Atlas of Sleep Medicine

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ATLAS OF SLEEP MEDICINE ISBN 0-7506-7398-2Copyright 2005, Elsevier, Inc. All rights reserved.

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BUTTERWORTHHEINEMANN

Jrme Argod, Ph.D.Department of Physiology and HP2 Laboratory (Hypoxia

Pathophysiology)Inserm EspriJoseph Fourier UniversitySleep LaboratoryClinical PhysiologyUniversity HospitalGrenoble, France

Alon Y. Avidan, M.D., M.P.H.Clinical Assistant ProfessorDepartment of NeurologyUniversity of Michigan Medical Center Director,Sleep Disorders ClinicSleep Disorders CenterAnn Arbor, Michigan

Meeta Bhatt, M.D., Ph.D.Assistant ProfessorDepartment of NeurologyNew York University Medical CenterDirectorNew York Sleep InstituteNew York, New York

Sudhansu Chokroverty, M.D., F.R.C.P., F.A.C.P.Professor and Co-Chair of NeurologyClinical Neurophysiology and Sleep MedicineNew Jersey Neuroscience Institute at JFK and

Seton Hall UniversityEdison, New Jersey

Deborah Dale, M.Sc.Department of Physiology and HP2 Laboratory (Hypoxia

Pathophysiology)Inserm EspriJoseph Fourier UniversitySleep LaboratoryClinical PhysiologyUniversity HospitalGrenoble, France

Tammy Goldhammer, R. EEGT, R. PSG-T, B.S.SupervisorDepartment of NeurologySaint Vincent Catholic Medical CenterNew York, New York

Timothy F. Hoban, M.D.Clinical Associate ProfessorDepartments of Pediatrics and NeurologyUniversity of MichiganAnn Arbor, Michigan

Patrick Lvy, M.D., Ph.D.Professor and HeadDepartment of Physiology and HP2 Laboratory (Hypoxia

Pathophysiology)Inserm EspriJoseph Fourier UniversityChiefSleep Laboratory and Department of Clinical

PhysiologyUniversity HospitalGrenoble, France

Contributors

viii Contributors

Pasquale Montagna, M.D.Professor of NeurologyDepartment of Neurological SciencesUniversity of Bologna Medical SchoolBologna, Italy

Liborio Parrino, M.D., Ph.D.University ResearcherDepartment of NeuroscienceUniversity of ParmaParma, Italy

Jean-Louis Ppin, M.D., Ph.D.Professor of Clinical PhysiologyPhysiology Department and HP2 Laboratory (Hypoxia

Pathophysiology)Inserm EspriJoseph Fourier UniversityClinical and Research PhysicianSleep Laboratory and Department of Respiratory

MedicineUniversity HospitalGrenoble, France

Stephen D. Pittman, MSBME, R. PSG-TResearch TechnologistDivision of Sleep MedicineBrigham and Womens HospitalBoston, Massachusetts

Arianna Smerieri, Ph.D.Department of Neuroscience and Sleep Disorders

CenterUniversity of ParmaParma, Italy

Mario Giovanni Teranzo, M.D.Professor of NeurologyDepartment of NeuroscienceUniversity of ParmaParma, Italy

Robert J. Thomas, M.D., M.M.Sc.Instructor in MedicineHarvard Medical SchoolInstructor in MedicineDepartment of Pulmonary, Critical Care and SleepBeth Israel Deaconess Medical CenterBoston, Massachusetts

Marco Zucconi, M.D.Assistant Professor of NeurologyDepartment of NeurologyVita-Salute San Raffaele UniversitySenior NeurologistSleep Disorders CenterHospital San RafaeleMilan, Italy

The importance of sleep has been reflected in the writings of Eastern and Western religions and civilizationsince time immemorial. The history of sleep research hasbeen a history of remarkable progress and remarkableignorance. In the 1940s and 1950s sleep had been in the forefront of neuroscience. Again in the 1990s there wasa resurgence in our understanding of the neurobiology ofsleep. Several textbooks of sleep medicine, including acouple of atlases, have been published attesting to suchgrowth. The basic knowledge in the text can be signifi-cantly augmented by including a number of illustrations,proving the old adage that a picture is worth a thousandwords. Hence the usefulness of an atlas encompassingsome textual materials accompanied by appropriate illus-trations that emphasize clinical-physiological correlationfor the sake of a correct diagnosis and treatment of a sleepdisorder. The best way to learn is to take a look at a tracing,identify the deviation from normal, and understand the sig-nificance in light of the clinical features. Laboratory tech-niques, particularly polysomnographic (PSG) study as wellas other related procedures, remain the cornerstone fordefinitive diagnosis of a number of sleep disorders. It mustbe remembered, however, that these laboratory techniquesmust be subservient to a careful evaluation of the clinicalhistory and physical examination. Thus, we have tried inthis atlas to provide correlation of the clinical features withthe physiologic findings as recorded by the PSG and otherlaboratory techniques. Our objective is to produce a com-prehensive and contemporary atlas illustrating numerousexamples of PSG and other tracings accompanied by suf-ficient clinical details so that the reader can formulate aclear view of the big picture.

The Atlas is divided into 13 chapters covering the tech-niques, clinical pattern recognition, continuous and bi-

level positive airway pressure titration (CPAP and BIPAP),and PSG changes, and a section on pediatric sleep medi-cine. Chapter 11, dealing with specialized techniques,includes recently introduced topics of pulse transit timeand peripheral arterial tonometry in addition to the recog-nition and usefulness of the cyclic alternating pattern(CAP), which has great potential for understanding themicrostructure of sleep.

The Atlas should be useful not only to all sleep special-ists in a variety of fields, including neurologists, pulmo-nologists, cardiologists and other internists, psychiatrists,psychologists, otolaryngologists, and dentists, but also toothers who may have an interest in advancing the practicalknowledge in sleep medicine, such as the EEG and PSGtechnologists, fellows, residents, and medical students.

We must express our gratitude to all the contributingauthors for their scholarly contributions. Wed like toacknowledge Susan Pioli, Executive Publisher of GlobalMedicine at Elsevier Science for her professionalism,helpful attitude and patience during all stages of produc-tion of the Atlas. We wish to thank also Laurie Anello atElsevier Science and the staff at Graphic World Publish-ing Services for their efforts in trying to speed up theprocess of publication. The senior editor would like toacknowledge his indebtedness to his wife, ManishaChokroverty, M.D., for her unfailing support, love andpatience during the long and arduous preparation of thisAtlas. Finally, all of us would like to thank our patients andthe trainees for motivating us to reach the highest level ofexcellence in patient care and education.

Sudhansu ChokrovertyRobert J. Thomas

Meeta Bhatt

Preface

ABD abdominal respiratory effortBiPAP bilevel positive airway pressureBKUP backup EMG channelCAPN capnogramCHIN chin EMGCMRR common mode rejection ratio CPAP continuous positive airway pressureEDS excessive daytime sleepinessEEG electroencephalogramEKG electrocardiogramEMG electromyogramEOG electrooculogramEPSP excitatory postsynaptic potentialsESS Epworth Sleepiness ScaleETCO2 average end-tidal CO2IPSP inhibitory postsynaptic potentialsLAT left anterior tibialis surface electromyogramLOC left electrooculogramMAs microarousalsMID mid-abdominal effortMSLT multiple sleep latency testMWT maintenance of wakefulness testN/O nasal and oral airflowNPRE nasal pressure transducer

OSA obstructive sleep apneaOSAS obstructive sleep apnea syndromePLEDS periodic or pseudoperiodic lateralized

epileptiform dischargesPES esophageal pressurePLMD periodic limb movement disorderPLMS periodic limb movements in sleepPSG polysomnogramPTT pulse transit timeRAT right anterior tibialis surface

electromyogramREM rapid eye movement sleepNREM non-rapid eye movement sleepROC right electrooculogramSaO2 hemoglobin oxygen saturationSNOR snore channelSNORE snore sensorSOREM sleep-onset REM periodSpO2 pulse oximetrySSS Stanford Sleepiness ScaleTIRDA temporal intermittent rhythmic delta activityTHOR thoracic respiratory effortUARES upper airway resistance episodesUARS upper airway resistance syndrome

Abbreviations

1Polysomnographic Recording Technique

SUDHANSU CHOKROVERTY, MEETA BHATT,

AND TAMMY GOLDHAMMER

Polysomnography (PSG) is the single most importantlaboratory technique for assessment of sleep and its disorders. PSG records multiple physiological charac-teristics simultaneously during sleep at night. These recordings allow assessment of sleep stages and wakeful-ness, respiration, cardiocirculatory functions, and bodymovements. Electroencephalography (EEG), electroocu-lography (EOG), and electromyography (EMG) of thechin muscles are recorded to score sleep staging. Respira-tory recording includes measurements of airflow and respiratory effort. PSG also records electrocardiography(EKG), finger oximetry, limb muscle activity, particularlyEMG of the tibialis anterior muscles bilaterally, snoring,and body positions. Special techniques, which are not usedroutinely, include measurements of intraesophageal pres-sure, esophageal pH, and penile tumescence for assess-ment of patient with erectile dysfunction. After recording,the data are then analyzed and interpreted by the sleep clinician. A single daytime nap study generally does notrecord rapid eye movement (REM) sleep, the stage inwhich most severe apneic episodes and maximum oxygendesaturation are noted. Hence, a daytime study cannotassess the severity of symptoms. There may be false-neg-ative studies as mild cases of sleep apnea may be missedduring daytime study. Furthermore, during continuouspositive airway pressure (CPAP) titration, an all night sleepstudy is essential to determine the optimum level of titra-tion pressure during both REM and non-REM (NREM)sleep.

It is important to remember that polysomnographicstudy is really an extension of the physical examination ofthe sleeping patient. Before the actual recording an ade-quate knowledge of patient preparation, laboratory envi-ronment, and some technical aspect of the equipment isneeded.

PATIENT PREPARATION ANDLABORATORY ENVIRONMENT

The technologist performing the study must have basicknowledge about the PSG equipment including the ampli-fiers, filters, sensitivities, and simple troubleshooting. Thetechnologist should also have an adequate knowledgeabout the PSG findings of important sleep disorders andmust have full clinical information so that he or she maymake the necessary protocol adjustments for the most effi-cient recording. On arrival in the laboratory, the patientshould be adequately informed about the entire procedureand should be shown through the laboratory. Ideally, thelaboratory should be located in an area that is free fromnoise and the room temperature should be optimal. Thesleeping room in the laboratory should be a comfortableroom, similar to the patients bedroom at home, so that itis easy for the patient to relax and fall asleep. The record-ing equipment should be in an adjacent room and thepatient should be advised about the audiovisual equip-ment. Some patients might feel comfortable if they areallowed to bring their own pillow as well as their pajamasand a book for reading.

Technical Considerations andPolysomnography Equipment

Equipment for recording PSG contains a series ofamplifiers with high- and low-frequency filters and differ-ent sensitivity settings. The amplifiers used consist of bothalternating current (AC) and direct current (DC) ampli-fiers. The AC amplifiers are used to record physiologicalcharacteristics showing high frequencies such as EEGs,EOGs, EMGs, and EKGs. The AC amplifier contains bothhigh- and low-frequency filters. Using appropriate filters,

1

2 Polysomnographic Recording Technique

one may record a specific band within the signal; for example, slow potentials can be attenuated by usinglow-frequency filters. DC amplifiers have no low-frequency filters and are typically used to record potentialswith slow frequency such as the output from the oximeter,the output from the pH meter, CPAP titration pressurechanges, and intraesophageal pressure readings. AC or DC amplifiers may be used to record respiratory flow and effort. An understanding of the amplifiers, along with the filters, and sensitivities is important for optimalPSG recording. The PSG equipment uses differentialamplifiers, which amplify the difference between the two amplifier inputs. The differential amplifier augmentsthe difference between two electrode inputs, which is an advantage because unwanted extraneous environmen-tal noise, which is likely to be seen at the two electrodes,is subtracted out and therefore cannot contaminate therecording. The ability of the amplifier to suppress an extraneous signal such as noise that is simultaneouslypresent in both electrodes is measured by the commonmode rejection ratio (CMRR). Ideally, this ratio mustexceed 1000 to 1, but most contemporary PSG amplifiersuse a ratio in excess of 10,000 to 1. The currents and volt-ages generated by the cerebral cortex, eyes, and heartduring the PSG recordings are extremely small andthrough amplification this tiny voltage is transformed intoan interpretable record by manipulating the sensitivityswitch. Sensitivity is expressed in microvolts per millime-ter or millivolts per centimeter. Sensitivity switches should be adjusted to obtain sufficient amplitude for interpretation. Sensitivity and filter settings vary accord-ing to the physiological characteristics recorded. (See Table 11.)

Following subtraction and amplification of the voltagesat the two inputs, the signal is then passed through a series of filters. Two types of filters are used in PSG record-ings: a high-pass filter (also known as a low-frequencyfilter) and a low-pass filter (also known as a high-frequencyfilter). A high-pass filter allows higher frequencies to pass unchanged while attenuating lower frequencies. A

low-pass filter, in contrast, allows lower frequencies to pass unchanged while attenuating higher frequencies. The other filter, present in most PSG amplifiers, is a 60-hertz notch filter, which attenuates main frequency whileattenuating activity of surrounding frequency less exten-sively. EEG recording is easily contaminated by 60-hertzartifact if the electrode application and impedance are suboptimal. The 60-hertz filter, however, should be usedsparingly because some important components in therecording, such as muscle activity and epileptiform spikes,may be attenuated by a notch filter. Most of the time the differential amplifier is sufficient to reject 60-hertz artifacts.

The standard speed for recording traditional PSG is 10 mm/sec, so that each monitor screen or page is a 30-second epoch. A 30-mm/sec recording speed is used for easy identification of epileptiform activities. Analogrecording using paper is being replaced in most of the lab-oratories by digital system recordings (see later).

It is important to have a facility for simultaneous videorecording to monitor behavior during sleep. It is advanta-geous to use two cameras to view the entire body. A lowlight level camera should be used to obtain good qualityvideo in the dark, and an infrared light source should beavailable after turning the laboratory lights off. The mon-itoring station should have remote control that can zoom,tilt, or pan the camera for adequate viewing. The camerashould be mounted on the wall across from the head endof the bed. An intercom from a microphone near thepatient should be available.

Electroencephalography

It is important for the polysomnographer and thepolysomnographic technologist to be familiar with theprinciples of EEG recording and should have adequateknowledge about normal waking and sleep EEG rhythmsin various age groups as well as major patterns of abnor-malities that may be encountered during PSG recording(see Section II).

Table 11. Filter and sensitivity settings for polysomnographic studies

High- Time Low-Frequency Constant Frequency

Characteristics Filter (Hz) (sec) Filter (Hz) Sensitivity

Electroencephalogram 70 or 35 0.4 0.3 57 mV/mmElectro-oculogram 70 or 35 0.4 0.3 57 mV/mmElectromyogram 90 0.04 5.0 23 mV/mmElectrocardiogram 15 0.12 1.0 1 mV/cm to start;

adjustAirflow and effort 15 1 0.1 57 mV/mm; adjust

Polysomnographic Recording Technique 3

Electrooculography

EOG records corneoretinal (relative positivity at thecornea and a relative negativity at the retina) potential dif-ference. Any eye movement changes the orientation of thedipole and it is the movement of the dipole that is recordedby the EOG. Gold cup or silver-silver chloride electrodescan be used to monitor the EOG. A typical electrodeplacement is 1 centimeter superior and lateral to the outercanthus of one eye with a second electrode placed one cminferior and lateral to the outer canthus of the oppositeeye. Both these electrodes are then connected to a singlereference electrode, either the same ear or the mastoidprocess of the temporal bone. Therefore, right outercanthus (ROC) and left outer canthus (LOC) electrodesare referred to as either A1 or A2. In this arrangement,conjugate eye movements produce out-of-phase deflec-tions in the two channels whereas the EEG slow activitiescontaminating the eye electrodes are in-phase. Both con-jugate horizontal and vertical eye movements are detectedby this placement scheme. The sensitivity and filter set-tings for EOG are similar to those used for EEG.

Several varieties of eye movements are recorded duringroutine PSG: waking eye movements (WEMs), slow eyemovements (SEMs), and REMs. During wakefulness botheye blinks and saccadic eye movements produce WEMs.During stage I sleep, SEMs are recorded consistently in the horizontal axis. SEMs generally disappear in thedeeper stages of NREM sleep. During REM sleep, char-acteristic REMs appear which are noted in all directions(horizontal, oblique, and vertical), although they are mostprominently seen in the horizontal axis. REMs typicallyoccur in bursts. In addition to finding these three types ofeye movements, Santamaria and Chiappa, using a sensitivemotion transducer, recorded small, fast, irregular eyemovements in about 60 percent of normal subjects in earlydrowsiness before the appearance of SEMs. They alsonoted small, fast, rhythmic eye movements generally asso-ciated with the traditional SEMs in about 30 percent ofnormal subjects.

Electromyography Recording DuringStandard Polysomnography

EMG activities are important physiological characteris-tics that need to be recorded for sleep staging as well asfor diagnosis and classification of a variety of sleep disor-ders. EMG represents electrical activities of muscle fibersas a result of depolarization of the muscles following trans-mission of electrical impulses along the nerves and neuro-muscular junctions. There is a fundamental tone in themuscle during wakefulness and NREM sleep, but this ismarkedly diminished or absent in major muscle groups

during REM sleep. EMG activities could be tonic, dystonicand phasic (including myoclonic bursts), or rhythmic. Adystonic muscle burst refers to prolonged EMG activitieslasting for 500 to 1000 milliseconds or longer. Phasic burstsmay include EMG activities phasically related to inspira-tory bursts. Myoclonic muscle bursts are also phasic bursts,which are characteristically noted during REM sleep andmay be seen as excessive fragmentary myoclonus alsoduring NREM sleep in many sleep disorders. Myoclonicbursts refer to EMG activities lasting for a brief durationof generally 20 to 250 milliseconds, sometimes up to 500milliseconds. In patients with tremor, EMG may recordrhythmic activities.

In a standard PSG recording, EMGs are recorded frommentalis or submental and right and left tibialis anteriormuscles. Mental or submental EMG activity is monitoredto record axial muscle tone, which is significantly decreasedduring REM sleep and, therefore, an important physiolog-ical characteristic for identifying REM sleep. EMG isrecorded using a gold cup or a silver-silver chloride elec-trode applied to a clean surface using a tape or electrodeglue. For chin EMG recordings, at least three EMG elec-trodes are applied so that in the event of a problem withone of the electrodes the additional electrode can be con-nected during the recording without disturbing the patient.Generally, the mentalis muscle on one side is connected tothe mentalis muscle on the other side, and an additionalelectrode is placed in the submental muscle. The electrodeimpedance should be less than 5 K. The high- and low-frequency filter settings for the EMG recordings are dif-ferent from those used for EEG and EOG, and are listedin Table 11. The sensitivity should be at least 20 micro-volts per centimeter for mental or submental EMG activ-ity. Additional electrodes may be needed in patients withbruxism (tooth grinding) over the masseter muscles to doc-ument associated bursts of EMG activities.

For recording from tibialis anterior muscles surfaceelectrodes are used over tibialis anterior muscles and thedistance between the two electrodes is 2 to 2.5 centime-ters. Bilateral tibialis anterior EMG is important to recordin patients suspected of restless legs syndrome (RLS)because the periodic limb movements in sleep (PLMS),which are noted in 80 percent of such patients, may alter-nate between the two legs. Ideally, the recording shouldalso include one or two EMG channels from the upperlimbs in patients with RLS as occasionally PLMS are notedin the upper limbs. For patients with suspected REMbehavior disorder, multiple muscle EMGs from all fourlimbs are essential as there is often a dissociation of theactivities between upper and lower limb muscles in suchpatients. The upper limb surface electrodes could beplaced over extensor digitorum communis muscles with aseparation of distance of 2 to 2.5 centimeters. If the upper


limbs are not included in the EMG in patients suspectedof REM behavior disorder, REM sleep without atonia maybe missed in some cases.

Other EMG recordings include intercostal anddiaphragmatic EMG to record respiratory muscle activi-ties. The intercostal EMG recorded from the seventh toninth intercostal space with active electrodes on the ante-rior axillary line and the reference electrodes on the midax-illary line may also include some diaphragmatic muscleactivity in addition to the intercostal activity. The diaphrag-matic activities can be recorded by placing surface elec-trodes over the right or left side of the umbilicus or overthe anterior costal margin, but these are contaminated by a mixture of intercostal activity and such noninvasivetechniques are unreliable for quantitative assessment ofdiaphragmatic EMG. The true diaphragmatic activities aretypically recorded by intraesophageal recording.

EMG shows progressively decreasing tone from wake-fulness through stages I to IV of NREM sleep. In REMsleep, the EMG is markedly diminished or absent. In REMbehavior disorder, a characteristic finding is absence ofmuscle atonia during REM sleep in the EMG recordingand the presence of phasic muscle bursts repeatedlyduring REM sleep.

Electrocardiography

A single channel of EKG is sufficient during PSGrecording by placing one electrode over the sternum andthe other electrode at a lateral chest location. This record-ing detects bradytachyarrhythmias or other arrhythmiasseen in many patients with obstructive sleep apnea syn-drome (OSAS). Gold cup surface electrodes are used torecord the EKG and Table 11 lists the filter settings andsensitivities for such recording.

RESPIRATORY MONITORING TECHNIQUE

Respiratory monitoring during PSG recording is a veryimportant procedure. In fact, the common reason forreferring a patient to a sleep laboratory for PSG is toexclude a diagnosis of sleep apnea-hypopnea syndrome.Therefore, the PSG recording must routinely includemethods to monitor airflow and respiratory effort ade-quately to correctly classify and diagnose sleep-relatedbreathing disorders.

Recording of Respiratory Effort

Respiratory effort can be measured by mercury-filled orpiezoelectric strain gauges, inductive plethysmography,impedance pneumography, respiratory magnetometers,and respiratory muscle EMG. Most commonly, piezoelec-

tric strain gauges and inductive plethysmography are usedto monitor respiratory effort.

Strain Gauges Strain gauges are used to record thoracic and abdomi-

nal and, thus, respiratory movements. A piezoelectricstrain gauge consists of a crystal that emits an electricalsignal in response to changes in length or pressure. For allthese devices, one belt is placed around the chest andanother one is placed around the abdomen, which allowsdetection of the paradoxical movements indicating upperairway OSAS. Sometimes strain gauges may not be able todifferentiate central from obstructive apneas. The otherdisadvantage of strain gauge includes displacement bybody position and body movements interfering with theaccuracy of measurements.

Respiratory Inductive Plethysmography This measures changes in thoracoabdominal cross-

sectional areas and the sum of these two compartments isproportional to airflow. Inductance refers to resistance tocurrent flow. Transducers across the chest and abdomendetect changes in the cross-sectional areas of the thoraxand abdomen during breathing. Similar to strain gauges,body movements and changes in body positions may dis-place the transducers, causing inaccuracy in measurementsof the respiratory effort.

Impedance PneumographyThis technique may not precisely measure the respira-

tory pattern and the volume. Furthermore, there may beelectrical interference and therefore, this technique is not generally used.

Respiratory MagnetometersRespiratory magnetometers record the chest and

abdominal motions in both the anteroposterior and lateraldirections. This was used as a research technique but hasnot been popular in practical PSG.

Respiratory Muscle ElectromyographyIntercostal and diaphragmatic EMG activities may be

recorded to measure indirectly effort of breathing byattaching surface electrodes over the intercostal spaces andupper abdomen near the margins of the rectus abdominismuscles. It is really not possible to detect pure uncontami-nated intercostal or diaphragmatic muscle activities bythese techniques.

MEASUREMENT OF AIRFLOW

Airflow can be measured by thermistors, thermocou-ples, or nasal cannulapressure transducers recording


nasal pressure. A thermistor or thermocouple devicebetween the nose and mouth is commonly used to monitorairflow to detect changes in temperature (e.g., cool airflows during inspiration and warm air flows during expira-tion). A thermistor consisting of wires records changes inelectrical resistance, and thermocouples consisting of dis-similar metals (e.g., copper and constantan) registerchanges in voltage that result from temperature variation.Thermistors or thermocouples are not as sensitive as nasalpressure transducers for detecting airflow limitations and,hence, may miss hypopnea. For these reasons, nasal pres-sure technique to detect airflow should be used routinelyduring PSG recording. The other problems with thermis-tors or thermocouples are that they are easily displaced,causing false changes in airflow. Also, certain precautionsshould be taken while using thermistors and thermocou-ples. The thermistor or thermocouple temperature mustbe below body temperature in order to sense the tem-perature difference between expired and inspired air.Oronasal transducers also must not be in contact with theskin, otherwise the transducer temperature will not bebelow the body temperature.

Nasal Pressure Monitoring

During inspiration nasal airway pressure decreases andduring expiration it increases. This alternate decrementand increment of nasal pressure produces electricalsignals, which indirectly register airflow. In nasal pressuremonitoring, a nasal cannula is connected to a pressure-sensitive transducer, which measures the pressure differ-ence. Nasal pressure monitoring is more sensitive thanthermocouples or thermistors in detecting airflow limita-tion and hypopnea. If there is airflow limitation andincreased upper airway resistance, the nasal pressuremonitor will register a plateau indicating a flow limitation.During nasal pressure recording, a DC amplifier or an ACamplifier with a long time constant should be used. Onedisadvantage is that nasal pressure cannula cannot be usedto measure airflow in the mouth breathers and in patientswith nasal obstruction.

Pneumotachography

This is an excellent technique to measure quantitativelythe tidal volume and direct airflow measurement. How-ever, this requires a sealed face mask, creating patient discomfort and sleep disturbance. Hence, it is not used inmost of the laboratories.

Intraesophageal Pressure Monitoring

This is the best technique and, in fact, the gold standardfor measuring respiratory effort. However, this is some-

what uncomfortable and invasive, requiring special exper-tise in acquiring the technique. It requires placing a naso-gastric balloon-tipped catheter into the distal esophagus,which is very uncomfortable for the patient. Recently smallfluid-filled esophageal catheters became available whichare more comfortable and cause less disturbance of sleep.This technique is not used in most laboratories. Nasal pres-sure monitoring can also detect respiratory flow limitationand increased upper airway resistance. Whether nasalpressure monitoring is as good as esophageal pressuremonitoring remains somewhat controversial. Esophagealpressure monitoring registers pleural pressure changes. In normal individuals during wakefulness, the pressurechange is less than 5 centimeters of water and in sleep itis between 5 and 10 centimeters of water. Inspirationcauses more negative pleural and hence esophageal pres-sure than expiration. Indications for esophageal pressuremonitoring include correct classification of apneas ascentral or obstructive, detection of increased upper airway resistance, diagnosis of upper airway resistance syndrome, and correct classification of hypopnea asobstructive type.

Expired Carbon Dioxide

Capnography detects the expired carbon dioxide (CO2)level, which closely approximates intra-alveolar CO2.Capnography does detect both airflow and the partial pressure of CO2 in alveoli, which is useful for evaluatingOSAS, sleep hypoventilation, and an underlying pul-monary disease. An infrared analyzer over the nose andmouth detects CO2 in the expired air, which qualitativelymeasures the airflow. This is the best noninvasive methodto detect alveolar hypoventilation. The method is costlyand therefore not used in most laboratories, but it shouldbe used in children with suspected OSA.

Oxygen Saturation

The best way to detect arterial O2 content (PaO2) is byinvasive method using an arterial cannula. This is not viablefrom the practical standpoint and in any case intermittentsampling of blood through the cannula may not reflect the severity of hypoxemia during a particular disorderedbreathing event. Therefore noninvasive method by fingerpulse oximetry is routinely used to monitor arterial oxygensaturation (SaO2) or arterial oxyhemoglobin saturation,which reflects the percentage of hemoglobin that is oxy-genated. The difference in light absorption between oxyhemoglobin and deoxyhemoglobin determines oxygensaturation. Continuous monitoring of SaO2 is crucialbecause it provides important information about the sever-ity of respiratory dysfunction.


ESOPHAGEAL pH

This technique is a specialized procedure and is not usedin standard PSG laboratories. Esophageal pH is monitoredby asking patients to swallow a pH probe. Recording theoutput using a DC amplifier detects nocturnal gastro-esophageal reflux disease, which may be mistaken for sleepapnea or nocturnal angina as the patient may wake upchoking or have severe chest pain as a result of acid eructation.

BODY POSITION MONITORING

Body position is monitored by placing sensors over oneshoulder and using a DC channel. Snoring and apneas aregenerally worse in the supine position and therefore CPAPtitration must include observing patients in the supineposition for evaluating optimal pressure for titration.

SNORING

This can be monitored by placing a miniature micro-phone on the patients neck. There is no generally acceptedstandardized technique to record quantitatively the inten-sity of snoring.

MONITORING OF PENILE TUMESCENCE

This is not used routinely in most of the sleep laborato-ries. Study of sleep-related penile erections remainslimited to those researchers interested in understandingphysiology of penile erections and occasionally used insome sleep laboratories to differentiate difficult and con-fusing cases of organic versus psychogenic impotence, andalso for legal purposes to settle compensation claims.Strain gauges are used to measure penile tumescence. Innormal adult men, penile tumescence occurs during REMsleep and this persists in psychogenic but not in organicimpotence.

Table 12 lists the EEG montages and other monitorsas recorded routinely for an overnight PSG study in ourlaboratory.

Figure 11 is a representative sample of an overnightPSG recording from our laboratory showing PLMS.

POLYSOMNOGRAPHY DOCUMENTATION

Important information should be clearly documented bythe technologist before beginning the recording. This mustinclude the patients name, age, date of study, identifica-

tion number, purpose of the recording (before referringthe patient to the laboratory for PSG study, sleep cliniciansshould have performed complete history and physicalexamination formulating a provisional diagnosis), and thename of the technician. Also lights out and lights onmust be clearly marked, and a detailed description of anyunusual behavior or motor events should be clearly indicated.

EQUIPMENT CALIBRATION

It is important to calibrate the PSG equipment and alsoto perform physiologic calibration prior to beginningtesting in order to ensure that all amplifiers are function-ing adequately. All-channel calibration followed by indi-vidual channel calibration is the first step. A known signalis sent through all the amplifiers, which are set to the samelow- and high-frequency filter settings and sensitivities,thus testing proper functioning of all amplifiers. Appropri-ate filter settings and sensitivities are then set for eachchannel recording the various physiological characteristics,documenting individual channel calibration.

Table 12. Typical overnight polysomnographicmontage with continuous positive airway pressure titration used in our laboratory*

Channel Number Name

1 F3-C32 F7-T33 T3-T54 T5-O15 F4-C46 F8-T47 T4-T68 T6-O29 C3-A2

10 C4-A111 Left electro-oculogram12 Right electro-oculogram13 Chin electromyogram (EMG)14 Left tibialis EMG15 Right tibialis EMG16 Oronasal17 Chest18 Abdomen19 Snoring20 Electrocardiogram21 Heart rate22 Position23 Arterial oxygen saturation24 Continuous positive airway pressure

*Channels 110 record electroencephalogram using the 1020 Interna-tional System of electrode placement. Channel 16 records airflow. Channels 17 and 18 record respiratory effort.


Physiologic Calibration

This is performed after application of the electrodes andother monitors to document proper functioning of thesedevices. Table 13 lists the instructions given for physio-logic calibrations. On completion of all calibration proce-dures, the lights are turned out in the room and the patientis asked to lie down and try to sleep. Throughout therecording, the technologist should watch for any artifactsand make appropriate adjustments to obtain a goodquality, relatively artifact-free recording for proper scoringand interpretation of the PSG.

ENDING THE TEST

When the patient is awakened in the morning (eitherspontaneously or at a set time), equipment and physiologic calibrations should be performed again to ensure that theequipment and all the devices have been continuouslyfunctioning properly throughout the night. The patient isthen asked to fill out a post-study questionnaire, whichincludes estimation of time to fall asleep, total sleep time,number of awakenings, and the quality of sleep. For thesafety of the patient, it is important to inquire about the

FIGURE 11. A 30-second epoch of a representative sample of an overnight PSG recording from our laboratory showing PLMS. Thesample is taken in stage II NREM sleep as shown by the presence of sleep spindles best defined in C3- and C4-derived EEG channels.Few slow waves of sleep are recorded on the EOC channels. Two PLMS are recorded in the Lt./Rt. Tib. EMG channel. The EKG arti-fact is noted to contaminate the chin EMG, Lt./Rt. Tib. EMG, and snoring channels. The in-phase deflections of respiratory channelsare noted. A snore artifact is also recorded on the snoring channel. EEG, Top 10 channels; Lt. and Rt. EOG, left and right electroocu-lograms; chin EMG, electromyography of chin; Lt./Rt. Tib. EMG, left and right tibialis anterior electromyography; oronasal thermistor;chest and abdomen effort channels; snore monitor; EKG, electrocardiography; SaO2, oxygen saturation by finger oxymetry.


EMG burst should be 0.5 to 5 seconds; the intervalbetween bursts should be 4 to 90 seconds; the amplitudeof the EMG bursts, although variable, should be more than25 percent of the EMG bursts recorded during thepresleep calibration recording. PLMS (Figures 13through 15) may or may not be associated with arousalsand they should be scored separately. To score PLMS asso-ciated with arousal, the arousal must occur within 3seconds of onset of PLMS. PLMS is expressed as an indexconsisting of number of PLMS per hour of sleep. To be ofpathological significance, the PLMS index should be 5 ormore. Leg movements may be noted occurring periodi-cally associated with resumption of breathing follow-ing recurrent episodes of apneas or hypopneas. These respiratory-related leg movements should not be countedas PLMS. PLMS are generally seen during NREM sleepbut they can occur rarely during REM sleep. In patientswith RLS, however, PLMS may also occur during wake-fulness (Figure 16) when they are termed periodic limbmovements in wakefulness (PLMW).

INDICATIONS FOR POLYSOMNOGRAPHY

The guidelines proposed by the AASM for performingovernight in-house attended PSG include the following:

PSG is routinely indicated for the diagnosis of sleep-related breathing disorders.

PSG is indicated for CPAP titration in patients withsleep-related breathing disorders.

PSG is indicated in patients to evaluate for the presence of OSAS before undergoing laser-assisted uvulopalatopharyngoplasty (LAUP).

PSG is indicated for assessment of therapeutic benefitsafter dental appliance treatment or after surgical treat-ment with moderately severe OSAS, including in pa-tients whose symptoms reappear despite an initial goodresponse.

If the patient has lost or gained substantial weight, afollow-up PSG is indicated for assessment of treatmentwhen the clinical response is insufficient or when symp-toms reappear despite a good initial response to treat-ment with CPAP.

An overnight PSG followed by a multiple sleep latencytest is routinely indicated in patients suspected to havenarcolepsy.

PSG is indicated in patients with parasomnias that areunusual or atypical or behaviors that are violent or inju-rious to the patient or others. In uncomplicated andtypical parasomnias, however, PSG is not routinely indicated.

Table 13. Instructions for physiologic calibrations

Open eyes and look straight for 30 secsClose eyes for 30 secsLook left, right, up, and downBlink eyes five timesClench teethInhale and exhaleHold breath for 10 secsExtend right hand and then left handDorsiflex right foot and then left foot

mode of transportation that will be used by the patientbefore leaving the sleep laboratory.

SCORING OF AROUSALS

When EEG shows waking activities for 15 seconds orlonger in a 30-second epoch, the scoring stage is wakeful-ness. Arousals are transient phenomena causing fragmen-tation of sleep and lasting less than 15 seconds. TheAmerican Academy of Sleep Medicine (AASM) estab-lished a task force to develop preliminary scoring rules for arousal. According to these preliminary rules, an EEG arousal is an abrupt shift in EEG frequency that may include alpha, theta, or beta activities but no spindleor delta waves (Figure 12) lasting for 3 to 14 seconds. The subject must be asleep for at least 10 continuousseconds before an arousal can be scored. Similarly, aminimum of 10 continuous seconds of intervening sleep is needed to score a second arousal. Arousals during REM sleep are scored only when accompanied by con-current increase in submental EMG amplitude. It should be noted that in NREM sleep, arousals may occurwithout an increase in submental EMG amplitudes.Therefore, based only on submental EMG amplitude augmentation, an arousal cannot be scored. Delta waves,K complexes, and artifacts are not counted as arousalsunless they are accompanied by frequency shifts. Anarousal index is defined as the number of arousals per hour of sleep and an index above 10 can be consideredabnormal.

SCORING OF PERIODIC LIMBMOVEMENTS IN SLEEP

PLMS are counted from the right and left tibialis ante-rior EMG recordings. The scoring criteria for PLMS canbe summarized as follows: the movements must occur aspart of four consecutive movements; the duration of each


PSG is indicated in patients suspected of nocturnalseizures.

PSG is not routinely performed to diagnose RLS butovernight PSG is indicated for patients with suspectedPLMS.

In patients with insomnia, PSG may be indicated if theresponse to a comprehensive behavioral or pharmaco-logical treatment program for the management ofinsomnia is unsatisfactory. If, however, there is a strongsuspicion of an associated sleep-related breathing disor-der or associated PLMS in patients with insomnia, anovernight PSG is indicated.

There is some controversy regarding the diagnosis ofperiodic limb movement disorder (PLMD) causing sleepfragmentation, arousals, and excessive daytime sleepiness.PLMS have been noted in a number of sleep disorders aswell as in normal individuals, particularly in patients over60 to 65. Although the specificity of PLMS is not defined,at least 80 percent of patients with RLS show PLMS onPSG recordings. Therefore, to document PLMS in RLS,PSG may be indicated. However, the diagnosis of RLS isa clinical one and has been based on international studygroup criteria. PLMS are not part of these criteria nor is asleep disturbance part of the minimal or essential criteria

Montage

F3-C3

F7-T3

T3-T5

T5-O1

F4-C4

F8-T4

T4-T6

T6-O2

C3-A2

C4-A1

LEFT EOG

RT EOG

CHIN

Lt/Rt Tib EMG

Oronasal

Chest

Abdomen

Snoring

EKG

Heart Rate

Position

SaO2

FIGURE 12. PSG recording shows two brief periods of arousals in the left and right segments of the recording, each lasting about 6to 7 seconds. Note 9- to 10-hertz alpha activities during arousals in the EEG channels and muscle bursts in the tibialis EMG. Also notemovement artifacts in O1 and T6 electrodes. There is more than a 10-second segment of sleep between two arousals. EEG, Top 10 chan-nels; Lt. and Rt. EOG, left and right electrooculograms; chin electromyography, Lt./Rt. Tib. EMG, left and right tibialis anterior elec-tromyography; oronasal airflow, chest respiratory effort; abdomen respiratory effort; snoring; EKG, electrocardiography; SaO2, oxygensaturation by finger oxymetry. (From Chokroverty S. An overview of normal sleep. In Sleep and Movement Disorders, Figure 37.Philadelphia: Butterworth Heinemann, 2003, with permission.)


for the diagnosis of RLS. PSG indications, therefore, forRLS-PLMS remain somewhat dubious and contentious.Some investigators do not believe in the existence ofPLMD as a separate sleep disorder causing sleep dys-function and excessive daytime somnolence (ESD). Theindications for pure PLMS or PLMD currently remainundetermined and further investigations including out-come studies are needed to document that PLMS orPLMD may cause sleep disturbance and EDS.

Recently the German Sleep Society gathered a group ofexperts and developed consensus criteria for performingovernight PSG study in patients with RLS and these have

been recently published. These indications, however, havenot been accepted generally outside the German SleepSociety at the present time.

VIDEO-POLYSOMNOGRAPHYRECORDING AND INDICATIONS

Video-PSG, which combines PSG with multiple EEGchannels and simultaneous video recording, can help assessabnormal movements and behavior occurring during sleep.Such recordings can differentiate parasomnias from noc-

FIGURE 13. A 120-second excerpt in stage II NREM sleep from an overnight PSG recording in a 29-year-old man with complaint ofsnoring since adolescence recently getting worse. He denies sensation of choking or difficulty breathing at night or excessive sleepinessin the daytime. Overnight PSG study did not show any evidence of sleep apnea, but showed an excessive number of periodic limb move-ments in sleep (PLMS) with a moderately increased periodic limb movement index of 28 and four sequential bursts of PLMS. The PLMSare not preceded by respiratory events or followed by electroencephalographic arousals. The montage used is the same as in Figure 11without oxymetry display.


turnal seizures, psychogenic dissociative disorders, andother diurnal movement disorders persisting during sleepat night. Clear documentation of abnormal motor manifes-tations, unusual and complex behaviors, characteristicEEG features of seizures, and other abnormal movementsis possible. The video recording can include multiplexedanalog signals captured on a tape, but many commerciallyavailable systems include digital video directly synchroniz-

ing and time-locking the abnormal behavior to the PSGsignals. Depending on the availability of the channels andthe electrode inputs in the equipment, multiple channelsof EEGs (e.g., for a suspected nocturnal seizure disorder)and EMGs to include additional muscles (e.g., to recordfrom forearm flexor and extensor muscles, masseter, andother muscles for patients with suspected REM behaviordisorder [RBD] and bruxism) are recommended.

FIGURE 14. A 33-year-old man with history of loud snoring for many years, frequent awakenings at night, and occasional gasping forbreath in sleep. Additionally he complains of uncomfortable sensation in the legs at sleep onset with relief on movement or massage forthe last 2 years. Clinical suspicion of mild upper airway obstructive sleep apnea and restless legs syndrome were raised. A 120-secondexcerpt from stage IV NREM sleep showing the presence of alternating limb movements on the left and right leg electromyography (EMG)channels. The limb movements occur at a frequency of approximately two per minute, each lasting about 3.9 to 6.7 seconds with alter-nation of sides, which has been well described. However, alternating leg muscle activation (ALMA) in sleep has been described recentlyand future studies may throw further light on the subject. Incidentally an electrocardiographic (EKG) artifact is noted superimposed onthe electroencephalography (EEG) and oculogram channels. A modified montage shows the following: EEG, Top 4 channels; LOC/A2,left outer canthus referred to right ear electrode; ROC/A1, right outer canthus referred to left ear electrode; chin EMG, electromyogra-phy of chin; Lt. Leg EMG, left leg electromyography; Rt. Leg EMG, right leg electromyography; EKG, electrocardiography.


COMPUTERIZED POLYSOMNOGRAPHY

The advantages of computerized PSG include easy dataacquisition, display, and storage. It is easy to review therecord on-line (i.e., the ability to look back at early tracingduring progression of recording). The other advantagesinclude the ability to manipulate large quantity of data forreview and storage for permanent record keeping. It is easyto click on a particular event of interest or a particular sleepstage to simultaneously display all the physiologic data. It

is also easy to perform review using a variety of filter set-tings, sensitivities, monitor speeds, and reformatted mon-tages (i.e., new montages may be created retrospectivelyfrom the electrode derivations used during actual record-ing). In a particular segment with a question of a poten-tially epileptiform event (e.g., spikes, sharp waves, spikeand waves, and sharp and slow waves), the standard PSGspeed of 10 millimeters per second can be quickly changedto the usual EEG speed of 30 millimeters per second forrecognizing the evolving pattern of activation for a correct

FIGURE 15. A 30-second excerpt from an overnight PSG in 44-year-old man with complaints of unusual electrical sensation in the legsfollowed by twitching and jerking in the legs and frequent awakenings at night. He also reports occasional similar involvement in thearms. There is no accompanying history of urge to move or relief on movement. Three sequential bursts of periodic limb movements inwaking (PLMW) are seen on tibialis anterior electromyography channel. Blink artifacts recur in the frontal and anterior temporal elec-trodes bilaterally. PLMW have been described in periodic limb movement disorder (PLMD) and restless legs syndrome (RLS). PLMWhave the same definition as PLMS except the duration may be longer (0.5 to 10 seconds). EEG, Top 10 channels; left and right EOG,electrooculograms; electromyography of chin; Lt./Rt. Tib EMG, left/right tibialis anterior electromyography; oronasal thermistor; chestand abdomen effort channels; snore monitor; EKG, electrocardiography.


diagnosis. These capabilities help identify an abnormalEEG pattern and distinguish artifacts from true cerebralevents.

Computerized PSG recording replaces paper and makesPSG paperless, thus making it easy to store the data andconserve space. Computerized PSG makes it easy to doc-ument all events during recording digitally and makes iteasy to edit and report. The computerized PSG can storethe raw data on relatively inexpensive CD-ROM or othersuitable media, making it easy to keep a database andaccess raw data. European data format (EDF) is the mostcommon format for exchange of digital PSGs between dif-ferent laboratories in different countries.

There are certain minimal requirements for digital PSG.The sampling rate for EEG, EOG, and EMG should

ideally be 200 hertz. A 12-bit analog-digital conversion isa suggested minimum acceptable for the digital amplituderesolution. It is important to have scroll-back modewithout interrupting data acquisition so that a particularchange during recording can be compared with the previ-ous signals. The computer screen must be of sufficient sizeand have high resolution.

Simultaneous video monitoring during PSG recording is essential to obtain patients behavior and motor manifestations, particularly in patients with parasomnias and nocturnal seizures. Recently introduced digital videoincorporated into the computerized PSG system is animportant advance over the traditional videotape record-ing. Digital video recordings, however, use a lot of spaceon the hard disk and one way to handle this problem is to

FIGURE 16. A 120-second excerpt from an overnight PSG recording is from the same subject as in Figure 15. It shows the presenceof periodic limb movements in waking not only on tibialis anterior electromyography channel but also on some occasions both simulta-neously and independently on the chin electromyography recording. Montage used is the same as in Figure 15.


save only small segments of digital video. The latest digitalversatile disk (DVD) may solve the storage problems in the future.

COMPUTER-ASSISTED SLEEP SCORING

Manual scoring of sleep using Rechtschaffen-Kales (R-K) technique and scoring of other physiological char-acteristics (e.g., breathing events, arousals, PLMS) arelabor intensive and time consuming, show wide interratervariability, and can be very difficult to score abnormalrecords. In spite of the fact that R-K system remains thegold standard for scoring sleep, there are severe limitationsto R-K scoring system. In the original recommendation,only one central EEG derivation (C3-A2 or C4-Al) was suggested. It was also suggested that epoch-by-epochscoring be undertaken, and this method of scoring missesshort stage changes and many short periods of wakefulnessafter sleep onset. The R-K system is meant for normaladult sleep patterns. In abnormal sleep with repeated stage and state changes, scoring results do not reflect thereal sleep architecture. The R-K system is based on theassumption that the signals are stationary, which is not true from a physiological standpoint. Another serious deficiency in the R-K system is that the amplitude and frequency criteria for delta waves are arbitrarily deter-mined and no justification is given for the use of those criteria. Amplitude criteria are easy to apply in youngadults but in the elderly subjects the amplitude of the deltawaves decreases and hence the amount of slow-wave sleepwill decrease in old age because many of the these deltawaves do not reach 75 microvolts of amplitude criteria.One suggestion has been to ignore amplitude criteria inthe elderly PSG recording but this has not been generallyaccepted.

The minimum duration of 0.5 second for sleep spindlesand K-complexes is also arbitrary and no justification was given in the original rules of the R-K system. Also, R-K rules do not take into consideration spindle intensityand sleep microstructure. REM sleep scoring in thissystem implies certain complicated rules. The beginningand ending of REM sleep are often difficult to score. Insleep apnea with repeated arousals, it is difficult to esti-mate the correct percentage of wakefulness and a par-ticular sleep stage. In the R-K system, sleep scoring is fallacious in patients with OSAS and parasomnias including RBD, narcolepsy, and seizure disorder. Theinterrater agreement in the R-K system is around 90percent, which is not quite satisfactory. The R-K sleepmanual also does not address scoring and recording of arousals, sleep-disordered breathing events, snoring,sleep-related penile erections, leg muscle activities, orEKG events.

In order to overcome the limitations and fallacies of theR-K system and to reduce the time needed for scoring,automatic computer-assisted scoring techniques have beenproposed and are commercially available. The proposedsleep scoring techniques have included period/amplitudeanalysis, EEG spectral analysis based on fast Fourier trans-form (FFT), the hybrid system concept combining analogand digital techniques, pattern recognition, the interval histogram method, the bayesian approach, an expertsystem approach, the neural network model, adaptive sleep stage scoring, and segmentation and self-organizationto cluster the different PSG patterns. Some of the lattermethods are still evolving, but none of the techniques havereceived wide popularity because of serious limitations in obtaining an acceptable scoring and because of lack ofstandardization, validation, and precision in the methods.Some of the problems in computer-assisted scoring includeartifact recognition, differentiating stage I NREM sleepfrom REM sleep, discriminating different sleep stages,inability to differentiate eye movements from high-amplitude delta waves, and failure to detect upper airwayresistance. Attempts have also been made to develop com-puter methods to identify obstructive, central and mixedapneas, and hypopneas as well as arousals and PLMS, butthe methods remain ambiguous, imprecise, and variable,resulting in lack of universal acceptance at this time.

Computerized scoring has no real gold standard tocompare the data. Another disadvantage of computerscoring is that there is no standardized procedure forscoring of various physiological characteristics. Further-more, for comparison between visual and computerizedscoring, sampling remains a problem. R-K manual scoringstill remains today the gold standard in clinical practice.

ARTIFACTS DURINGPOLYSOMNOGRAPHY RECORDING

The extraneous electrical activities not recorded fromthe regions of interest (e.g., the brain, muscles, eyes, andheart) generate artifacts in the PSG recording. Theseextraneous electrical activities may obscure the biologicalsignals of interest and, therefore, recognition and correc-tion of these artifacts is an important task for the PSG tech-nologist. The artifacts can be divided into three categories:physiologic, environmental, and instrumental.

Physiologic Artifacts

These include myogenic potentials, artifacts resultingfrom movements of the head, eyes, tongue, mouth, andother body parts, sweating, pulse and EKG artifacts, aswell as rhythmic tremorogenic artifacts. Potentials origi-nating from the scalp muscles may obscure the EEG activ-ities and may simulate beta rhythms, and obscure


low-amplitude cerebral activities. Figure 17 documentsmuscle activities during PSG recordings. Movements ofthe head, eyes, tongue, mouth, and other body parts willproduce movement artifacts (Figure 18) and sometimesresemble slow waves in the EEG or may obscure the EEGactivities, causing difficulty in scoring the different sleepstages. The rhythmic movements sometimes generated byrhythmic movements of the head, tongue, and legs as wellas rhythmic movements generated by tremor in a patientmay produce apparent slow waves, thus causing difficultyin scoring the slow-wave sleep. Sweating may cause exces-sive baseline swaying, producing a very slow-frequencywave lasting for 1 to 3 seconds, which is noticed promi-

nently in the frontal electrodes (Figures 19 and 110).The electrical potentials result from salt content of thesweat glands. Eye movement artifacts can be confusedwith actual cerebral activities (Figure 111). Pulse artifactoccurs when the electrode is placed over the scalp arter-ies. Slow waves are generated by electrode movementcaused by the pulsations. Temporal relation of these wavesto the EKG recording helps identify such extracerebralactivity. EKG artifacts can contaminate the EEG record-ings, particularly in patients who are obese with short neck(Figure 112). Tongue movements may produce char-acteristic glossokinetic potentials that obscure the EEGactivities.

FIGURE 17. A 30-second excerpt of PSG recording shows spike-like potentials of varying amplitude and frequency denoting muscleactivity not only on the chin electromyography channel but also in all electroencephalography (EEG) channels, particularly prominentin T3, T4, A2 and A1 electrode derivations. EEG, Top 10 channels; Lt. and Rt. EOG, left and right electrooculograms; electromyogra-phy of chin; Lt. and Rt. Tib. EMG, left and right tibialis anterior electromyography; P. Flow, peak flow; oronasal thermistor; chest andabdomen effort channels; snore monitor; EKG, electrocardiography.

Text continued on p. 25


FIGURE 18. A: A 30-second excerpt from overnight PSG recording shows body movement artifact following an obstructive apnea asdemonstrated by variable, high-voltage, asymmetrical and asynchronous slow activity on electroencephalography (EEG) channels andincreased electromyography activity on chin and tibialis anterior electrode channels. EEG, Top 10 channels; Lt. and Rt. EOG, left andright electrooculograms; electromyography of chin; Lt. and Rt. Tib. EMG, left and right tibialis anterior electromyography; P. Flow, peakflow; oronasal thermistor; chest and abdomen effort channels; snore monitor; EKG, electrocardiography.


FIGURE 18, contd B: A 64-year-old man was referred for evaluation of upper airway obstructive sleep apnea syndrome. The abdomeneffort channel in the PSG recording reveals a double-peaked waveform in the post-apnea recovery breaths. This may be related to theclinical finding of a large protuberant abdomen with a ventral midline hernia. EEG, Top 10 channels; Lt. and Rt. EOG, left and rightelectrooculograms; electromyography of chin; Lt./Rt. Tib. EMG, left and right tibialis anterior electromyography; oronasal thermistor;chest and abdomen effort channels; snore monitor; EKG, electrocardiography; SaO2, oxygen saturation by finger oxymetry.


FIGURE 19. A 58-year-old man with history of loud snoring for several years, witnessed apneas in sleep for the past 5 years, and exces-sive daytime sleepiness. Overnight PSG showed severe obstructive sleep apnea with an apnea-hypopnea index of 75.6. A 30-second excerptfrom the study shows the presence of sweat artifact characterized by chaotic, asynchronous, high-voltage slow activity in delta range,randomly distributed over all the EEG channels and electrooculogram electrodes. Sweating occurs commonly in NREM sleep and mayneed to be differentiated from focal or diffuse slow activity of cerebral origin. EEG, Top 10 channels; Lt. and Rt. EOG, left and rightelectrooculograms; electromyography of chin; Lt. and Rt. Tib. EMG, left and right tibialis anterior electromyography; oronasal thermistor; chest and abdomen effort channels; snore monitor; EKG, electrocardiography.


FIGURE 110. Overnight PSG shows evidence of severe upper airway obstructive sleep apnea with an apnea-hypopnea index of 51.3in a 71-year-old man with history of snoring, difficulty breathing in sleep, and excessive daytime sleepiness despite uvulopalatoplastyperformed 2 years ago for treatment of obstructive sleep apnea. Medical history is significant for hypertension. Additionally, the PSGshows the presence of sweating recurrently and exclusively during REM sleep while being conspicuously absent during non-REM sleep.This is an atypical finding, raising the suggestion of REM sleep dysregulation. This figure shows a 30-second excerpt in REM sleep takenfrom overnight PSG recording showing sweat artifact as described for Figure 19. EEG, Top 10 channels;, Lt. and Rt. EOG, left andright electrooculograms; electromyography of chin; Lt. and Rt. Tib. EMG, left and right tibialis anterior electromyography; P. flow, peakflow; oronasal thermistor; chest and abdomen effort channels; snore monitor; EKG, electrocardiography.


FIGURE 111. A: A 30-second excerpt from a multiple sleep latency test showing the presence of slow rolling eye movements with theonset of stage I non-REM sleep. The slow rolling eye movements are not only recorded on the left and right electrooculogram channelsbut are also noted on the anterior temporal electroencephalography (EEG) electrode recordings. This can be confused with focal ordiffuse slow activity of cerebral origin. EEG, Top eight channels; Lt. and Rt. EOG, left and right electrooculograms; electromyographyof chin; EKG, electrocardiography.


FIGURE 111, contd B: A 30-second epoch from overnight PSG showing repetitive blink artifact best noted in the electrooculogramchannels and on the frontal and anterior temporal EEG electrode recordings. This can be misinterpreted for abnormal cerebral activity. EEG, Top 10 channels; Lt. and Rt. EOG, left and right electrooculograms; electromyography of chin; Lt. and Rt. Tib. EMG,left and right tibialis anterior electromyography; P. flow, peak flow; chest and abdomen effort channels; snore monitor; EKG, electrocardiography.


FIGURE 111, contd C: A 30-second epoch from overnight PSG showing rapid eye flutter artifact best recorded in F3 and F4 elec-trode recordings during the latter half of the epoch. A blink artifact is noted in the initial part of the recording and is best seen on theelectrooculogram channels. Well-formed alpha rhythm is noted in the background on all EEG channels when eyes are closed.


FIGURE 111, contd D: A 50-year-old man is referred for exclusion of upper airway obstructive sleep apnea syndrome. Past medicalhistory is significant for an artificial left eye and limited vision in the right eye secondary to severe uveitis. A 30-second excerpt in REMsleep from overnight PSG shows the presence of unilateral eye movement artifact in right electrooculogram and anterior temporal EEGelectrodes but not in the corresponding channels on the left side. Myoclonic bursts of electromyographic activity are recorded in the tib-ialis electromyography channels and a mixed apnea is recorded on the oronasal thermistor and effort channels. A superimposed elec-trocardiography artifact is also noted. EEG, Top 10 channels; Lt. and Rt. EOG, left and right electrooculograms; electromyography ofchin; Lt. and Rt. Tib. EMG, left and right tibialis anterior electromyography; P. flow, peak flow; oronasal thermistor; chest and abdomeneffort channels; snore monitor; EKG, electrocardiography.


FIGURE 112. This is a 30-second excerpt from an overnight PSG selected to show electrocardiography artifact, which is character-ized by its morphology, rhythm, and synchrony with electrocardiographic recording. It is seen throughout the epoch on electroen-cephalography, electrooculography, and tibialis electromyography channels. Incidentally an obstructive apnea is seen in the airflow and effort channels. EEG, Top 10 channels; Lt. and Rt. EOG, left and right electrooculograms; electromyography of chin; Lt./Rt. Tib.EMG, left and right tibialis anterior electromyography; oronasal thermistor; chest and abdomen effort channels; snore monitor; EKG,electrocardiography.


Environmental Sources of Electrical Signals

These may simulate electrocerebral activity or obscureEEG activities. The 60-hertz (or 50-hertz) signals fromtelephone or pager systems can result in artifacts. Electro-static artifacts result from movements of the subject in theenvironment. Sixty-hertz electromagnetic radiation fromAC current in power lines may contaminate the recording(Figure 113). The main frequency is 50 hertz in manyother countries. The 60-hertz artifact may be mistaken formyogenic artifact in the slow paper speed of 10 millimeters

per second commonly used for PSG recording. The PSGtechnician should identify this problem and locate thesource of this artifact and try to eliminate the artifact. Mostimportant is keeping the impedance below 5 K.

Instrumental Artifacts

These arise from faulty electrodes, electrode wires,switches, and the polygraph machine itself. Electrodepops may produce transient sharp waves or slow waves

FIGURE 113. A: A 30-second excerpt from an overnight PSG recording shows a uniform monorhythmic artifact at 60 hertz on left/righttibialis electromyography channel, which is further superimposed by an electrocardiography artifact. The 60-Hz artifact is due to elec-trical interference from power lines and equipment occurring at a frequency of 60 hertz in North America (but at 50 hertz in many othercountries). Maximum interference is seen in the presence of poor electrode contact. Incidentally an obstructive apnea is noted in theairflow and respiratory effort channels. EEG, Top 10 channels; Lt. and Rt. EOG, left and right electrooculograms; electromyography ofchin; Lt./Rt. Tib. EMG, left and right tibialis anterior electromyography; oronasal thermistor; chest and abdomen effort channels; snoremonitor; EKG, electrocardiography; SaO2, oxygen saturation.


FIGURE 113, contd B: The 60-hertz artifact described in A is shown at a paper speed of 2 seconds per epoch.

limited to one electrode (see Figure 114 and Figure213). These artifacts are very common and result fromfaulty electrode placement of insufficient electrode gelcausing abrupt changes in impedance. The electrodeshould be reset and gel applied. If the artifact persists, thenelectrodes need to be changed. Other sources of artifactsare the electrode wires, the cables, and the switches. In thePSG machine, random fluctuations of charges result insome instrumental noise artifacts. If the sensitivity isgreater than 2 microvolts per millimeter, which is not gen-erally used in PSG recordings, then these instrument arti-facts may interfere with the recording. Loose contacts inswitches or wires may also cause sudden changes in voltageor loss of signal.

PSG artifacts must be eliminated or reduced to aminimum for proper identification of the EEG, EMG, and

other bioelectrical potentials for correct staging of thesleep and for diagnosis and classification of sleep disorders.Identification of all these artifacts is best made at the timeof the recording by the skilled PSG technologist. Particu-lar attention should be paid to the electrodes, electrode gelapplication, and impedance. Preparation of the patient andelectrode application is fundamental for recording a goodquality PSG.

PITFALLS OF POLYSOMNOGRAPHY

PSG is the single most important laboratory test forassessment of sleep disorders, particularly in patients pre-senting with excessive daytime somnolence. However, PSGhas considerable limitations. First, there is no standardized


FIGURE 114. A 30-second excerpt from an overnight PSG recording shows the presence of a C3 electrode pop artifact. Near the middleof the epoch it is accentuated by a movement artifact simultaneously recorded on the chin electromyography, left and right tibialis ante-rior electromyography, and snoring channel. EEG, Top 10 channels; Lt. and Rt. EOG, left and right electrooculograms; electromyogra-phy of chin; Lt./Rt. Tib. EMG, left and right tibialis anterior electromyography; oronasal thermistor; chest and abdomen effort channels;snore monitor; EKG, electrocardiography.

uniform protocol used in all sleep laboratories, and this maymake the comparison of the data from one laboratory toanother somewhat misleading. The most serious limitationis that the overnight in-laboratory PSG is labor intensive,time consuming, and expensive. A single nights PSG maymiss the diagnosis of mild OSAS, PLMS, parasomnias, ornocturnal seizures. PSG data and the patients clinical find-ings may not be concordant. Standard PSG study cannotdiagnose upper airway resistance syndrome definitively.PSG cannot determine etiology of apnea-hypopnea syn-drome. PSG is not helpful in the diagnosis of insomnia, themost common sleep disorder in the general population.PSG is not useful in the diagnosis or treatment of circadian

rhythm sleep disorders. PSG data may be confounded bythe first night effects (e.g., increased wakefulness and stageI NREM sleep and decreased slow-wave and REM sleep).Standard PSG does not measure PaCO2 and thus may misshypoventilation, which is an early abnormality (particularlyREM-related hypoventilation) in neuromuscular disorders.Standard PSG does not adequately measure cardiac func-tion (one channel of EKG is inadequate), which may affectthe prognosis of OSAS. Furthermore, cardiorespiratorysleep studies, which do not include EEG, may producefalse negative findings in mild to moderate OSAS patients.Standard PSG does not include autonomic monitoring,which may be important for assessment of autonomic


arousal as well as for assessing autonomic changes whichare intense during sleep. Finally, lack of a standardized def-inition of hypopnea remains a serious limitation to the diag-nosis and outcome studies of patients with sleepapnea-hypopnea syndrome.

BIBLIOGRAPHY

1. Keenan S. Polysomnographic technique: An overview. In:Chokroverty S, ed. Sleep Disorders Medicine: Basic Science,

Technical Considerations, and Clinical Aspects, 2nd Ed.Boston: Butterworth-Heinemann, 1999:151174.

2. Chokroverty S. Polysomnography and related procedures. In: Hallett M, ed. Movement Disorders. Handbook of ClinicalNeurophysiology, Vol. 1, Amsterdam: Elsevier, 2003:139151.

3. Broughton RJ. Polysomnography: Principles and applicationsin sleep and arousal disorders. In: Niedermeyer E, Lopes DaSilva, eds. Electroencephalography: Basic Principles, ClinicalScience, and Related Fields, 4th Ed. Philadelphia: Lippincott,Williams and Wilkins, 1999:858895.

2Electroencephalography for the

Sleep Specialists

SUDHANSU CHOKROVERTY, MEETA BHATT,

AND TAMMY GOLDHAMMER

Electroencephalography (EEG) records the difference in electrical potentials between two electrode sites over the scalp. The EEG recorded over the scalp results fromextracerebral current flow due to the summated excita-tory postsynaptic potentials (EPSP) and inhibitory postsy-naptic potentials (IPSP). Rhythmic oscillations of the thalamocortical neuronal projections cause synchronoussynaptic EPSP and IPSP over the areas of the cortex. Thescalp EEG recording voltages are attenuated and reflectabout one tenth of the voltage recorded over the corticalsurface.

The electrical signals are transmitted by the electrodesand conducting gel through the electrode wires, which areattached between the electrodes and the jack box of thepolysomnography (PSG) equipment. Different types ofelectrodes are available but for recording EEG activities,the best electrodes to use are the gold cup electrodes withholes in the center and the silver-silver chloride electrodes.Silver-silver chloride electrodes need repeated chloridingfor proper maintenance. Electrode paste or conducting gel is used to secure the electrodes. Positive and negativechargers are generated between the scalp and recordingelectrode as a result of ionic dissociation. The electrode-electrolyte interface is the most critical link in the PSGmachine, as most of the artifacts originate at this site and careful preparation is therefore very important. Theimpedance in a pair of electrodes should be measured byan impedance meter and should not be greater than 5000(5K) ohms. High impedance impairs the ability of the elec-trical signal to reach the amplifier and interferes with thecapacity of the amplifier to eliminate environmental noise,thus increasing the artifacts. All electrode wires terminatein a pin, which is plugged into the electrode box, called a

jack. All the wires from the head are gathered into a pony-tail in the back of the head and then secured by tying themtogether. A wrap around the scalp secures the electrodesand the wires. The arrangement is therefore as follows:electrodes from the surface of the scalp are connected byelectrode wires into the jacks, which are numbered, andthen the wires from the jack box are connected through ashielded conductor cable to the electrode montage selec-tor containing rows of switches in pairs corresponding tothe inputs of the amplifier.

The placement of the electrodes is determined by the10-20 electrode placement system which is recommendedby the International Federation of Societies for EEG andClinical Neurophysiology, and was published by Jasper in1958. The 10-20 system is based on definable anatomicallandmarks (Figure 21). The system consists of lettersdenoting the parts of the brain underneath the area of thescalp and numbers denoting specific location. The oddnumbers refer to the left side and the even numbers referto the right side. In the original Rechtschaffen-Kales (R-K) scoring technique, only one channel (C3-A2 or C4-Al) was recommended. One channel may be sufficient torecord the background waking activities as well as vertexsharp waves, K complexes, and sleep spindles. However,most of the laboratories use at least four channels (C3-A2,C4-Al, O1-A1, O2-A2) to clearly document the onset ofsleep. Some laboratories use eight to ten channels of EEG(see Figure 11) to cover the parasagittal and the tempo-ral areas of the brain to record possible focal or diffuseEEG abnormalities as well as for recording epileptiformactivities. The technician should be familiar with themeasurement technique for placement of the electrodesaccording to the 10-20 international system. Important

29

30 Electroencephalography for the Sleep Specialists

landmarks for measuring include the inion, the nasion, andthe right and left preauricular points. The distance fromthe nasion to inion along the midline through the vertexshould be measured. FPz in the midline is 10 percentabove the nasion of the total distance between the inionand nasion. The electrodes marked FP1 and FP2 arelocated laterally 10 percent above the nasion of the totaldistance between the inion and nasion measured along thetemporal regions through the preauricular points. Ozdenotes an electrode placed at a distance of 10 percentabove the inion of the total distance between the nasionand inion. T3 and T4 electrodes are placed in a location 10percent above the preauricular points of a total distancebetween the two preauricular points. The rest of the elec-trodes are located at a distance of 20 percent measuredfrom inion to nasion anteroposteriorly or laterally throughthe ears as well as transversely between the ears. Themontage refers to an arrangement of two electrodes (der-ivations). Both bipolar montage (connection of the elec-trodes between two relatively active sites over the scalp)and referential montage (connection of the electrodes

between an active and relatively inactive site, e.g., A1, A2,Cz, Pz) are recommended.

NORMAL WAKING AND SLEEPELECTROENCEPHALOGRAPHY

RHYTHMS IN ADULTS

The dominant rhythm in adults during wakefulness isthe alpha rhythm, which consists of 8 to 13 hertz and is distributed synchronously and symmetrically over theparieto-occipital regions (Figure 22). The frequency ofthe rhythm between the two hemispheres should not varyby more than 1 hertz and the amplitude should not varyby more than 50 percent. The alpha rhythm is best seenduring quiet wakefulness with eyes closed and is signifi-cantly attenuated by eye opening or mental concentration.A small percentage of normal adults show no alpha rhythmand their EEG is characterized by the dominant rhythmof low-amplitude EEG in the beta frequencies (greaterthan 13 hertz) with amplitude varying between 10 and 25microvolts (Figure 23). In most adults, the beta rhythmsare seen predominately in the frontal and central regionsintermixed with the posterior alpha rhythms.

There are characteristic changes in the backgroundEEG rhythms as an individual goes into nonrapid eyemovement (NREM) sleep consisting of four stages andREM sleep consisting of tonic and phasic stages. Thevarious characteristics during different sleep stages aredescribed under the section of sleep scoring technique(Chapter 3).

EEG in elderly subjects shows a progressive slowing of the alpha frequency during wakefulness and diminutionof alpha blocking and photic driving responses. Focal temporal slow waves, particularly in the left temporalregion, often called benign temporal delta transients(Figure 24) of the elderly, are seen in many apparentlynormal elderly individuals and are sometimes associatedwith sharp transients. Transient bursts of anteriorly domi-nant rhythmic delta waves may also be seen in some elderly subjects in the early stage of sleep. Other changesin older adults consist of sleep fragmentation with frequentawakenings, including early morning awakening and multiple sleep stage shifts. Another important finding inthe sleep EEG of older adults is the reduction of ampli-tude of the delta waves and therefore, according to R-Kscoring technique, they do not meet the amplitude crite-ria and slow-wave sleep is thought to be reduced in thesesubjects.

FIGURE 21. (From Keenan S. Polysomnographic technique: An overview. In Chokroverty S, ed. Sleep Disorders Medicine.Boston: Butterworth-Heinemann, 1999, with permission.)


Electroencephalography for the Sleep Specialists 31

FIGURE 22. A: Normal awake EEG pattern characterized by symmetrical, sinusoidal posterior dominant alpha rhythm at 11 to 12hertz, 82 to 105 microvolts in a 40-year-old woman at a conventional EEG paper speed of 10 seconds per page. Eye movement artifactis noted in channels linked to frontal FP1 and FP2 electrodes.


FIGURE 22, contd B: Same subject data as in A viewed at a 30-second epoch.


FIGURE 23. A: Normal awake EEG pattern characterized by diffuse low-voltage beta rhythm in a 23-year-old man at a conventionalEEG paper speed of 10 seconds per page.


FIGURE 24. A: EEG shows transient burst of delta activity in the left temporal, and occasionally also in the right temporal, region ina 94-year-old woman with history of syncope.



ABNORMAL EEG PATTERNS

The abnormal pattern may consist of focal or diffuseslow waves and epileptiform discharges. The importanceof multiple channel EEG recordings is to document thosefocal (Figure 25) or diffuse (Figure 26) slow waves andepileptiform discharges. Many patients are referred to the PSG laboratory for a possible diagnosis of nocturnalseizures. The standard recommended EEG recordings of one to two channels or even four channels of record-ings miss most of the epileptiform discharges during all-night recording. Therefore, in patients suspected of nocturnal seizures, PSG study should include multiplechannels of EEG covering temporal and parasagittalregions and simultaneous video recording (video-PSGstudy) for correlation of the electrical activities with theactual behavior of the patients. In computerized PSG

recordings (digital PSG recordings), which are currentlyperformed in most laboratories, it is easy to change therecording speed from the standard 10 millimeters persecond of the usual sleep recording to 30 millimeters persecond of the standard EEG recording for proper identi-fication of the epileptiform discharges. Full complementof electrodes and special electrode placements (e.g., Tl andT2 electrodes) should be used.

RECOGNITION OF EPILEPTIFORM PATTERNS IN THEPOLYSOMNOGRAPHIC TRACING

First, it should be remembered that epilepsy is a clini-cal diagnosis; therefore history must be obtained from pa-tients, relatives, witnesses, or medical personnel if a patient



FIGURE 25. A: EEG shows focal slowing at 3 to 6 hertz over the left temporoparietal region in an 85-year-old man with history of astroke. Normal alpha rhythm at 10 to 11 hertz is noted on the right side.


FIGURE 26. A: EEG shows mild diffuse slowing of the background rhythm, dominated by 6- to 7-hertz theta activity in an 80-year-old woman with history of hypertension, diabetes mellitus, and bacteremia.


FIGURE 26, contd B: Same subject data as in A viewed at a 30-second epoch. Additionally, brief bursts of rhythmic muscle artifactare noted on the left side toward the end of the epoch. Mild diffuse slowing of the background rhythm may be missed during conven-tional polysomnography scoring. The clue is in that the epoch looks like an awake recording without the characteristic alpha rhythm.This necessitates viewing the segment as a 10-second epoch.


FIGURE 26, contd C: A 27-year-old man with a 3-week history of sudden severe headaches diagnosed as pituitary adenoma. EEGshows evidence of a brief burst of frontal intermittent rhythmic delta activity (FIRDA), synchronously recorded over both hemispheres.FIRDA was originally believed to arise from deep midline pathology, which is demonstrated in this case. However, FIRDA is now believedto be anatomically nonlocalizing and indicates diffuse cerebral dysfunction secondary to metabolic or structural involvement.


FIGURE 26, contd D: Same subject data as in C viewed at a 30-second epoch.

is referred to the sleep laboratory with a suspected diag-nosis of nocturnal seizure. The history must include ictal,preictal, postictal, and interictal periods, family and drughistories, as well as history of any significant medical or sur-gical illnesses that might be responsible for triggering theseizures. Physical examination must be conducted to findany evidence of neurological or other medical disordersbefore PSG recording in the laboratory.

EEG SIGNS OF EPILEPSY

EEG is the single most important diagnostic laboratorytest for patients with suspected seizure disorders. Certaincharacteristic EEG waveforms correlate with a high percentage of patients with clinical seizures and therefore

can be considered of potentially epileptogenic significance.These epileptiform patterns consist of spikes, sharp waves,spike and waves, sharp- and slow-wave complexes, as wellas evolving pattern of rhythmic focal activities, particularlyin neonatal seizures. Another pattern that correlates highlywith complex partial seizure is the temporal intermittentrhythmic delta activity (TIRDA) (Figure 27).

Recognition of a spike or a sharp wave depends on thepresence of certain characteristics. A spike is defined as awaveform, which suddenly appears out of the backgroundrhythm with a brief duration of 20 to 70 millisecondsshowing a field of distribution and is often followed by anafter-going slow wave (Figure 28A). The characteristicfeature of a true epileptiform spike or sharp wave is abiphasic or triphasic appearance with a sharp ascendinglimb followed by a slow descending limb in contrast to aug-

Electroencephalography for the Sleep Specialists

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