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Polysomnography: Overview and Clinical Application

Author: Carmel Armon, MD, MSc, MHS, Professor of Neurology, Tufts University School of Medicine; Chief, Division of Neurology, Baystate Medical Center
Coauthor(s): Asim Roy, MD, Assistant Clinical Professor of Neurology, Tufts University School of Medicine; Consulting Staff, Division of Neurology, Baystate Medical Center; William J Nowack, MD, Associate Professor, Epilepsy Center, Department of Neurology, University of Kansas Medical Center
Contributor Information and Disclosures

Updated: Mar 30, 2007

Introduction

Nocturnal, laboratory-based polysomnography (PSG) is the most commonly used test in the diagnosis of obstructive sleep apnea syndrome (OSAS). It is often considered the criterion standard for diagnosing OSAS, determining the severity of the disease, and evaluating various other sleep disorders that can exist with or without OSAS. PSG consists of a simultaneous recording of multiple physiologic parameters related to sleep and wakefulness (see Image 1). PSG can directly monitor and quantify the number of respiratory events (ie, obstructive, central, or complex) and the resultant hypoxemia and arousals related to the respiratory events or even independent of the respiratory events.

A single-night PSG is usually adequate to determine if OSAS is present and the degree of the disorder. However, night-to-night variability may exist in patients who have a high probability but a low apnea index. In addition, variability in laboratory equipment, scoring technique, and interscorer reliability may also play roles. As is well known, PSG scoring also usually varies from laboratory to laboratory.

PSG is used to evaluate abnormalities of sleep and/or wakefulness and other physiologic disorders that have an impact on or are related to sleep and/or wakefulness.

Parameters Monitored

Assessment of sleep stages requires 3 studies: electroencephalography (EEG), electrooculography (EOG), and surface electromyography (EMG).

One EEG channel (central channel with an ear reference provides the best amplitude) is used to monitor sleep stage. However, most laboratories use 2 central channels and 2 occipital channels, with ear references as an adjunct to help identify sleep latency and arousals. A 10- to 20-electrode placement system is used to determine the location of these channels. Additional EEG channels can be used, particularly in patients with epilepsy (ie, a full 10-20 montage).

Two EOG channels are used to monitor both horizontal and vertical eye movements. Electrodes are placed at the right and left outer canthi, one above and one below the horizontal eye axis. The electrodes pick up the inherent voltage within the eye; the cornea has a positive charge and the retina has a negative charge. Evaluation of the eye movements is necessary for 2 reasons. First is for documentation of the onset of rapid eye movement (REM) sleep, and second is to note the presence of slow-rolling eye movements that usually accompany the onset of sleep.

One EMG channel (usually chin or mentalis and/or submentalis) is used to record atonia during REM sleep or lack of atonia in patients with REM-related parasomnias. To assess bruxism, the EMG electrodes can be placed over the masseter. The EMG recording from other muscle groups is assessed for other sleep disorders. For example, the anterior tibialis EMG is helpful for assessing periodic limb movements during sleep and the intercostal EMG is used as adjunctive help for determining effort during respiratory events.

Other parameters that can be monitored in a sleep study include the following:

• Airflow (nasal and/or oral)
• Electrocardiography
• Pulse oximetry
• Respiratory effort (thoracic and abdominal)
• Sound recordings to measure snoring
• Continuous video monitoring of body positions

Optional parameters that can be monitored in a sleep study include the following:

• Core body temperature
• Incident light intensity
• Penile tumescence
• Pressure and pH at various esophageal levels

Procedures

In 1992, the Office of Technology Assessment of the Agency of Health Care Policy and Research recommended, in an evidence-based assessment, 2 tests as having been studied sufficiently. Both tests are performed in a sleep laboratory. The first is overnight PSG, which is an overnight recording of the patient's sleep. The second is multiple sleep latency testing (MSLT), which records multiple naps throughout a day (usually four 20-min naps separated by 90 min).

Standard sleep studies usually use the overnight PSG (may be performed over several nights). If daytime sleepiness is an issue and cannot be fully explained by the overnight study results, an MSLT should be performed the next day. Limitations usually stem from the fact that recording conditions may not reflect what happens during a regular night in the patient's home.

Although diagnosing a sleep problem on the basis of a recording over a single night is common practice, some authorities caution that more than one night of recording may be necessary so the patient can become comfortable with unfamiliar surroundings and sleep more naturally. This effect is greatest on the first night in the sleep laboratory (ie, first-night effect).

Sporadic events may be missed with a single-night PSG. External factors that disturb the subject's sleep may be present in the home but absent from the controlled environment of the sleep laboratory.

Patient preparation is important so that the patient sleeps naturally. Patient instructions include the following:

• Maintain regular sleep-wake rhythm
• Avoid sleeping pills
• Avoid alcohol
• Avoid stimulants, including medications for narcolepsy
• Avoid strenuous exercise on the day of the PSG

High costs and long waiting lists have prompted the exploration of alternative methods of evaluation. Although the following studies may have usefulness in specific clinical situations, Bloch concludes that their role compared with conventional sleep studies remains controversial.

• Ambulatory monitoring with portable equipment
• Daytime PSG
• Simplified sleep studies with limited subsets of monitored parameters

Staging of Sleep

EEG background

• Alpha EEG
  • Frequency of 8-13 cps (cycles per second)
  • Produced in occipital region
  • Crescendo-decrescendo appearance
• Theta EEG
  • Frequency of 3-7 cps
  • Produced in the central vertex region
  • No amplitude criteria
  • Most common sleep frequency
• Delta EEG
  • Frequency of 0.5-2 cps
  • Clinical EEG frequency of greater than 0.5-4 cps
  • Seen predominantly in frontal region
  • Amplitude of greater than 75 mN

Sleep spindle

• Frequency of 12-14 cps
• Produced in central-vertex region
• Greater than 0.5-3 seconds in duration
• 0.5-second spindles with 6-7 cycles
• Indicative of stage 2 sleep

K complexes

• Sharp, slow waves with a negative, then positive, deflection
• No amplitude criteria
• Duration must be at least 0.5 seconds
• Predominantly produced in central-vertex region
• Indicative of stage 2 sleep
• May occur with or without stimuli

Awake stage

• Greater than 50% of each epoch contains alpha activity
• Slow-rolling eye movements or eye blinks seen in EOG channels
• Relatively high EMG muscle tone

Stage 1

• Greater than 50% of the epoch contains theta activity (3-7 cps)
• Alpha activity possible within less than 50% of the epoch
• Slow-rolling eye movements in EOG channels
• Relatively high submental EMG tone

Stage 2

• Theta activity (3-7 cps)
• K-complexes and sleep spindles occur episodically
• High tonic submental EMG

Stage 3

• Accounts for 20-50% of each epoch and must contain delta activity
• Submental muscle tone may be slightly reduced

Stage 4

• Greater than 50% of the epoch has scorable delta activity
• Submental EMG activity slightly reduced from that of light sleep

REM sleep

• Rapid eye movements
• Mixed frequency EEG (similar to awake pattern)
• Low tonic submental EMG
• May see saw-tooth waves

Respiratory Events and Leg Movement Scoring

Basic rules

• All respiratory events counted are at least 10 seconds in duration
• All events need to have at least a 3% or greater oxygen saturation (SaO₂) decrease
• EEG arousals occur with most respiratory events

Obstructive apnea

• No airflow for greater than 10 seconds
• Increasing respiratory effort; usually seen as paradoxical
• SaO₂ decrease greater than 3% (may be adjusted)

Hypopnea

• Reduction in airflow to approximately 50% of baseline value
• SaO₂ decrease of greater than 3%
• Usually is a steadily increasing effort signal and usually is associated with arousal

Mixed apnea

• Complete absence of nasal and oral airflow
• Total absence of respiratory effort at the beginning of the event, followed by a gradual increase in effort, which eventually breaks the apnea (usually paradoxical)
• SaO₂ decrease of greater than 3%

Central apnea

• Absence of airflow at nose and mouth for greater than 10 seconds
• Complete absence of respiratory effort
• SaO₂ decrease of greater than 3%

Periodic limb movement

• Each jerk must be greater than 0.5 seconds but less than 5 seconds in duration
• Each jerk must be greater than 0.25 amplitude of physiological calibration
• Must have 4 jerks separated by no more than 90 seconds (Some protocols allow for 120-second event variability.)

Summary

Standard analysis still consists of reviewing each of the parameters recorded. Overnight parameters (eg, times of lights on/off, total time in bed, total sleep time, sleep latency, REM latency) are collected. The overnight recording is divided into epochs of approximately 30 seconds. The standard EEG, EMG, and EOG recordings are evaluated, and the predominant stage of sleep (according to the manual of Rechtschaffen and Kales) is then assigned to the entire epoch.

Total time and relative proportion of the night spent in each of the stages and in REM and non-REM sleep are calculated. Latencies to REM and slow-wave sleep are reported.

Stages of sleep, any abnormalities noted with EEG, and periodic limb movements are reported. Respiratory activity (eg, apneic or hypopneic episodes, oxygen desaturations) is correlated with sleep stages. Other parameters, such as body position, are recorded. If needed, esophageal pH or penile tumescence can also be recorded.

If a sleep apnea syndrome is diagnosed, the patient undergoes a trial and titration of positive airway pressure either (1) in a partial-night PSG titration study if he or she meets criteria based on individual laboratory criteria (generally, apnea-hypopnea index >30) or (2) in a full-night PSG titration study.

Disorders Evaluated With Polysomnography

Dyssomnias (disorders of initiating or maintaining sleep)

• Circadian rhythm disorders
• Narcolepsy
• Idiopathic hypersomnia
• Inadequate sleep hygiene
• Sleep-related respiratory disorders
  • Sleep apnea syndrome
  • Upper airway resistance syndrome

Parasomnias

• Disorders of arousal
• Disorders of sleep-wake transition
• Disorders that occur during REM sleep
  • Nightmares
  • REM behavior disorder
• Medical-psychiatric sleep disorders
  • Medical - Sleep-related asthma
  • Psychiatric - Depression, panic disorder
  • Neurologic - Sleep-related epilepsy
• Others
  • Bruxism
  • Restless legs syndrome and periodic limb movement disorder

Treatment

Treatment is determined by the disorder diagnosed using PSG and/or MSLT.

Patient education

For excellent patient education resources, visit eMedicine's Sleep Disorders Center. In addition, see eMedicine's patient education article Disorders That Disrupt Sleep (Parasomnias).

Multimedia

(Enlarge Image)

Media file 1: Typical polysomnogram tracing. The burst of electromyogram activity recorded from the left tibialis anterior muscle was caused by a periodic movement of sleep.

Keywords

PSG, polysomnogram, sleep study, rapid eye movement study, REM study, sleep, wakefulness, multiple sleep latency testing, MSLT, electroencephalography, EEG, electrooculogram, EOG, electromyography, EMG, overnight polysomnography, oPSG, REM sleep, slow-wave sleep, SWS, non-REM sleep, dyssomnias, parasomnias, sleep disorders, abnormalities of sleep, obstructive sleep apnea, sleep disordered breathing, central sleep apnea, mixed sleep apnea, complex sleep apnea

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