Polysomnography: recording and sleep staging


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Polysomnography: recording and sleep staging

  1. 1. Polysomnography: Recording and Sleep staging
  2. 2. Introduction <ul><li>“ Sleep is a reversible behavioral state of perceptual disengagement from and unresponsiveness to the environment”. </li></ul><ul><li>Complex and active state. </li></ul><ul><li>Distinctive changes in physiology including muscle tone, temperature, and endocrine, gastrointestinal, and cardio-respiratory activity. </li></ul><ul><li>Sleep disorders are common, underdiagnosed, have significant morbidity and are treatable. </li></ul>
  3. 3. Introduction <ul><li>Term “Polysomnography” was proposed by Holland ,Dement and Raynal (1974). </li></ul><ul><li>“ Recording, analysis and interpretation of multiple, simultaneous, physiological parameters over the course of several hours, mainly in the night”. </li></ul><ul><li>Utility: </li></ul><ul><li>To diagnose sleep related disorders. </li></ul><ul><li>To study abnormalities of sleep & wakefulness. </li></ul><ul><li>Sleep staging. </li></ul><ul><li>To study physiological functions of other organ systems during sleep. </li></ul>
  4. 4. Recording a polysomnogram
  5. 5. Indications for PSG <ul><li>Sleep related breathing disorders. </li></ul><ul><li>Assessment of treatment results- OSA. </li></ul><ul><li>CPAP titration in OSA. </li></ul><ul><li>Sleep related behavioral disorders. </li></ul><ul><li>Atypical or unusual parasomnias. </li></ul><ul><li>Narcolepsy. </li></ul><ul><li>Neuromuscular disorder & sleep related symptoms. </li></ul><ul><li>Paroxysmal arousal or seizure phenomenon. </li></ul><ul><li>Periodic Leg Movements of Sleep. </li></ul><ul><li>Parasomnias not responding to conventional therapy. </li></ul>American Academy of Sleep Medicine 2005
  6. 6. Initial Assessment <ul><li>Detailed medical & psychiatric history. </li></ul><ul><li>Medication, smoking, alcohol, activities on that day, last meal prior to the study. </li></ul><ul><li>Assessment of sleep: </li></ul><ul><li>Sleep history for the last 24 hrs. </li></ul><ul><li>Sleep scoring systems: </li></ul><ul><li>1. Stanford Sleepiness scale </li></ul><ul><li>2. Epworth Sleepiness scale </li></ul><ul><li>3. Pittsburgh Sleep Quality Index (PSQI) </li></ul>
  7. 7. Recording Room <ul><li>The study patient and the monitoring apparatus and technologist are housed in adjacent rooms. </li></ul><ul><li>Recording room should be as homely and comfortable. </li></ul><ul><li>It should be sound-proofed and air-conditioned. </li></ul><ul><li>Intercom system to interact with the patient. </li></ul><ul><li>Rheostatically controlled lighting. </li></ul><ul><li>Toilet and restroom. </li></ul>
  8. 8. Recording room with study subject
  9. 9. Recording video camera
  10. 10. Monitoring room for the technician
  11. 11. Patient instructions <ul><li>Reporting time: 1 hour prior to the usual time of sleep. </li></ul><ul><li>A prior visit to familiarize with the lab is useful. </li></ul><ul><li>Patient should have had a relaxed day, without daytime sleep. </li></ul><ul><li>Headwash, adequate food. </li></ul><ul><li>Once electrodes are attached patient goes to bed. </li></ul><ul><li>Time of “lights out” – for measurement of latency to sleep onset and other variables. </li></ul>
  12. 12. Briefing Hair wash, Avoid afternoon nap Stop nonessential medications Nocturnal Polysomnography PSG components 4 channel EEG EOG Chin EMG Tibial EMG ECG Airflow monitor Pulse oximeter Respiratory effort Body position
  13. 13. PSG monitor with all the channels
  14. 15. Specific channels and Utility Male impotence Nocturnal penile tumescence Cardiac arhrythmias ECG Oxygen desaturation Transcutaneous oxymetry Chest/ abdominal respiratory effort Inductive plethysmography Disorders of respiration Thermistor/ thermocouple Respiratory effort Intercostal EMG Movement disorders Superficial EMGs: Tibialis anterior, gastrocnemius, wrist extensor
  15. 16. EEG Monitoring <ul><li>Rechtschaffen staging manual: single central EEG lead. </li></ul><ul><li>Often insufficient to separate wakefulness from Stage 1. </li></ul><ul><li>4 commonly used channels: </li></ul><ul><li>8-16 channels or more may be used if required. </li></ul><ul><li>Paper speed- 10mm/sec. </li></ul><ul><li>Better visualization of alpha waves , sleep spindle, saw-tooth appearance and eye movements. </li></ul>O2- A1 C4- A1 O1- A2 C3- A2
  16. 17. EEG Electrodes
  17. 18. EEG electrodes: occipital electrodes
  18. 19. EEG central electrodes, EOG electrodes
  19. 20. Electro-Oculogram <ul><li>For routine recordings, referential recording from each outer canthus to the ipsilateral ear: </li></ul><ul><li>1. Presence or absence of eye movements in sleep staging. </li></ul><ul><li>2. Detailed analysis of separate horizontal and vertical eye movements. </li></ul><ul><li>Shows horizontal eye movements as out-of-phase potentials in the two channels </li></ul><ul><li>However, artifacts are noted, especially in slow-wave sleep when the EEG reaches maximum amplitude. </li></ul>
  20. 21. Electro- Oculogram <ul><li>A bipolar linkage from an electrode 1 cm lateral and 1 cm superior to one outer canthus to a second electrode 1 cm lateral and 1 cm inferior to the other outer canthus. </li></ul><ul><li>The electrode toward which the eyes move becomes relatively positive, the other relatively negative. </li></ul><ul><li>Frontal EEG activity tends to be equipotential between these two periocular electrodes and cancel out. </li></ul>< 0.3 sec > 1 Hz Rapid eye movements > 0.5 sec 0.25- 0.5 Hz Slow eye movements Slope duration Frequency
  21. 24. EMG Monitoring <ul><li>Regular EEG electrodes are placed submentally on the skin overlying the mylohyoid muscle. </li></ul><ul><li>Electrode placement: </li></ul><ul><ul><li>Pairs of electrodes on the tip of the jaw or on the side of the face over the masseter muscles. </li></ul></ul><ul><ul><li>Three electrodes attached, one just behind the tip of the jaw and the other two more posteriorly and laterally. </li></ul></ul><ul><li>Tonic EMG level in axial muscles usually decreases from wakefulness through stages 1, 2, 3, and 4 of NREM sleep, and is normally absent in REM sleep. </li></ul>
  22. 25. EMG Monitoring <ul><li>Factors for variability in resting chin EMG: </li></ul><ul><ul><li>Amount of subcutaneous adipose tissue. </li></ul></ul><ul><ul><li>Muscle tone. </li></ul></ul><ul><ul><li>Age, and other factors. </li></ul></ul><ul><li>Tonic EMG activity may rarely be absent in all stages other than wakefulness, stage 1 drowsiness and body movements. </li></ul><ul><li>Minimal filtering permits passage of the higher frequency EMG activity. </li></ul>
  23. 26. EMG Monitoring <ul><li>Limb EMGs: Extra long leads are required. </li></ul><ul><li>Utility of limb EMG: </li></ul><ul><li>Periodic limb movements in sleep. </li></ul><ul><li>Restless leg syndrome. </li></ul><ul><li>Other movement disorders. </li></ul><ul><li>To document the hand and arm gestures of REM sleep behavior disorder. </li></ul><ul><li>Record convulsive movements during nocturnal epileptic seizures. </li></ul>
  24. 27. Limb EMG electrode placement
  25. 29. Respiratory Monitoring <ul><li>To measure upper airway exchange and respiratory effort. </li></ul><ul><li>Routine procedure in diagnostic polysomnography. </li></ul><ul><li>Differentiating various forms of apnea requires monitoring of: </li></ul><ul><li>Upper airway airflow. </li></ul><ul><li>Thoracoabdominal movement. </li></ul><ul><li>Endoesophageal (intrathoracic) pressure recording. </li></ul><ul><li>Snoring Monitors. </li></ul><ul><li>Indirect Arterial Blood Gas Monitoring. </li></ul>
  26. 30. Upper airway airflow <ul><li>Thermistor/ thermocouple: records upper airway flow rate. </li></ul><ul><li>Thermistor resistance fluctuations are induced by temperature changes of air passing in and out through the mouth and nostrils- generates a potential. </li></ul><ul><li>Two thermocouples are attached at the nostrils and a third is placed in front of the mouth. </li></ul><ul><li>A small plastic mask funneling upper airway air movement to a single sensor can be substituted, but may be less comfortable for the patient. </li></ul>
  27. 31. Thermistor
  28. 32. Thoracoabdominal Movement <ul><li>Common method of monitoring respiratory movement.  </li></ul><ul><li>Improved method of spirometry. </li></ul><ul><li>Separates chest and abdominal movement and then adds them together, mimicking total spirometric volume. </li></ul><ul><li>Sensors are two wire coils, one placed around the chest and the other around the abdomen. </li></ul><ul><li>A change in mean cross-sectional coil area produces a proportional variation in coil inductance, which is converted into a voltage change by a variable frequency oscillator. </li></ul>
  29. 33. Sensors for plethysmography
  30. 34. Sensors for respiratory effort
  31. 35. Transducer for thoracoabdominal movement
  32. 36. Transducer for thoracoabdominal movement
  33. 37. Thoracoabdominal Movement <ul><li>Impedance Plethysmography: </li></ul><ul><li>Measures transthoracic and transabdominal impedance changes with respiration. </li></ul><ul><li>Magnetometry: </li></ul><ul><li>Measures the magnetic field generated between two sides of a body by chest and abdominal movement. </li></ul><ul><li>Apparatus : ferrite wire coils held in rubber sleeves and attached to aluminum plates. </li></ul><ul><li>Cumbersome technique. </li></ul>
  34. 39. Apnea and hypopnea <ul><li>Apnea: </li></ul><ul><li>Absence of or > 90% decrease in airflow compared to baseline lasting ≥ 10 sec. </li></ul><ul><li>Classified as central, obstructive or mixed. </li></ul><ul><li>Hypopnea: </li></ul><ul><li>Any of the following respiratory events lasting ≥ 10 sec: </li></ul><ul><li>1. ≥ 50% reduction of airflow (<90%). </li></ul><ul><li>2. ≥ 30% reduction of airflow (but <50%) but associated with ≥ 4% oxygen desaturation. </li></ul>
  35. 46. Snoring Monitors <ul><li>Snoring detected by: </li></ul><ul><li>Room microphone and recorded directly on the polysomnogram tracing. </li></ul><ul><li>Vibration sensor - a small rubber disk containing a piezocrystal that is taped to the patient's throat area </li></ul><ul><li>Signal is appropriately filtered, rectified, and integrated before display. </li></ul><ul><li>May pick up snoring of patient’s attender. </li></ul><ul><li>Utility: </li></ul><ul><li>OSA </li></ul>
  36. 47. Snoring monitors
  37. 48. Chin EMG, airflow and snoring sensors
  38. 49. Snoring monitor
  39. 51. Intrathoracic Pressure Monitoring <ul><li>Endoesophageal pressure probe: most sensitive detection of heightened respiratory effort. </li></ul><ul><li>An endoesophageal tube is passed nasally till the probe is about 5 cm above the esophageal-gastric junction. </li></ul><ul><li>Measures increases in intrathoracic negative pressure to overcome increased upper airway resistance. </li></ul>
  40. 52. Indirect Arterial Blood Gas Monitoring <ul><li>Severity of the various sleep-related breathing impairment can be assessed by: </li></ul><ul><li>1. O 2 saturation (%) </li></ul><ul><li>2. O 2 tension (mm Hg, torr) </li></ul><ul><li>3. Transcutaneous TcCO 2 </li></ul><ul><li>Useful in pulmonary or muscular diseases (e.g., COPD, myotonic dystrophy). </li></ul>
  41. 53. Oxygen Saturation <ul><li>Continuous oxygen saturation monitoring is routine. </li></ul><ul><li>Finger or ear probe: </li></ul><ul><li>1. Contains two fiberoptic bundles enclosed in Teflon or light-emitting diodes and a light sensing apparatus. </li></ul><ul><li>2. Based on the absorption of certain wavelengths of light. </li></ul><ul><li>Change in SaO2 associated with a respiratory event appears 20-40 sec after the actual event. </li></ul><ul><li>The skin is cleaned with 70% isopropyl alcohol and may be arterialized by brisk rubbing. </li></ul>
  42. 54. Oxymetry sensor
  43. 55. Oxymetry sensor
  44. 57. Oxygen Tension <ul><li>Transcutaneous oxygen tension is measured. </li></ul><ul><li>Detects arterial oxygen tension in mmHg of mercury or torr. </li></ul><ul><li>Skin over chest wall is the most often used site. </li></ul><ul><li>The electrode contains a thermistor-regulated heating resistor to ensure necessary skin arterialization. </li></ul><ul><li>Electrode site may have to be altered during the night to avoid heat blistering. </li></ul><ul><li>It is accurate, but cumbersome and uncomfortable. </li></ul><ul><li>Not used commonly. </li></ul>
  45. 58. Capnography <ul><li>Transcutaneous CO 2 measures may be monitored. </li></ul><ul><li>pO 2 and CO 2 detections are performed by two parts of the same probe. </li></ul><ul><li>May rarely be used to determine respiratory rate. </li></ul><ul><li>To document CO 2 retention in various sleep-related respiratory disorders. </li></ul><ul><li>1. COPD </li></ul><ul><li>2. Hypoventilation syndromes. </li></ul>
  46. 59. Electrocardiogram <ul><li>ECG abnormalities in sleep apnea patients: </li></ul><ul><li>1. Marked sinus arrhythmia. </li></ul><ul><li>2. Extra systoles. </li></ul><ul><li>3. Prolonged asystolic episodes. </li></ul><ul><li>4. Atrial or ventricular fibrillation. </li></ul><ul><li>5. Nocturnal angina may show ST segment deviation. </li></ul><ul><li>Also in sleep terrors, idiopathic CNS hypersomnia, or psychophysiological (stress-related) insomnia. </li></ul>
  47. 60. Electrocardiogram
  48. 61. <ul><li>Esophageal pH </li></ul><ul><li>Gastro-esophageal reflux of acidic stomach contents into the lower esophagus may cause insomnia. </li></ul><ul><li>pH probe is introduced nasally and swallowed to about 5 cm above the esophageal sphincter. </li></ul><ul><li>Penile Tumescence </li></ul><ul><li>Psychogenic Vs organic causes of impotence. </li></ul><ul><li>Psychogenic cases: Normal REM sleep-related erections. </li></ul><ul><li>Strain gauge technique: to calibrate, record, score NPT. </li></ul><ul><li>Supplemented by bulbocavernosus EMG activity. </li></ul>
  49. 62. Core Body Temperature <ul><li>Rectal body temperature approximates core body temperature. </li></ul><ul><li>Define several disorders of the circadian sleep-wake cycle. </li></ul><ul><li>May be abnormal in various neurological lesions: - damage to the suprachiasmatic nuclei. </li></ul><ul><li>A number of commercially available recording devices exist, usually as part of an ambulant monitoring system. </li></ul>
  50. 63. Artifacts
  51. 70. Sleep Staging
  52. 71. Sleep staging NREM REM Stage 1 Stage 2 Stage 3 Stage 4 CAP Non-CAP Phase B Phase A
  53. 72. Proportion of sleep stages <ul><li>NREM: 75-80% </li></ul><ul><ul><li>Stage 1: 10-12% </li></ul></ul><ul><ul><li>Stage 2: 45-55% </li></ul></ul><ul><ul><li>Stage 3 and 4: 15-20% </li></ul></ul><ul><li>REM: 20-25% </li></ul><ul><li>Sleep is entered through NREM. </li></ul><ul><li>NREM-REM cycle every 90 minutes. </li></ul><ul><li>4-6 cycles per major sleep episode. </li></ul><ul><li>REM duration progressively lengthens. </li></ul><ul><li>Early cycles dominated by slow wave sleep. </li></ul>
  54. 73. Stage W: Awake state Eyes open: slow eye movements when drowsy. Eyes closed: REMs or none. Relatively high tonic activity. Voluntary control. Voluntary control Eyes closed: rhythmic α , occipital dominant; disappears on eye opening. Eyes open: low voltage mixed frequency. EMG EOG EEG
  55. 75. Stage 1 Vertex sharp waves: biphasic sharp transients, maximal centrally. Tonic activity is less compared to awake state. Decreased blinking rate. SEMs ( 0.25-0.5Hz) lasting several seconds. <ul><li>Attenuation of α . </li></ul><ul><li>Relatively low voltage, mixed frequency (2- 7/s). </li></ul><ul><li>Vertex sharp waves in late stage. </li></ul><ul><li>Synchronous high voltage theta waves in children. </li></ul>EMG EOG EEG
  56. 77. Stage 2 Tonic activity low No eye movements, or occasional SEMs. Background: Relatively low voltage, mixed frequency(3-7/sec). Sleep spindles: K complex: Biphasic; maximum at vertex. Delta activity <20% EMG EOG EEG
  57. 78. <ul><li>Sleep spindles </li></ul><ul><ul><li>Waxing and waning pattern. </li></ul></ul><ul><ul><li>12- 14 Hz central bursts. </li></ul></ul><ul><ul><li>Should last > 0.5 sec to be scored. </li></ul></ul><ul><ul><li>Amplitude criteria: 15 µV. </li></ul></ul><ul><ul><li>Voltage tends to become smaller with age. </li></ul></ul><ul><li>K complexes consist of all or any 2 of 3 main components </li></ul><ul><ul><li>Negative vertex sharp-wave. </li></ul></ul><ul><ul><li>Following positive slow-wave maximal frontally </li></ul></ul><ul><ul><li>Sleep spindle maximal in the central regions. </li></ul></ul><ul><ul><li>Should last > 0.5-sec to be scored. </li></ul></ul>Sleep spindle and K complex
  58. 82. Three Minute Rule <ul><li>In Stage 2 if six successive epochs (or 3 minutes) look like Stage 1, these have to be coded as Stage 2 as long as a sleep spindle or K-complex is seen during this time-frame. </li></ul><ul><li>If, after the three minutes (6 epochs), no sleep spindles or K-complex are seen, then retrace to the last epoch of Stage 2 and code the six epochs as Stage 1. </li></ul>
  59. 83. Stage 4 Stage 3 Low level tonic activity. None. Delta activity (2/sec or less), high amplitude(> 75 µV). 20%-50% of epoch. Maximum in central. Sleep spindles may be seen. Low level tonic activity. None. Delta activity with high amplitude(> 75 µV) comprising >50% of epoch. Sleep spindles and K complex occur rarely. EMG EOG EEG
  60. 87. Stage REM Tonic: suppression. Phasic: twitches. Phasic: intense REM. Tonic: none or few. Relatively low voltage, mixed frequency saw tooth waves; theta activity; slow alpha. Initial REM period at times contains some low-voltage spindles. EMG EOG EEG
  61. 89. Stages of sleep
  62. 90. Proportion of sleep stages
  63. 91. Sleep stages and age
  64. 92. Sleep terminology Time spent in sleep/ time in bed (Normal >85%) Sleep efficiency (REM+ NREM)- awake time Total sleep time Light off to sleep onset. Latency of sleep onset 3 consecutive epochs of Stage 1 NREM, or first epoch of any other Stage. Sleep onset Total minutes spent in bed. Time in bed End of recording. Light on Start of recording. Lights out Fixed time domain of 30 sec. Epoch
  65. 93. Sleep terminology <ul><li>Movement time (MT): is scored for each epoch in which more than 50% of the epoch is obscured by movement artifacts that make staging impossible. </li></ul><ul><ul><li>1. Must be preceded and followed by epochs of definite EEG sleep. </li></ul></ul><ul><ul><li>2. Distinguish from movement during arousals and during wakefulness. </li></ul></ul><ul><li>Wake percentage: is the percentage of wake period from lights out to lights on. </li></ul><ul><li>REM latency: time from sleep onset to first appearance of REM sleep. </li></ul>
  66. 94. <ul><li>Arousal: </li></ul><ul><li>Differs from awakening. </li></ul><ul><li>EEG frequency shift lasting >3 sec (alpha,theta or beta) preceded by > 10 sec of continuous sleep. </li></ul><ul><li>REM sleep: arousal requires increased chin EMG activity. </li></ul><ul><li>May or may not be associated with body movements or respiratory events. </li></ul><ul><li>More than 10-15/hr is abnormal. </li></ul><ul><li>Microarousals: </li></ul><ul><li>Consist of brief arousals typically lasting 5 to 10 seconds. </li></ul><ul><li>No change of sleep stage. </li></ul><ul><li>Increased day time somnolence. </li></ul>
  67. 96. Limb movements <ul><li>Periodic limb movements are scored in sleep only when there are ≥ 4 limb movements in sequence occuring > 5 sec but < 90 sec apart. </li></ul><ul><li>A limb movements is an increase in the limb activity lasting 0.5 to 5 sec with an amplitude of > 25% of the burst of EMG activity recorded during biocalibration. </li></ul>
  68. 97. Generating a report
  69. 98. Sleep hypnogram <ul><li>Summary of the entire night’s PSG data in a graphic form. </li></ul><ul><li>Helpful to open a window for the hypnogram simultaneously while reviewing PSG. </li></ul><ul><li>Allows to easily select the part of the study one wishes to see. </li></ul><ul><li>Switch the epochs rapidly for comparison of sleep stages. </li></ul>
  70. 101. Polysomnography report <ul><li>Patient: </li></ul><ul><li>Test Date: 8/28/2009 </li></ul><ul><li>Age/Sex: Pt ID no: </li></ul><ul><li>Height: Weight: EEG# PSG </li></ul><ul><li>Technician: Referring Doctor: </li></ul><ul><li>Comments: </li></ul><ul><li>History: </li></ul><ul><li>Medications: </li></ul><ul><li>Test Information: </li></ul><ul><li>Referral Reason: </li></ul>
  71. 102. Summary Of Sleep Parameters <ul><li>Study Start Time: 9:49:57 PM Lights Off Time: 9:49:57 PM </li></ul><ul><li>Study End Time: 5:35:07 AM Lights On Time: 5:35:07 AM </li></ul><ul><li>Total Study Time (TIB): 465.2 Minutes 7.8 Hours 930.3 Epochs </li></ul><ul><li>Sleep Period Time (SPT): 383.5 Minutes 6.4 Hours 767.0 Epochs </li></ul><ul><li>Total Sleep Time (TST): 290.5 Minutes 4.8 Hours 581.0 Epochs </li></ul><ul><li>Sleep Efficiency: 62.5 % </li></ul><ul><li>Latency to Stage 1: 30.1 minutes </li></ul><ul><li>REM sleep latency: 7.0 minutes </li></ul><ul><li>Sleep Onset: 25.5 minutes </li></ul>
  72. 103. Sleep parameters 0.0 60.1 174.7 Stage Wake 25.5 85.5 248.5 Non-REM 32.5 14.5 42.0 Stage REM 51.5 0.2 0.5 Stage 4 43.0 15.1 44.0 Stage 3 26.0 56.1 163.0 Stage 2 25.5 14.1 41.0 Stage 1 Latencies %TST Minutes
  73. 104. Arousal analysis 26 0 5.4 26 Arousal associated with Snores 1 0 0.2 1 Arousal associated with Hypopnea 0 0 0.0 0 Arousal associated with Central Apneas 0 0 0.0 0 Arousal associated with mixed Apneas 1 0 0.2 1 Arousal associated with OSA 1 0 0.2 1 Arousal associated with resp. Events 10 0 2.1 10 Arousal associated with periodic Movement 0 0 0.0 0 Arousal associated with Desaturations 52 1 10.9 53 Arousals NREM REM Index No
  74. 105. Leg movements 0.6 3 3 0 Isolated with arousal 3.9 19 16 3 Isolated (Total) 2.1 10 10 0 Periodic with arousal 28.1 136 124 12 Periodic (Total) Index Total No of Movements NREM REM Movement Types
  75. 106. Respiratory events and sleep Stages 12.8 10.8 12.8 Max Duration 12.8 10.8 12.8 Min Duration 11.3 10.8 11.5 Mean Duration 3 1 2 # HYPOPNEA - - - Max Duration - - - Min Duration - - - Mean Duration 0 0 0 # OBSTRUCTIVE - - - Max Duration - - - Min Duration - - - Mean Duration 0 0 0 # MIXED - - - Max Duration - - - Min Duration - - - Mean Duration 0 0 0 # CENTRAL Total REM NREM
  76. 107. Limitations of PSG <ul><li>First Night Effect: </li></ul><ul><ul><li>Reduced sleep efficiency. </li></ul></ul><ul><ul><li>Increased awakenings and arousals. </li></ul></ul><ul><ul><li>Prolonged sleep and REM latency. </li></ul></ul><ul><ul><li>Decreased percentage of REM and slow-wave sleep. </li></ul></ul><ul><ul><li>Increased percentage of light sleep. </li></ul></ul><ul><li>Technology, technician, technique dependant. </li></ul><ul><li>Episodic disorders may be missed: eg seizures, parasomnias. </li></ul><ul><li>Night- to- night variability: eg in case of apnea. </li></ul>