Electroencephalography

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Electroencephalography

• doc. MUDr. Valja Kellerová, DrSc.
• Department of Neurology

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Principle of EEG

• EEG examines the spontaneous electrical activity
  of the brain by means of scalp electrodes
• recording procedure amplification
• recording (20 minutes at least)
• storage (on CD, DVD,
• NAS, cloud )
•  

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Electrode placement the ten-twenty electrode
system
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Principle of registration

• EEG curve is registered/recalculated as the
  difference of electric potentials between two
  electrodes (varying in the course of time)
• derivation recording from a pair of electrodes
  (two electrodes are connected to a single
  amplifier, to one EEG channel)
• bipolar
• monopolar

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Montages

• montage a particular electrode arrangement,
• a number of different derivations is diplayed
  simultaneously
• commonly used and preset montages
• longitudinal
• transverse
• ( referential )

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EEG reading

• Morphology
• Amplitude
• Frequency

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EEG reading morphology (wave shape)

• sharp waves
• spikes epileptiform patterns,
• spike-and-wave complexes they accompany epilepsy

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EEG reading amplitude

• voltage in microvolts
• a peak-to-peak measurement
• compared with the calibration signal
• commonly 20 -100 µV (in normal EEG)
• amplitude changes may be pathological

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EEG reading frequency

• the rhythmic EEG activity is classified as
• delta - less than 4 Hz
• theta 4 to 8 Hz
• alpha 8 to 13 Hz
• beta more than 13 Hz

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Normal rhythms

• Alpha rhythm
• sinusoidal waves (8-13 Hz)
• maximal over the posterior head region
  (occipitally)
• occurs during wakefulness and relaxation
• best seen with the patients eyes closed
• blocked or attenuated with eye opening

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Normal rhythms

• Beta rhythm
• is not prominent
• has low amplitude
• is best seen frontally
• not affected by eye opening
• Theta rhythm
• a small amount, of low amplitude, frontally
• accompanies drowsiness
• Delta rhythm
• is normal during deep sleep
• is normal in childhood
• is abnormal in the other cases

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The normal EEG awake EEG in adults

• typical normal EEG
• alpha rhythm
• the posterior dominant rhythm
• blocked with eyes opening
• Frontally beta and theta low amplitude activity

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The normal EEG awake EEG in adults

• low voltage EEG (atypical flat EEG)
• a variety of normal finding
• 10 of normal healthy adults
• no activity over 20 µV
• various frequencies may be present
• alpha rhythm may be absent

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The normal EEG during drowsiness and NREM sleep
Drowsiness - somnolence
disappearance of the alpha rhythm low voltage
beta activity or flat recording low voltage
theta (5-6 Hz) 3-4 Hz (moderate
amplitude) paradoxical reaction to eye opening
(alpha rhythm appears)
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The normal EEG EEG during drowsiness and sleep -
NREM sleep

• Sleep stage 2
• slow waves 3-4 Hz continue

sleep spindles (12-14-16 Hz) and K complexes
(reaction to stimuli)
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The normal EEG EEG during drowsiness and sleep -
NREM sleep

• Sleep stage 3
• slow waves of 2 Hz or less, of high amplitude
  (20-50 of the recording time)
• Sleep stage 4
• slow activity of about 1 Hz (over 50)

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The normal EEG EEG during drowsiness and sleep -
REM sleep

• REM sleep (paradoxical sleep)
• associated with dreaming
• asynchronous low voltage waves of mixed frequency
• may resemble the pattern of stage 1

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Polysomnography

• For reliable determination of REM stage
  polysomnography is necessary with recording also
• rapid eye movements
• heart rate and breathing
• EMG of the mental muscles (postural) tonic
  activity disappears in the REM stage

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Polysomnographyin NREM and REM sleep
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Polysomnography in REM sleep (patient with
narcolepsy)
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Sleep cycles

• Sleep cycles
• last about 80-120 minutes
• NREM sleep lasts about 60-90 minutes
• REM sleep lasts about 10-30 minutes (it is longer
  in the morning)

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Abnormal EEG patterns

• abnormalities of the background rhythms (areal
  differentiation, hemispheral organization)
• abnormal sleep patterns narcolepsy
• paroxysmal epileptiform abnormalities
  (generalized, focal) - epilepsy
• abnormal slow activity (generalized, focal)
  structural cerebral lesions
• amplitude changes

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Narcolepsy
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Abnormal sleep patterns - narcolepsy

• short latency to sleep onset (less than 5 min)
• sleep onset REM periods
• multiple sleep latency test (MSLT)
• gives a patient 5 opportunities to fall asleep
  during the day
• at 2-hour intervals EEG is recorded and the
  patient is given 20 min to fall asleep
• if 2 or more sleep periods contain REM sleep,
  then a diagnosis of narcolepsy is highly likely

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Abnormal EEG amplitude changes

• local - reduction of amplitude due to
• superficial lesions in the cortex, rare
• change of material between cortex and electrodes
  subdural hematoma - flattening

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Abnormal EEG amplitude changes

• generalized in deep coma
• burst-suppression pattern or flat record

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EEG in epilepsy

• interictal patterns in intervals without
  seizures
• epileptiform patterns sharp waves, spikes,
  spike-and-wave complexes
• local, as a focus
• or generalized
• ictal patterns during seizure

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Interictal patterns in epilepsy localfocus
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Interictal patterns in epilepsy generalized
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Ictal patterns depend on the type of the seizure
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Simple partial seizures - consciousness is not
impaired- Jacksonian motor or somatosensory
seizure
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Simple partial seizures -single repetitive sharp
waves
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Complex partial seizurestemporal lobe seizure
with automatisms
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Complex partial seizures -sharp theta waves 6Hz
temporal region
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Generalized absence seizures
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Absence seizures 3/sec generalized
spike-and-wave discharges
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Absence seizures
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Generalized tonic-clonic seizures
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Generalized tonic-clonic seizures

• Tonic phase generalized hypersynchronous 8-12Hz
  sharp waves and muscle artifacts
• clonic phase generalized rhythmic spikes form
  groups, followed by a slow wave
• postictal coma lower voltage arrhythmic
  activity, attenuation and slowing or flat EEG

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Generalized tonic-clonic seizure tonic and
clonic phase
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Generalized tonic-clonic seizure clonic and
postictal phase
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EEG in structural cerebral lesions

• Tumours
• Abscesses
• Contusions
• Strokes
• cause mainly local slow waves (theta or delta),
  they form a focus of slow waves

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EEG in structural cerebral lesions
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Brain tumours

• supratentorial
• focal slow waves (irregular, persistent delta)
• principles of localization of large tumours
  phase reversal
• deep or infratentorial
• episodes of synchronous slow waves theta or delta

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Phase reversal
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Phase reversal
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Activation procedures

• Hyperventilation
• in all patients, 4 minutes
• absence of any change is normal
• abnormal responses include epileptiform activity
• intermittent photic stimulation
• sleep recordings after sleep deprivation

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intermittent photic stimulation

• rhythmic flashes, 1 - 30 Hz
• normal response photic driving, reproduction of
  the rhythm - mostly frequencies of alpha,
  partially beta

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intermittent photic stimulation

• photic driving increases
• Hyperthyroidism
• tumours in the posterior fossa
• Migraine (fast beta)
• abnormal photo convulsive response
  (epileptiform activity)

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sleep recordings after sleep deprivation

• drowsiness and light sleep (stages 1 and 2 of
  NREM sleep) accentuate epileptiform abnormalities
• all night sleep deprivation is required prior to
  taking the EEG

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Easy, isnt it ?