Introduction to EEG for non-epileptologists working in seizure prediction and dynamics

Richard Wennberg, MD, FRCPC

University of Toronto

IWSP4, Kansas City, 2009

1. EEG source

• cortical pyramidal cells

• voltage fluctuations in space/time

• summated EPSPs/IPSPs

• dependent on neural “synchrony”

2. EEG oscillations

• Normal– (alpha, beta, mu, gamma, sleep spindles/delta)– generated in cortex– varying degrees of thalamocortical

interdependence

• Abnormal– (seizures, burst-suppression)

3. EEG sharp transients

• Normal– Vertex sharp waves, positive occipital sharp

transients of sleep (POSTS), benign epileptiform transients of sleep (BETS) or small sharp spikes; eye blinks, EKG, EMG

• Abnormal– Epileptiform spikes, polyspikes, spike and waves,

sharp waves, sharp and slow waves– Periodic complexes (lateralized and generalized),

triphasic waves

International (10-20) Electrode Placement

Gloor, J Clin Neurophysiol, 1985

Gloor, J Clin Neurophysiol, 1985

Gloor, J Clin Neurophysiol, 1985

Pedley and Traub. In: Daly and Pedley, eds. Current Practice of Clinical EEG, 1990

Pedley and Traub. In: Daly and Pedley, eds. Current Practice of Clinical EEG, 1990

Steriade. In: Niedermeyer and Lopes da Silva, eds. Electroencephalography, 1993

EEG examples

Subject awake, resting. Normal posterior alpha rhythm disappears with eye opening (*). High frequency activity at end of figure after eye opening is muscle artifact. Anterior-posterior bipolar montage. LFF 0.5 Hz, HFF 70 Hz, this and all other figures.

Stage II sleep. K-complex (*); Sleep spindles (**). Anterior-posterior bipolar montage.

Burst of generalized 3 Hz spike and wave activity (*). Primary generalized epilepsy. Anterior-posterior bipolar montage.

Generalized, bilaterally synchronous, 3 Hz spike and wave activity in a different patient with primary generalized epilepsy. Referential montage; reference = linked ears.

Primary generalized epilepsy: spike and wave bursts

Juvenile myoclonic epilepsy

Generalized, bilaterally synchronous bursts of spike and wave activity in another patient with primary generalized epilepsy, subtype juvenile myoclonic epilepsy. Referential montage; reference = linked ears.

Primary generalized epilepsy: transition to tonic-clonic seizure

Juvenile myoclonic epilepsy

In this condition, bursts of spike and wave activity increase in frequency in the morning hours after awakening in a true “pre-ictal period” that may – or may not – result in a transition to a generalized tonic-clonic seizure.

Primary generalized epilepsy: transition to tonic-clonic seizure

High amplitude “hypersynchrony”

Same seizure transition as previous figure, shown here at slower sweep speed.

Bilateral temporal lobe (“focal”, “partial”) interictal epileptiform activity. Independent sharp and slow wave complexes over right (*) and left (**) anterior-mid temporal regions. Temporal lobe epilepsy. Anterior-posterior bipolar montage.

Ictal EEG showing focal rhythmic seizure pattern localized to right temporal region (“equipotentiality” at F8-T4). Temporal lobe epilepsy. Anterior-posterior bipolar montage.

Interictal EEG during drowsiness in a different patient showing unilateral right anterior temporal lobe spikes (“phase reversing” at Zg2, F8, F10)

Patient with bilateral hippocampal sclerosis, global developmental delay, medically-refractory complex partial seizures

Patient with bilateral hippocampal sclerosis, global developmental delay, medically-refractory complex partial seizures

Ictal EEG showing unilateral right temporal lobe seizure (with “equipotentiality” at Zg2-T4, F8-T4, F10-T10)(note different sensitivity and time scale compared with preceding, interictal EEG figure from same patient)

Artifacts

Reference electrodes

Why should the non-epileptologist care about artifacts and reference electrodes?

Two examples– EEG studies of beta and gamma oscillations in

cognition would appear to have been analyzing mainly muscle artifact

• Whitham et al. Clin Neurophysiol 2008;119:1166-75 and Clin Neurophysiol 2007;118;1877-88

– The need for a reference electrode in EEG affects phase synchronization studies; resulting amplitude variations influence the phase locking analyses

• Guevara et al. Neuroinformatics 2005;3:301-13

CZ-C3

C3-T3

T3-SP1

SP1-SP2

SP2-T4

T4-C4

C4-CZ

EOG1-EOG2

FP1-F7

F7-T3

T3-T5

T5-O1

FP2-F8

F8-T4

T4-T6

T6-O2

FP1-F3

F3-C3

C3-P3

P3-O1

FP2-F4

F4-C4

C4-P4

P4-O2

EKG

Comment Eye Blink C3 pulse artifact C3 pulse artifact C3 pulse artifact Eyes left Eyes right 100 uV

1 sec

Eye blink, horizontal eye movements, frontalis and temporalis EMG, lateral rectus EMG, pulse artifacts.

Combined circular and anterior-posterior bipolar montage.

F7-Pz

T3-Pz

T5-Pz

Fp1-Pz

F3-Pz

C3-Pz

P3-Pz

O1-Pz

Fz-Pz

Cz-Pz

Pz-Pz

Fp2-Pz

F4-Pz

C4-Pz

P4-Pz

O2-Pz

F8-Pz

T4-Pz

T6-Pz

Comment Eye Blink C3 pulse artifact C3 pulse artifact C3 pulse artifact Eyes left Eyes right 100 uV

1 sec

Eye blink, horizontal eye movements, frontalis and temporalis EMG, lateral rectus EMG, pulse artifacts.

Referential montage; reference = Pz.

F7-Fz

T3-Fz

T5-Fz

Fp1-Fz

F3-Fz

C3-Fz

P3-Fz

O1-Fz

Fz-Fz

Cz-Fz

Pz-Fz

Fp2-Fz

F4-Fz

C4-Fz

P4-Fz

O2-Fz

F8-Fz

T4-Fz

T6-Fz

Comment Eye Blink C3 pulse artifact C3 pulse artifact C3 pulse artifact Eyes left Eyes right 100 uV

1 sec

Eye blink, horizontal eye movements, frontalis and temporalis EMG, lateral rectus EMG, pulse artifacts.

Referential montage; reference = Fz.

F7-C3

T3-C3

T5-C3

Fp1-C3

F3-C3

C3-C3

P3-C3

O1-C3

Fz-C3

Cz-C3

Pz-C3

Fp2-C3

F4-C3

C4-C3

P4-C3

O2-C3

F8-C3

T4-C3

T6-C3

Comment Eye Blink C3 pulse artifact C3 pulse artifact C3 pulse artifact Eyes left Eyes right 100 uV

1 sec

Eye blink, horizontal eye movements, frontalis and temporalis EMG, lateral rectus EMG, pulse artifacts.

Referential montage; reference = C3.

F7-Av12

T3-Av12

T5-Av12

Fp1-Av12

F3-Av12

C3-Av12

P3-Av12

O1-Av12

Fz-Av12

Cz-Av12

Pz-Av12

Fp2-Av12

F4-Av12

C4-Av12

P4-Av12

O2-Av12

F8-Av12

T4-Av12

T6-Av12

Comment Eye Blink C3 pulse artifact C3 pulse artifact C3 pulse artifact Eyes left Eyes right 100 uV

1 sec

Eye blink, horizontal eye movements, frontalis and temporalis EMG, lateral rectus EMG, pulse artifacts.

Referential montage; reference = common average (of electrodes F3, F4, T3, C3, C4, T4, T5, P3, P4, T6, O1, O2).

F7-aF7

T3-aT3

T5-aT5

Fp1-aFp1

F3-aF3

C3-aC3

P3-aP3

O1-aO1

FZ-aFz

CZ-aCz

PZ-aPz

Fp2-aFp2

F4-aF4

C4-aC4

P4-aP4

O2-aO2

F8-aF8

T4-aT4

T6-aT6

Comment Eye Blink C3 pulse artifact C3 pulse artifact C3 pulse artifact Eyes left Eyes right100 uV

1 sec

Eye blink, horizontal eye movements, frontalis and temporalis EMG, lateral rectus EMG, pulse artifacts.

Referential montage; reference = Laplacian.

EEG cannot “see” deep into the brain

Spontaneous activity in, e.g.,

mesial temporal regions,

interhemispheric frontal lobe structures, thalamus

is NOT apparent on scalp EEG

Comparison of intracranial interictal epileptiform activity recorded during sleep with simultaneous scalp EEG. Focal spikes in left and right hippocampus (LH and RH), electrode contacts LHD1 and RHD1, show no scalp EEG correlates; more diffuse right temporal spike and wave complexes (RT) apparent at multiple contacts of right temporal depth electrode (RHD1-4) are associated with visible epileptiform potentials on scalp EEG (channels F8, T4). Referential montage; reference = common average 10-20 electrodes. Top 16 channels = scalp EEG. Channels 17-20 and 21-24 = left and right, respectively, temporal depth electrode recordings. Sensitivity = 15μV/mm for scalp EEG, 50 μV/mm for intracranial recordings.

• Alarcón et al. JNNP 1994;57:435-49.– Scalp/FOE or depth/subdural or

scalp/subdural/depth– Mesial temporal focal spike voltage gradient ~

750μV/2.5mm– Estimated depth current dipole 2 nA·m would

produce scalp voltage of 0.45μV – A typical 100μV scalp spike would require a mesial

temporal focal dipole strength ~ 100-600 nA·m (an 80 mV hippocampal spike!)

• Nayak et al. Clin Neurophysiol 2004;115:1423-35.– Scalp/FOE– Only 9% of temporal spikes seen intracranially

visible on scalp w/o averaging

LHD1-Ref

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750 uV1 sec

*

*

Comparison of intracranial ictal epileptiform activity recorded during sleep with simultaneous scalp EEG. Focal electrographic seizure in right hippocampus (rhythmic activity at intracranial depth electrode contact RHD1) has no scalp EEG correlate. Referential montage; reference = common average 10-20 electrodes. Top 16 channels = scalp EEG. Channels 17-20 and 21-24 = left and right, respectively, temporal depth electrode recordings. Sensitivity = 15μV/mm for scalp EEG, 50 μV/mm for intracranial recordings.

Intracranial EEG (ECoG)

Intracranial EEG of one mesial temporal lobe seizure (continuous recording from top left to bottom right). EEG recorded from a depth electrode contact situated within the right anterior hippocampus in a patient with medically-refractory temporal lobe epilepsy. Referential montage, scalp FCz reference.

Temporal lobe epilepsyLeft regional hippocampal/parahippocampal seizure onset

(Intracranial depth electrode recording)

Temporal lobe epilepsySeizure “spread” to right mesial temporal region

(Do seizures “spread” or “jump”?)

An individual patient may have more than one morphology and/or localization of seizure onset

Next seizures all from same patient, now seizure free >1 year after right anterior temporal lobe resection

RHD1-Ref

750 uV

500 msec

RHD1-Ref

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RHD1-Ref

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LHD1-Ref

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Five different seizure onsets recorded from intracranial depth electrodes in one patient over 24 hours

LHD1-Ref

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500 msec

LHD1-Ref

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RHD1-Ref

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RHD1-Ref

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…Continuation of the five seizures from previous figure

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Time

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Focal onset Left Hippocampus

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Regional onset Left Hippocampus Parahippocampus

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Regional onset Right UncusHippocampus

Regional onset Right UncusHippocampus “spread” to Generalized

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Intraoperative ECoG

Pre-resection ECoG in a patient with right temporal lobe epilepsy, hippocampal sclerosis and a posterior middle temporal gyrus gliotic lesion.C1-4 inferior temporal gyrus, C4 anterior. C5-8 middle temporal gyrus, C8 anterior. C11-14 superior temporal gyrus; C1, C5 over lesion, C11 superior to lesion. C14-17 above Sylvian fissure. Propofol bolus given 3 minutes earlier with maximal activation of inferior temporal spikes.

ECoG 2 min after bolus of alfentanil: activation of amygdala/hippocampal spikes and suppression of temporal neocortical spikes

ECoG 10 min later: return to pre-activation baseline of amygdala/hippocampal spikes and reappearance of independent temporal neocortical spikes