a comfortable bedroom. The technologist then painlessly attaches various sensors and electrodes to specific areas of the body, including the scalp, face (near the eyes and on the chin), chest, and legs. A small clip is also placed on a finger or earlobe.
The room is equipped with a low-light video camera and a microphone, allowing the technologist to monitor the patient from a separate control room throughout the night. Wires from the sensors are long enough to permit normal movement, including turning over in bed. If a patient needs assistance, such as for a bathroom break, they can communicate with the technologist, who will temporarily disconnect and reconnect the necessary wires.
Key Measurements Taken
During nocturnal polysomnography, several physiological parameters are simultaneously recorded to provide a detailed picture of sleep.
Brain Activity (EEG)
Electrodes placed on the scalp measure brain activity, known as an electroencephalogram (EEG). This helps identify different sleep stages, including light, deep, and rapid eye movement (REM) sleep, allowing specialists to assess sleep architecture and determine time spent in each stage.
Eye Movements (EOG)
Sensors around the eyes record eye movements (electrooculogram or EOG). These are important for pinpointing REM sleep, which is characterized by rapid eye movements.
Muscle Activity (EMG)
Electrodes on the chin and legs monitor muscle activity (electromyogram or EMG). The chin EMG helps differentiate sleep stages, noting the muscle relaxation typical of REM sleep. Leg electrodes detect involuntary limb movements, which can indicate conditions like Periodic Limb Movement Disorder.
Heart Rate (ECG)
Electrodes on the chest monitor heart rate and rhythm (electrocardiogram or ECG) throughout the night, identifying any irregularities that may occur during sleep.
Breathing Patterns
Breathing patterns are assessed using belts placed around the chest and abdomen, which measure respiratory effort. Airflow is detected by sensors positioned under the nose, such as a nasal cannula or thermistor, to identify any reductions or cessations of breathing.
Blood Oxygen Levels
A pulse oximeter, clipped to a finger, continuously measures the oxygen saturation levels in the blood. Significant drops in blood oxygen can signal breathing disturbances, such as apneas, where breathing repeatedly stops or becomes shallow.
Interpreting the Results and Diagnosis
Following the overnight study, a board-certified sleep specialist analyzes the extensive data collected from the polysomnogram. This analysis involves reviewing recorded brain waves, eye movements, muscle activity, heart rhythms, breathing patterns, and blood oxygen levels to understand the individual’s sleep architecture and identify deviations.
A primary metric for diagnosing sleep-related breathing disorders is the Apnea-Hypopnea Index (AHI). This index quantifies the average number of apneas (complete cessation of airflow) and hypopneas (significant reduction in airflow) events per hour of sleep. An AHI value below 5 events per hour is considered within a normal range.
An AHI between 5 and 14 events per hour indicates mild sleep apnea, while 15 to 29 events per hour suggests moderate sleep apnea. A diagnosis of severe sleep apnea is made when the AHI is 30 or more events per hour. The polysomnography findings, including the AHI, are integrated with the individual’s reported symptoms and medical history to arrive at a precise diagnosis.
Nocturnal polysomnography is used to definitively diagnose conditions such as Obstructive Sleep Apnea (OSA), where the airway repeatedly collapses during sleep, and Central Sleep Apnea (CSA), where the brain temporarily fails to send signals to breathing muscles. It aids in diagnosing Narcolepsy, which involves excessive daytime sleepiness and a premature entry into REM sleep. Periodic Limb Movement Disorder (PLMD) and REM Sleep Behavior Disorder (RBD), where individuals physically act out their dreams, are also identified.