Polysomnography (PSG), also known as a
sleep study, is a
multi-parametricIn mathematics, statistics, and the mathematical sciences, a parameter is a quantity that serves to relate functions and variables using a common variable when such a relationship would be difficult to explicate with an equation...
test used in the study of
sleepSleep is a naturally recurring state of relatively suspended sensory and motor activity, characterized by total or partial unconsciousness and the inactivity of nearly all voluntary muscles. It is distinguished from quiet wakefulness by a decreased ability to react to stimuli, and it is more easily...
and as a diagnostic tool in
sleep medicineSleep medicine is a medical subspecialty devoted to the diagnosis and therapy of sleep disturbances and disorders. From the middle of the 20th century, research has provided increasing knowledge and answered many questions about sleep-wake functioning...
. The test result is called a
polysomnogram, also abbreviated PSG. The name is derived from
GreekGreek , an independent branch of the Indo-European family of languages, is the language of the Greeks. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. In its ancient form, it is the language of classical...
and Latin roots: the Greek 'poly' for multi-channel (many), the Latin 'somnus' (sleep), and the Greek 'graphein' (to write).
Polysomnography is a comprehensive recording of the biophysiological changes that occur during sleep. It is usually performed at night, when most people sleep, though some labs can accommodate shift workers and people with
circadian rhythm sleep disorderCircadian rhythm sleep disorders are a family of sleep disorders affecting, among other things, the timing of sleep. People with circadian rhythm sleep disorders are unable to sleep and wake at the times required for normal work, school, and social needs. They are generally able to get enough sleep...
s and do the test at other times of day. The PSG monitors many body functions including
brainThe brain is the center of the nervous system in all vertebrate, and most invertebrate, animals. Some primitive animals such as jellyfish and starfish have a decentralized nervous system without a brain, while sponges lack any nervous system at all...
(
EEGElectroencephalography is the recording of electrical activity along the scalp produced by the firing of neurons within the brain . In clinical contexts, EEG refers to the recording of the brain's spontaneous electrical activity over a short period of time, usually 20–40 minutes, as recorded from...
),
eyeEyes are organs that detect light, and send electrical impulses along the optic nerve to the visual and other areas of the brain. Complex optical systems with resolving power have come in ten fundamentally different forms, and 96% of animal species possess a complex optical system...
movements (
EOGElectrooculography is a technique for measuring the resting potential of the retina. The resulting signal is called the electrooculogram. The main applications are in ophthalmological diagnosis and in recording eye movements...
), muscle activity or
skeletal muscleSkeletal muscle is a form of striated muscle tissue existing under control of the somatic nervous system. It is one of three major muscle types, the others being cardiac and smooth muscle. As its name suggests, skeletal muscle is linked to bone by bundles of collagen fibers known as...
activation (
EMGElectromyography is a technique for evaluating and recording the activation signal of muscles. EMG is performed using an instrument called an electromyograph, to produce a record called an electromyogram. An electromyograph detects the electrical potential generated by muscle cells when these...
) and
heartThe heart is a muscular organ found in all vertebrates that is responsible for pumping blood throughout the blood vessels by repeated, rhythmic contractions...
rhythm (
ECGElectrocardiography is a transthoracic interpretation of the electrical activity of the heart over time captured and externally recorded by skin electrodes. It is a noninvasive recording produced by an electrocardiographic device...
) during sleep. After the identification of the
sleep disorderA sleep disorder is a medical disorder of the sleep patterns of a person or animal. Some sleep disorders are serious enough to interfere with normal physical, mental and emotional functioning...
sleep apnea in the 1970s, the breathing functions
respiratoryIn animal physiology, respiration is the transport of oxygen from the outside air to the cells within tissues, and the transport of carbon dioxide in the opposite direction...
airflow and respiratory effort indicators were added along with peripheral
pulse oximetryA pulse oximeter is a medical device that indirectly measures the oxygen saturation of a patient's blood and changes in blood volume in the skin, producing a photoplethysmograph. It is often attached to a medical monitor so staff can see a patient's oxygenation at all times...
.
Indications
Polysomnography is used to diagnose, or rule out, many types of
sleep disorderA sleep disorder is a medical disorder of the sleep patterns of a person or animal. Some sleep disorders are serious enough to interfere with normal physical, mental and emotional functioning...
s including narcolepsy, restless legs syndrome, REM behavior disorder, parasomnias, and sleep apnea. It is often ordered for patients with complaints of daytime fatigue or sleepiness that may be caused by interrupted sleep. Although it is not directly useful in diagnosing circadian rhythm sleep disorders, it may be used to rule out other sleep disorders.
Increasingly, polysomnography is being supplemented or replaced by
ActigraphyActigraphy is a relatively non-invasive method of monitoring human rest/activity cycles. A small actigraph unit, also called an actimetry sensor, is worn by a patient to measure gross motor activity. Motor activity often under test is that of the wrist, measured by an actigraph in a...
in cases where longitudinal or large scale data sets need to be generated, or when PSG is not a cost-efficient option.
Mechanism
A polysomnogram will typically record a minimum of eleven channels requiring a minimum of 22 wire attachments to the patient. Two channels are for the EEG, one or two measure airflow, one is for chin movements, one or more for leg movements, two for eye movements (EOG), one for heart rate and rhythm, one for oxygen saturation and one each for the belts which measure chest wall movement and upper abdominal wall movement.
Wires for each channel of recorded data lead from the patient and converge into a central box, which in turn is connected to a computer system for recording, storing and displaying the data. During sleep the computer monitor can display multiple channels continuously. In addition, most labs have a small video camera in the room so the technician can observe the patient visually from an adjacent room.
The electroencephalogram (EEG) will generally use six "exploring" electrodes and two "reference" electrodes, unless a seizure disorder is suspected, in which case more electrodes will be applied to document the appearance of seizure activity. The exploring electrodes are usually attached to the scalp near the frontal, central (top) and occipital (back) portions of the brain via a paste that will conduct electrical signals originating from the neurons of the cortex. These electrodes will provide a readout of the brain activity that can be "scored" into different stages of sleep (N1, N2, N3 which combined are referred to as NREM sleep, and Stage R which is rapid eye movement sleep or REM, and Wakefulness).
The electrooculogram (EOG) uses two electrodes; one that is placed 1 cm above the outer
canthusCanthus is either corner of the eye where the upper and lower eyelids meet.The bicanthal plane is the transversal plane linking both canthi and defines the upper boundary of the midface.-Commissures:...
of the right eye and one that is placed 1 cm below the outer canthus of the left eye. These electrodes pick up the activity of the eyes in virtue of the electropotential difference between the cornea and the retina (the cornea is positively charged relative to the retina). This determines when REM sleep occurs, of which rapid eye movements are characteristic, and also essentially aids in determining when sleep occurs.
The electromyogram (EMG) typically uses four electrodes to measure muscle tension in the body as well as to monitor for an excessive amount of leg movements during sleep (which may be indicative of
Periodic Limb Movement DisorderPeriodic Limb Movement Disorder , previously known as Nocturnal myoclonus, is a sleep disorder where the patient moves limbs involuntarily during sleep, and has symptoms or problems related to the movement....
, PLMD). Two leads are placed on the chin with one above the jaw line and one below. This, like the EOG, helps determine when sleep occurs as well as REM sleep. Sleep generally includes relaxation and so a marked decrease in muscle tension occurs. A further decrease in skeletal muscle tension occurs in REM sleep. A person becomes partially paralyzed to make acting out of dreams impossible, although people that do not have this paralysis can suffer from REM Behavior Disorder. Finally, two more leads are placed on the
anterior tibialisIn human anatomy, the tibialis anterior is a muscle that originates in the upper two-thirds of the lateral surface of the tibia and inserts into the medial cuneiform and first metatarsal bones of the foot...
of each leg to measure leg movements.
Though a typical electrokardiogram (ECG or EKG) would use ten electrodes, only two or three are used for a polysomnogram. They can either be placed under the collar bone on each side of the chest, or one under the collar bone and the other six inches above the waist on either side of the body. These electrodes measure the electrical activity of the heart as it contracts and expands, recording such features as the "P" wave, "QRS" complex, and "T" wave. These can be analyzed for any abnormalities that might be indicative of an underlying heart pathology.
Nasal and oral airflow can be measured using pressure transducers, and/or a thermocouple, fitted in or near the nostrils; the pressure transducer is considered the more sensitive. This allows the clinician/researcher to measure rate of respiration and identify interruptions in breathing. Respiratory effort is also measured in concert with nasal/oral airflow by the use of belts. These belts expand and contract upon breathing effort.
Pulse oximetryPulse oximetry is a non-invasive method allowing the monitoring of the oxygenation of a patient's hemoglobin.A sensor is placed on a thin part of the patient's anatomy, usually a fingertip or earlobe, or in the case of a neonate, across a foot, and a light containing both red and infrared...
helps determine changes in blood oxygen levels that often occur with sleep apnea and other respiratory problems. The pulse oximeter fits over a finger tip or an ear lobe.
SnoringSnoring is the vibration of respiratory structures and the resulting sound, due to obstructed air movement during breathing while sleeping. In some cases the sound may be soft, but in other cases, it can be rather loud and quite unpleasant. Generally speaking, the structures involved are the uvula...
may be recorded with a sound probe over the neck, though more commonly the sleep technician will just note snoring as "mild", "moderate" or "loud" or give a numerical estimate on a scale of 1 to 10.
Procedure
For the standard test the patient comes to a sleep lab in the early evening, and over the next 1-2 hours is introduced to the setting and "wired up" so that multiple channels of data can be recorded when he/she falls asleep. The sleep lab may be in a hospital, a free-standing medical office, or in a hotel. A sleep technician should always be in attendance and is responsible for attaching the electrodes to the patient and monitoring the patient during the study.
During the study, the technician observes sleep activity by looking at the video monitor and the computer screen that displays all the data second by second. In most labs the test is completed and the patient is discharged home by 7 a.m. unless a
Multiple Sleep Latency TestThe Multiple Sleep Latency Test is a sleep disorder diagnostic tool. It is used to measure the time it takes from the start of a daytime nap period to the first signs of sleep, called sleep latency...
(MSLT) is to be done during the day to test for
excessive daytime sleepinessExcessive daytime sleepiness is characterized by persistent sleepiness, and often a general lack of energy, even after apparently adequate night time sleep. Sudden involuntary sleep onset, and microsleeps are common complications...
.
Interpretation
After the test is completed a 'scorer' analyzes the data by reviewing the study in 30 second 'epochs'.
The score consists of the following information:
- Onset of sleep from time the lights were turned off; this is called 'sleep onset latency
In sleep science, sleep onset latency is the length of time that it takes to accomplish the transition from full wakefulness to sleep, normally to the lightest sleep stage non-REM....
' and normally is less than 20 minutes. (Note that determining 'sleep' and 'awake' is based solely on the EEG. Patients sometimes feel they were awake when the EEG shows they were sleeping.)
- Sleep efficiency: the number of minutes of sleep divided by the number of minutes in bed. Normal is approximately 85 to 90% or higher.
- Sleep stages; these are based on 3 sources of data coming from 7 channels: EEG (4 channels usually), EOG (2) and chin EMG (1). From this information each 30-second epoch is scored as 'awake' or one of 5 sleep stages: 1, 2, 3, 4 and REM or Rapid Eye Movement sleep. Stages 1–4 are together called non-REM
The sleep stages 1 through 3, previously known as stages 1 through 4, are collectively referred to as NREM, non-rapid eye movement, sleep. Rapid eye movement is not included. There are distinct electroencephalographic and other characteristics seen in each stage. Unlike REM sleep, there is...
sleep. Non-REM sleep is distinguished from REM sleep, which is altogether different. Within non-REMThe sleep stages 1 through 3, previously known as stages 1 through 4, are collectively referred to as NREM, non-rapid eye movement, sleep. Rapid eye movement is not included. There are distinct electroencephalographic and other characteristics seen in each stage. Unlike REM sleep, there is...
sleep, stages 3 and 4 are called "slow wave" sleep because of the relatively wide brain waves compared to other stages; another name for stages 3 and 4 is 'deep sleep'. By contrast, stage 1 and 2 are 'light sleep.'. The figures show Stage 4 sleep and REM sleep; each figure is a 30-second epoch from an overnight PSG.
is Stage 2. REM normally occupies about 20-25% of sleep time. Many factors besides age can affect both the amount and percentage of each sleep stage, including drugs (particularly anti-depressants and pain meds), alcohol taken before bed time, and sleep deprivation.)
- Any breathing irregularities; mainly apneas and hypopneas. Apnea is a complete or near complete cessation of airflow for at least 10 seconds followed by an arousal and/or 3% oxygen desaturation; hypopnea is a 50% decrease in airflow for at least 10 seconds followed by an arousal and/or 3% oxygen desaturation.
- 'Arousals' are sudden shifts in brain wave activity. They may be caused by numerous factors, including breathing abnormalities, leg movements, environmental noises, etc. An abnormal number of arousals indicates 'interrupted sleep' and may explain a person's daytime symptoms of fatigue and/or sleepiness.
- Cardiac rhythm abnormalities
- Body position during sleep
- Oxygen saturation during sleep
Once scored, the test recording and the scoring data are sent to the sleep medicine physician for interpretation. Ideally, interpretation is done in conjunction with the medical history, a complete list of drugs the patient is taking, and any other relevant information that might impact the study such as napping done before the test.
Once interpreted, the sleep physician writes a report which is sent to the referring physician, usually with specific recommendations based on the test results.
Example of summary report
'Split night' study
The above report mentions CPAP as treatment for obstructive
sleep apneaSleep apnea is a sleep disorder characterized by pauses in breathing during sleep. Each episode, called an apnea , lasts long enough so that one or more breaths are missed, and such episodes occur repeatedly throughout sleep...
. CPAP is continuous positive airway pressure, and is delivered via a tight fitting mask to the patient's nose or nose & mouth (some masks cover one, some both). CPAP is typically prescribed after the diagnosis of OSA is made from a sleep study (i.e., after a PSG test). To determine the correct amount of pressure, the right mask size, and also to make sure the patient is tolerant of this therapy, a 'CPAP titration study' is recommended. This is the same as a 'PSG', but with the addition of the mask applied, so the technician can increase the airway pressure inside the mask as needed, until all (or most all) of the patient's airway obstructions are eliminated.
The above report recommends Mr.J---- return for a CPAP titration study, which means return to the lab for a 2nd all night PSG (this one with the mask applied). Often, however, when a patient manifests OSA in the first 2 or 3 hours of the initial PSG, the technician will interrupt the study and apply the mask right then and there; the patient is woken up and fitted for a mask. The rest of the sleep study is then a 'CPAP titration.' When both the diagnostic PSG and a CPAP titration are done the same night, the entire study is called 'Split Night'.
The advantages of the split night study are: 1) the patient only has to come to the lab once, so it is less disruptive than coming two different nights; 2) it is 'half as expensive' to whomever is paying for the study. The disadvantages of a split night study are 1) less time to make a diagnosis of OSA (Medicare requires a minimum of 2 hours of diagnosis time before the mask can be applied); and 2) less time to assure an adequate CPAP titration. If the titration is begun with only a few hours of sleep left, the remaining time may not assure a proper CPAP titration, and the patient may still have to return to the lab.
Because of costs, more and more studies for 'sleep apnea' are attempted as split night when there is early evidence for OSA. Note that both types of study - with and without a CPAP mask - are still polysomnograms. When the CPAP mask is worn, however, the flow measurement lead in the patient's nose is removed, and a wire coming directly from the mask then measures air flow.
Example of summary report from a 'split night' study
External links