1.
Electroencephalography
doc. MUDr. Valja Kellerová, DrSc.
Department of Neurology

2.
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 …)

3.
Electrode placement – the „ten-twenty“
electrode system

4.
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

5.
Montages
• montage = a particular
electrode arrangement,
a number of different derivations
is diplayed simultaneously
• commonly used and preset
montages:
– longitudinal
– transverse
– ( referential )

6.
EEG reading
• Morphology
• Amplitude
• Frequency

7.
EEG reading: morphology (wave shape)
• sharp waves
• spikes = epileptiform patterns,
• spike-and-wave complexes… they accompany epilepsy

8.
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

9.
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

10.
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
patient’s eyes closed
– blocked or attenuated
with eye opening

11.
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

12.
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

13.
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

14.
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)
The normal EEG during drowsiness and NREM sleep
Drowsiness - somnolence

15.
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)

16.
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%)

17.
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

18.
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

19.
Polysomnography
in NREM and REM sleep

20.
Polysomnography in REM sleep
(patient with narcolepsy)

21.
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)

22.
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

23.
Narcolepsy

24.
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

25.
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

26.
Abnormal EEG – amplitude changes:
• generalized – in deep coma
burst-suppression pattern or flat record

27.
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

28.
Interictal patterns in epilepsy – local
focus:

29.
Interictal patterns in epilepsy –
generalized:

30.
Ictal patterns depend on the type of the seizure

31.
Simple partial seizures
- consciousness is not impaired
- Jacksonian motor or somatosensory seizure

32.
Simple partial seizures -
single repetitive sharp waves

33.
Complex partial seizures
temporal lobe seizure with automatisms

34.
Complex partial seizures -
sharp theta waves 6Hz – temporal region

35.
Generalized – absence seizures

36.
Absence seizures –
3/sec generalized spike-and-wave discharges

37.
Absence seizures

38.
Generalized tonic-clonic seizures

39.
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

40.
Generalized tonic-clonic seizure –
tonic and clonic phase

41.
Generalized tonic-clonic seizure –
clonic and postictal phase

42.
EEG in structural cerebral lesions
• Tumours
• Abscesses
• Contusions
• Strokes
cause mainly local slow waves (theta or delta),
they form a focus of slow waves

43.
EEG in structural cerebral lesions

44.
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

45.
Phase reversal

46.
Phase reversal

47.
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

48.
intermittent photic stimulation
– rhythmic flashes, 1 - 30 Hz
– normal response – photic driving, reproduction of the
rhythm - mostly frequencies of alpha, partially beta

49.
intermittent photic stimulation
– photic driving increases
• Hyperthyroidism
• tumours in the posterior fossa
• Migraine (fast beta)
– abnormal – photo convulsive response (epileptiform activity)

50.
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

51.
Easy, isn’t it ?