eeg

EEG Basics & Interpretation

A/Prof Ong Hian Tat Senior Consultant & Associate Professor Division of of Paediatric Paediatric Neurology & Developmental Paediatrics University Children’s Medical Institute National University Health System Singapore

Outline 1.

EEG and its uses and limitations

2.

EEG electrode placement, 10-20 international system

3.

EEG Rhythms

4.

Normal awake and sleep EEGs in adults and children

5.

Activation procedures for EEG

6.

Common benign variants in the EEG

7.

EEG artifacts

8.

Non-epileptiform abnormalities in the EEG

9.

Epileptiform discharges

Outline 1.


2.


3.

EEG Rhythms

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7.

EEG artifacts

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What is the EEG? • EE EEG G (el (elec ectr troe oenc nceph ephal alog ogra ram) m) dis displa plays ys the the elec electr tric ical al activity of the brain created by neurons generating the electrical signals • Th The e br brai ain’ n’s s el elec ectr tric ical al ac acti tivi vity ty is picked up by electrodes attached on the patient’s scalp and amplified on the EEG machine to be viewed as brain waves

Hans Berger (1873-1941), a German psychiatrist, discovered the electroencephalography (EEG) in 1929

Analog EEG

Digital EEG

Allows more accurate interpretation of the EEG record by applying different montages/references, gain/sensitivity, filters and time constant

Use of the EEG 

Helps to confirm Epilepsy Syndrome (diagnosis) Confirm seizures, classify seizures and epileptic syndrome Quantify seizures or epileptic bursts Localising epileptic focus – especially for pre-surgical evaluation



Help to indicate abnormalities that may suggest the following possibilities though definitive diagnosis is by other investigations/tests: Acute encephalitis/cerebral abscess Brain tumour Intracranial stroke – haemorrhage/ischemic infarction Any other non-specific encephalopathy e.g. Metabolic encephalopathy Brain death

Limitations of the EEG (sensitivity) 1. Some abnormal activity may not be detected when: The area is too small on the brain’s surface - need 4 cm2 surface area of cortex to be involved

Foci located too deep in the brain - mesial aspect and inferior aspect of the cortex (FLE or TLE)

Limited time sampling

2. Poor technical quality 3. Interpretation of the significance of the EEG abnormalities appropriate to the clinical setting

Limitations of the EEG (specificity) 1. Some non-specific EEG abnormalities in normal people - up to 10%

2. Paroxysmal epileptiform activity in the absence of any clinical seizures - ~ 1% of normal people - much higher in patients with non-epileptic neurological disorders e.g. migraine, cerebrovascular disorders

3. Benign focal epileptiform discharges in children without seizures - 2 to 4 %

Electrode Placement International 10-20 System

Fp1 = left frontal pole

Fp2 = right frontal pole

F3 = left frontal

F4 = right frontal

F7 = left fronto-temporal

F8 = right fronto-temporal

C3 = left central

C4 = right central

T3 = left temporal

T4 = right temporal

P3 = left parietal

P4 = right parietal

T5 = left postero-temporal

T6 = right postero-temporal

O1 = left occipital

O2 = right occipital

A1 = reference to left year

A2 = reference to right ear

G = ground Fz = mid frontal Cz = midline Pz = mid posterior

IPSI

AP Bipolar

EEG montages based on the 10-20 system Coronal

The Routine EEG • To include recordings (each at least 15min) of - awake (eye open/close) - drowsiness, sleep (following partial sleep deprivation) - awakening

• Activation procedures - intermittent photic stimulation - hyperventilation (with breath counting) - other forms of appropriate activation of reflex seizures e.g. reading, hot water, somatosensory stimuli, auditory stimuli

The Routine EEG Simultaneous video-EEG recordings should be made routine - confirms seizure(s) and diagnosis of epilepsy - excludes epileptic seizures and arrive at diagnosis of nonepileptic paroxysmal events, which are also very common in infants & children - clinical signs during the seizure may be subtle, and not able to be picked up or recognised immediately by the EEG technologist e.g. absences during HV, myoclonic jerks, focal seizures

EEG Rhythms Rhythm

Hertz

Description

Alpha

8-13 Hz

Posterior dominant rhythm

Beta

>13 Hz

Normal in sleep especially in infants and young children

Theta

4-7 Hz

Drowsiness and sleep

Delta

<4 Hz

Stage IV sleep

Alpha activity

Beta activity

Theta activity

Delta activity

Digital EEG • Interpretation of EEG can be enhanced by applying changes to the following: – Sensitivity – High frequency filter – Time constant

Normal Awake EEG in Adults & Children

Normal adult awake EEG

2-3 Hz posterior rhythm seen in a 3 month old baby

5 Hz posterior rhythm seen in a 6 month old baby

6 Hz posterior rhythm seen in a 1 year old child

7.5 Hz posterior rhythm seen in a 2 year old child

9 Hz posterior rhythm seen in a 6 years old child

10 Hz posterior rhythm seen in children >8 years old

Normal Sleep EEG in Adults & Children

Asynchronous sleep spindles seen in a 2 month old infant

Hypnogogic burst

EEG during Activation Procedures

Photic driving

Photic Driving • Rhythmic activity elicited by frequencies of 5-30 Hz that are time-locked to represent a harmonic of the stimulus frequency • Usually maximal over the posterior regions • Adults tend to drive best at frequencies near their alpha rhythm whereas children may drive at slower frequencies • An impressive driving response at very low flash frequencies of 0.5-3 Hz usually indicative of CNS dysfunction • Asymmetric or absence of photic driving may be seen in normal individuals

Photomyoclonic Response

Photomyoclonic Response • Consists of muscle spikes over the anterior regions in response to IPS • Spikes increase in amplitude as stimulation increases appearing maximal at flash frequencies of 12-18 Hz and disappearing abruptly with the end of stimulation • Photoconvulsive response is associated with anxiety, alcohol, or drug withdrawal states and parkinsonism • Considered a normal response

Photoconvulsive Response

slight hand jerk

Photoconvulsive Response •

Characterized by generalized or posteriorly dominant spike/polyspikewave complexes produced by photic stimulation

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Lapses of consciousness or myoclonic jerks may accompany the spike discharges

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Generalized discharges represent an abnormal response that suggests a predisposition to an epileptic disorder, particularly if sustained beyond the stimulus

•

Photoparoxysmal response is most frequently elicited at flash rates of 15-20 Hz and is elicited most often in people with primary generalized epilepsy

•

However, can also be seen in patients with toxic/metabolic or drug withdrawal states and is observed in 2% of normal persons

Comparison of Photomyoclonic and Photoconvulsive responses Photomyoclonic response

Photoconvulsive response

Effective stimulus frequency

8-20 flashes/sec

3-20 flashes/sec

Eyelids, position for maximal effect

Closed

Closed (and open)

Clinical accompaniments

Fluttering of eyelids

Eyes turning, speech arrest

Consciousness

Maintained

Often disturbed

Distribution of electric changes

Face, frontal regions

Diffuse over scalp

EEG response, type

Myoclonic spikes (polyspike)

Spike-wave ot atypical spike-wave

Recruitment

Marked

Less frequent

After-discharge

None

Frequent

Age group

Adult

All ages

Variability of threshold

Large

Slight

Muscle tension

Increases

No effect

Nervous tension

Increases

No effect

Occurrence

Normals frequent

Normals rare

Photoparoxymsal Response

Photoparoxysmal Response • Prolonged (self-sustained) which continues for a short period after the stimulus has been withdrawn • High incidence of epilepsy • Patients with photoparoxysmal response often had other epileptiform abnormalities in their resting EEG

Slow waves activated by hyperventilation

Common Benign Variants in the EEG

Mu rhythm, which disappears when patient is told to make a clenched fist

Mu Waves • Central rhythm of 7-12 Hz with arciform morphology • Observed in ~20% of normal young adults and less common in children and the elderly • Slightly more common in females than males • Mu rhythm does not attenuate with eye opening but does diminish with movement or tactile stimulation of the contralateral extremities, fatigue, arithmetic or problem solving

Lambda Waves

Lambda Waves • Sharp transients characterized as biphasic or triphasic waveforms having initial small positive phase followed by a prominent negative component • Appears over occipital regions bilaterally • Elicited by looking at a patterned design in a well-lit room • Most commonly seen in children 2-15 years of age • May be asymmetric and misinterpreted as occipital spikes • Eye closure, reducing illumination of the room or having the patient stare at a blank card will eliminate lambda waves

Posterior Slow Waves of Youth (PSWY) superimposed on Posterior Rhythm

Posterior Slow Waves of Youth • Also called sail waves and polyphasic waves • Moderate voltage, fused waves intermixed with the alpha rhythm that attenuate with eye opening and diminish with drowsiness • May not appear symmetric or synchronous, but asymmetries should not exceed 50% • Observed in children and young adults, maximal incidence between 8-14 years

Positive Occipital Sharp Transients (POSTS)

Positive Occipital Sharp Transients (POSTs) • Also referred to as occipital V-waves of sleep, lambdoids of sleep and rho waves • Surface positive sharp waves in stage I and II sleep most commonly in young adults (15-35 years) • Waves consist of sharp, surface-positive peak followed in some instances by low voltage surface-negative peak • Initial deflection has a slower duration than the ascending phase resulting in a checkmark morphology • Typically occur in runs of bilaterally synchronous 4-5 Hz waves that may be asymmetrical

14- and 6-Hz Positive Spikes

14- and 6- Hz Positive Spikes • Bursts consists of rhythmic arciform waveform of 0.5-1.0 second duration during drowsiness and light sleep • Maximal in amplitude over the posterior temporal regions • Usually unilateral or independent over both sides, but may be bilaterally synchronous • Appear most often in adolescents between 13-14 years but have been seen in children as young as 3-4 years

Phantom spike-waves

Phantom Spike-Wave • Char Charac acte teri rize zed d by bri brief ef bur burst st of of 5-7 5-7 Hz ge gene nera raliz lized ed spik spikeewave discharges seen primarily in young adults in relaxed wakefulness or drowsiness • Hugh Hughes es sub subdi divid vided ed thi this s patt patter ern n in 2 subg subgro roup ups: s: ‘FO ‘FOLD LD’’ & ‘WHAM’ • ‘FOLD’ oc occur urs s pr primaril ily y in in Females, maximal in the Occipital regions, L ow in amplitude and present in Drowsiness • ‘WHAM’ occurs in Waking state, is Higher in amplitude, more A nterior and observed primarily in Males • Ob Obse serv rved ed in in 2-3 2-3% % of of nor norma mall pers person ons s >50 >50 yea years rs

Wicket spikes

Wicket Spikes • Cons Consis ists ts of mo mono nopha phasi sic c arc arcif ifor orm m mumu-lilike ke 66-11 11 Hz transients occurring singly or in trains in the temporal regions during wakefulness or sleep • Typic Typical ally ly bila bilate tera rall and and inde indepe pende ndent nt,, usua usually lly ha havi ving ng a unilateral predominance • Se Seen en in adu adult lts s >30 >30 ye year ars s in in ~1~1-3% 3% of no norm rmal al po popu pula lati tion on • Wick Wicket et sp spik ikes es are are not not as asso soci ciat ated ed wit with h an af afte terc rcom omin ing g slow wave or background slowing

Small Sharp Spikes (SSS)

Small Sharp Spikes (SSS) •

Also referred to as benign epileptiform transients of sleep (BETS) and benign sporadic sleep spikes (BSSS)

•

Benign transients of low amplitude (<50uV) and short duration (<50msec) seen in adults during drowsiness and light sleep

•

Monophasic or diphasic spike with abrupt ascending and a steep decending limb occasionally followed by a lower amplitude slow wave

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Seen best in the anterior to mid-temporal regions

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Unilateral or bilateral and may demonstrate opposite polarity on the two hemispheres

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Seen in 20-25% of normal individuals and have no clinical significance

Breech Rhythm (Skull Defect)

Breech Rhythm (Skull Defect) • Associated with skull defects are focal “mu-like” rhythms in Rolandic or temporal region with sporadic slow waves and spiky or sharp transients • Rhythms are unrelated to epilepsy and do not indicate recurrence of a tumor

Rhythmic Mid-Temporal Theta of Drowsiness

Rhythmic Mid-Temporal Theta of Drowsiness • Also known as rhythmic midtemporal discharges (RMTDs) • Characterized by burst or trains of rhythmic 4-7 Hz theta waves in the temporal region • Often have flat-topped or notched appearance due to the superimposition of faster harmonic frequencies • Burst may be unilateral or bilateral, but often appear independently with shifting emphasis in the relaxed or drowsy state • Pattern evolves in amplitude, increasing at the beginning and gradually decreasing at the end • Found in 0.5 to 2% of normal adults

Central Theta Description: The prominent feature here is the rhythmic 6 Hz activity at the vertex (arrow). This discharge spreads to the paracentral regions. The background frequencies are in the theta range with superimposed beta. Significance: Normal, state transition

Mitten Description: This shows a mitten discharge during stage 2 sleep (arrows). The sharp component precedes the slow wave, with resulting appearance similar a sharp and slow wave complex. These may appear spontaneously or with auditory stimulation, and are best demonstrated with an ear reference Significance: Normal, may be mistaken for pathological bifrontal sharp and slow wave complex

EEG Artifacts

EEG Artifacts • Physiological – Eye movements – Lateral rectus spikes (muscle action potential) – Eyelid flutter – Sweat – Glossokinetic – ECG

• Non-physiological – Electrode pop – Movement – 60 Hz artifacts

Fp1

Fp2

F7

F8

Eye movements

Asymmetrical slow eye movements of drowsiness (F7, F8), not to be mistaken for focal delta activity

Lateral Rectus Spikes (muscle action potentials)

Eyelid flutter

Sweat artifact

Prominent glossokinetic potentials resembling frontal intermittent rhythmic delta activity

Chewing

ECG artifact

Electrode Pop

Head movement

60 Hz Artifact

60 Hz Artifact (after using filter)

Non Epileptiform Abnormalities

Non Epileptiform Abnormalities • Intermittent slowing – Generalized – Focal

• Intermittent rhythmic slowing – Generalized – Focal

• Continuous slowing – Focal (regional)

Non Epileptiform Abnormalities • Intermittent slowing – generalized – focal

• Intermittent rhythmic slowing – generalized – focal

Generalized intermittent slowing

Focal intermittent slowing

Non Epileptiform Abnormalities • Intermittent slowing – generalized – focal

• Intermittent rhythmic slowing – generalized – focal

Occipital Intermittent Rhythmic Delta Activity (OIRDA)

Frontal Intermittent Rhythmic Delta Activity (FIRDA)

Focal intermittent rhythmic slowing Left temporal region (F7-T3)

Non Epileptiform Abnormalities • Intermittent slowing – Generalized – Focal

• Intermittent rhythmic slowing – Generalized – Focal

• Continuous slowing – Focal (regional)

background asymmetry, continuous slowing right hemisphere

Epileptiform Discharges

Definition of Epileptiform Discharges • Should be unarguably discrete events, not just accentuation of part of an ongoing sequence of waves • Should be clearly separable from ongoing background activity, not only by their higher amplitude but also by their morphology and duration • Have bi- or triphasic waveform and have more complex morphology than even high-voltage background rhythms • Not sinusoidal but rather show asymmetric, rising and falling phases • Most spikes and sharp waves followed by slow wave • Should have a physiological potential field involving more than one electrode that helps distinguish them from electrode-related artifacts or muscle potentials

Bipolar montage showing phase reversal

Right centro ‐temporal spikes (BFEDC)

BFEDC

staring

Characteristics of ‘Ictal’ pattern • Repetitive EEG discharges with relatively abrupt onset and termination • Pattern of evolution lasting at least several seconds • Generally rhythmic, frequency displays increasing amplitude, decreasing frequency and spatial spread during the seizure

Grading of EEG Abnormalities in Diffuse Encephalopathy Grade I (almost normal)

Dominant activity is alpha rhythm with minimal theta activity

Grade II (mildly abnormal)

Dominant theta background with some alpha and delta activities

Grade III (moderately abnormal)

Continuous delta activity predominates, little activity of faster frequencies

Grade IV (severely abnormal)

Low-amplitude delta activity or burst-suppression pattern

Grade V (extremely abnormal)

Nearly “flat” tracing or electrocerebral inactivity

Continuous Slow Waves

Triphasic Waves

Periodic Lateralized Epileptiform Discharges (PLEDS)

BiPLEDs

Bilateral Independent Periodic Lateralized Epileptiform Discharges (BiPLEDS)

Generalized Suppression (low amplitude delta)

Burst Suppression Pattern

Burst Suppression Pattern Differential Diagnosis 1.

Severe diffuse encephalopathy e.g. hypoxia, metabolic disorders

2.

Iatrogenic e.g. general anesthesia e.g. thiopentone treatment for GCSE

3.

Normal pattern in prematurity

4.

Early infantile epileptic encephalopathy: - Othahara syndrome - Early myoclonic encephalopathy - West syndrome (EEG in sleep)

Summary 1.


2.


3.

EEG Rhythms

4.


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EEG artifacts

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Take Home Message 1.

Not all sharply contoured transients or rhythmic activity are epileptiform, but may represent normal findings or benign variants with no clinical significance.

2.

Use various montages and references for analysis of the field potential (maximum negativity) instead of relying on visual pattern recognition.

3.

Do not miss the wood for the trees, being over-interested in each individual spike but missing the big picture, especially in the presence of encephalopathy.

4.

Conservative reading and interpretation should be the rule, as more “damage” is done by over-reading or overinterpretation of an EEG record.

References • Comprehensive Clinical Neurophysiology Cleveland Clinic Foundation

• Atlas of Electroencephalography Ed: MR Sperling & R Clancy

• EEG Activation & Artifacts American Society of Electroneurodiagnostic Technologists

• Pediatric EEG American Society of Electroneurodiagnostic Technologists

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