Polysomnography

# Overview
Polysomnography or PSG is a multi-parametric test used in the study of sleep; the test is called a polysomnogram. The name is derived from Greek and Latin roots: 'Polys' (many), 'somnus' (sleep), and  'graphein' (to write).
Normal patterns of human sleep were first discovered by Loomis, Harvey and Hobart in 1935. In 1952, the discovery and documentation of REM (Rapid Eye Movement) sleep was accomplished by William C. Dement, Eugene Aserinsky, Nathaniel Kleitman and graduate students at the University of Chicago in the 1950s. It has since been codified by the adoption in 1968 of A Manual of Standardized Terminology, Techniques, and Scoring System for Sleep Stages of Human Subjects, edited by Allan Rechtschaffen and Anthony Kales.
Aserinsky and Kleitman's discovery of REM sleep was published Sept 4, 1953 (Aserinsky E, Kleitman N. Regularly Occurring Periods of Eye Motility, and Concomitant Phenomena, during Sleep. Science, 118:3062; 273-274).  Surprisingly, no other sleep laboratory findings were published until 1959.
Polysomnography is a comprehensive recording of the biophysiological changes that occur during sleep. The polysomnogram, or PSG, is usually performed at night, when most people sleep [[some labs can accommodate shift workers and do the test at other times).  The PSG monitors many body functions including brain (EEG), eye movements (EOG), muscle activity or skeletal muscle activation (EMG), heart rhythm (ECG), and breathing function or respiratory effort during sleep.
For a polysomnogram, the EEG will generally consist of four "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 central and occipitial 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 (1, 2, 3, 4, REM, and Wakefulness).

# What is Measured
The EOG or electrooculogram consists of two electrodes; one that is placed slightly out from and above the outer canthus of the right eye and one that is placed slightly out and below the outer canthus of the left eye. These electrodes will 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 essentially aids in determining when sleep occurs, as well as when REM Sleep occurs; of which rapid eye movements are characteristic.
The EMG or electromyogram 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 Disorder). 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. This is because when we fall asleep, we generally become more relaxed and so a marked decrease in muscle tension will occur. Also, we become partially paralyzed when we enter REM sleep so that we do not act out our dreams (of course people that do not have this paralysis can suffer from REM Behavior Disorder). Finally, two more leads are placed on the anterior tibialis of each leg to measure leg movements.
Though a typical ECG (or EKG for Electrokardiogram) would use twelve 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 record the electrical activity of the heart as it contracts and expands, resulting in several indentifiable features such as the "P" wave, "QRS" complex, and "T" wave that 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. This allows the clinician/researcher to measure rate of respiration or help to diagnose sleep apnea. 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 Oxymetry helps determine changes in blood oxygen levels that often occur with sleep apnea and other respiratory problems.
In the 1970s, respiratory airflow and respiratory effort indicators were added along with peripheral pulse oximetry following the identification of the sleep disorder Sleep apnea.  Polysomnography is used to diagnose many types of sleep disorders including narcolepsy, restless legs syndrome, REM behavior disorder, parasomnias, and sleep apnea.
Increasingly, polysomnography is being supplemented or replaced by Actigraphy in cases where longitudinal or large scale data sets need to be generated, or when PSG is not a cost-efficient option
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