eeg

EEGBasics&Interpretation

A/Prof Ong Hian Tat

SeniorConsultant&AssociateProfessorDivisionofPaediatricNeurology&DevelopmentalPaediatrics

UniversityChildrensMedicalInstituteNationalUniversityHealthSystem

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

What is the EEG?

EEG (electroencephalogram) displays the electrical activity of the brain created by neurons generating the electrical signals

The brains electrical activity is picked up by electrodes attached on the patients 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 burstsLocalising 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 abscessBrain tumourIntracranial stroke haemorrhage/ischemic infarctionAny 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 brains 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

Fp2 = right frontal pole

F4 = right frontal

F8 = right fronto-temporal

C4 = right central

T4 = right temporal

P4 = right parietal

T6 = right postero-temporal

O2 = right occipital

A2 = reference to right ear

Fp1 = left frontal pole

F3 = left frontal

F7 = left fronto-temporal

C3 = left central

T3 = left temporal

P3 = left parietal

T5 = left postero-temporal

O1 = left occipital

A1 = reference to left year

G = ground

Fz = mid frontal

Cz = midline

Pz = mid posterior

EEG montages based on the 10-20 system

IPSI AP Bipolar

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 non-epileptic 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

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

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

Photic Driving

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

Photomyoclonic Response

slighthandjerk

Photoconvulsive Response

Characterized by generalized or posteriorly dominant spike/polyspike-wave complexes produced by photic stimulation

Lapses of consciousness or myoclonic jerks may accompany the spike discharges

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

Photoconvulsive Response

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

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

Photoparoxysmal Response

Slow waves activated by hyperventilation

Common Benign Variants in the EEG

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

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

Mu 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

Lambda Waves

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

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

Posterior Slow Waves of Youth

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

Positive Occipital Sharp Transients (POSTs)

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

14- and 6- Hz Positive Spikes

Phantom spike-waves

Characterized by brief burst of 5-7 Hz generalized spike-wave discharges seen primarily in young adults in relaxed wakefulness or drowsiness

Hughes subdivided this pattern in 2 subgroups: FOLD & WHAM

FOLD occurs primarily in Females, maximal in the Occipital regions, Low in amplitude and present in Drowsiness

WHAM occurs in Waking state, is Higher in amplitude, more Anterior and observed primarily in Males

Observed in 2-3% of normal persons >50 years

Phantom Spike-Wave

Wicket spikes

Consists of monophasic arciform mu-like 6-11 Hz transients occurring singly or in trains in the temporal regions during wakefulness or sleep

Typically bilateral and independent, usually having a unilateral predominance

Seen in adults >30 years in ~1-3% of normal population

Wicket spikes are not associated with an aftercoming slow wave or background slowing

Wicket Spikes

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 (

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

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

Central Theta

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

Mitten

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

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

Eye movements

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


Intermittent slowing Generalized Focal

Intermittent rhythmic slowing Generalized Focal

Continuous slowing Focal (regional)


Intermittent slowing generalized focal

Intermittent rhythmic slowing generalized focal

Generalized intermittent slowing

Focal intermittent slowing


Intermittent slowing generalized focal

Intermittent rhythmic slowing generalized focal

Occipital Intermittent Rhythmic Delta Activity (OIRDA)

Frontal Intermittent Rhythmic Delta Activity (FIRDA)

Focal intermittent rhythmic slowingLeft temporal region (F7-T3)


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

Rightcentrotemporalspikes(BFEDC)Bipolarmontageshowingphasereversal

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-suppressionpattern

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 Diagnosis1. 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. 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

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 over-interpretation of an EEG record.

References

Comprehensive Clinical Neurophysiology Cleveland Clinic Foundation

Atlas of ElectroencephalographyEd: MR Sperling & R Clancy

EEG Activation & Artifacts American Society of Electroneurodiagnostic Technologists

Pediatric EEG American Society of Electroneurodiagnostic Technologists

Thank You

University Childrens Medical Institute

Documents

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