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

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 …)  

Electrode placement – the „ten-twenty“ electrode system

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

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

EEG reading Morphology Amplitude Frequency

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

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

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

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

Normal rhythms Beta rhythm Delta rhythm Theta 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

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

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

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)

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)

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%)

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

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

Polysomnography in NREM and REM sleep

Polysomnography in REM sleep (patient with narcolepsy)

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)

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

Narcolepsy

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

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

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

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

Interictal patterns in epilepsy – local focus:

Interictal patterns in epilepsy – generalized:

Ictal patterns depend on the type of the seizure

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

Simple partial seizures - single repetitive sharp waves

Complex partial seizures temporal lobe seizure with automatisms

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

Generalized – absence seizures

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

Absence seizures

Generalized tonic-clonic seizures

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

Generalized tonic-clonic seizure – tonic and clonic phase

Generalized tonic-clonic seizure – clonic and postictal phase

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

EEG in structural cerebral lesions

Brain tumours supratentorial deep or infratentorial 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

Phase reversal

Phase reversal

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

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

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

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

Easy, isn’t it ?