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 …)
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 )
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
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)
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
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
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
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
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
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
