EEG Lecture Normal EEGs

7/28/2019 EEG Lecture Normal EEGs

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Normal EEG Patterns

Dr Lim Shih Hui

Senior Consultant NeurologistSingapore General Hospital


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EEG Interpretation

Normal

Lack of Abnormality Abnormal

Non-epileptiform Patterns

Epileptiform Patterns


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Alpha Rhythm

The starting point of analysing awake EEG

8-13 Hz activity occurring during wakefulness

20-60 mV, max over posterior head regions

Present when eyes closed; blocked by eye opening or alerting

the patient 8 Hz is reached by 3 years of age and progressively increases in

a stepwise fashion until 9-12 Hz is reached by adolescence

Very stable in an individual, rarely varying by more than 0.5 Hz.

With drowsiness, alpha activity may decrease by 1-2 Hz

A difference of greater than 1 Hz between the two hemispheresis significant.

10% of adult have little or no alpha


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Normal Alpha Rhythm


5/48Normal Alpha Rhythm


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Alpha Rhythm: Reactivity

Should attenuate bilaterally with

eye opening

alerting stimuli

mental concentration

Some alpha may return when eyes remain open for

more than a few seconds.

Failure of the alpha rhythm to attenuate on one side

with either eye opening or mental alerting indicatesan abnormality on the side that fails to attenuate


7/48Normal Alpha Reactivity


8/48Normal Alpha Reactivity

Eyes Closed


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Beta Activity

Frequency of over 13 Hz; if >30-35 Hz gamma

activity or exceedingly fast activity by Gibbs.

Average voltage is 10-20 microvolts

Two main types in adults:

Often enhanced during drowsiness or when present

over a skull defect

Should not be misinterpreted as a focus of

abnormal fast activity.


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Beta Activity

Frequency of over 13 Hz; if >30-35 Hz gamma activity or exceedingly

fast activity by Gibbs.



The precentral type: predominantly over the anterior andcentral regions; related to the functions of the

sensorimotor cortex and reacts to movement or touch.

The generalized beta activity: induced or enhanced by

drugs; may attain amplitude over 25 microvolts.

Often enhanced during drowsiness or when present over a skull defect

Should not be misinterpreted as a focus of abnormal fast activity.


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Generalized Beta Activity


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Beta Activity

Frequency of over 13 Hz; if >30-35 Hz gamma

activity or exceedingly fast activity by Gibbs.



Often enhanced during drowsiness or when present

over a skull defect

Should not be misinterpreted as a focus of

abnormal fast activity.


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Theta Activity

The term theta was coined by Gray Walter in 1944when it was believed that this rhythm was related tothe function of the thalamus.

Occurs as a normal rhythm during drowsiness In young children between age 4 months 8 years: predominance over

the fronto-central regions during drowsiness

In adolescents: sinusoidal theta activity can occur over the anterior head

regions during drowsiness. In adults, theta components can occur diffusely or over the posterior head

regions during drowsiness.

Single transient theta waveforms or mixed alpha-theta waves can bepresent over the temporal regions in older adults.


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Theta Activity

The term theta was coined by Gray Walter in 1944 when it was believedthat this rhythm was related to the function of the thalamus.

Occurs as a normal rhythm during drowsiness

In young children between age 4 months 8

years: predominance over the fronto-central regionsduring drowsiness

In adolescents: sinusoidal theta activity can occurover the anterior head regions during drowsiness.

In adults, theta components can occur diffusely or over the posteriorhead regions during drowsiness.

Single transient theta waveforms or mixed alpha-theta waves can bepresent over the temporal regions in older adults.


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Theta Activity

The term theta was coined by Gray Walter in 1944 when it was believedthat this rhythm was related to the function of the thalamus.

Occurs as a normal rhythm during drowsiness

In young children between age 4 months 8 years: predominance overthe fronto-central regions during drowsiness

In adolescents: sinusoidal theta activity can occur over the anterior head

regions during drowsiness.

In adults: theta components can occur diffusely orover the posterior head regions during drowsiness.

Single transient theta waveforms or mixed alpha-theta waves can be present over the temporalregions in older adults.


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Temporal Slowing Of The Elderly

Occur chiefly over the age of 60 years

Confined to the temporal regions and are usually maximalanteriorly

Occur more frequently on the left side

Do not disrupt background activity

Usually have a rounded morphologic appearance

Voltage is usually less than 60-70 microvolts

Attenuated by mental alerting and eye opening and increased bydrowsiness and hyperventilation

Occur sporadically as single or double waves but not in longerrhythmic trains

Present for only a small portion of the tracing (up to 1%) of therecording time when the patient is in a fully alert state


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EEG of Drowsiness

(Stage I Sleep)

In adults, most sensitive signs of drowsiness is the

disappearance of eye blinks and the onset of slow

eye movements

Slowing, dropout or attenuation of the background

Occurrence of theta activity over the posteriorregions


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Drowsy


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Drowsy


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Drowsy


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EEG of Drowsiness Alpha Activity

may be occurrence or persistence over the temporalregions after a disappearance of the occipital alpha

may be asymmetric

Mu activity may persist

Beta activity

over the fronto-central regions may become more prominent during drowsiness

20-30 Hz; occasional bursts of 30-40 Hz activity


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Other Activities During

Stage I Sleep

Vertex Sharp Transients

Positive Occipital Sharp

Transients of Sleep (POSTs)


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Vertex Sharp Transient -

V-Wave In young adults, the V-waves may have sharp or spiky

appearance and attain rather high voltages

During the earlier stages of sleep these may occur in anasymmetric fashion

Should be careful not to mistake V-waves for abnormalepileptiform activity

Sometimes trains or short repetitive series, clusters, or

bursts of V-waves may occur in quick succession In older adults the V-waves may have a more blunted

appearance


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Vertex Sharp Transients


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Post Occipital Sharp

Transients of Sleep (POSTs)

Sharp-contoured, mornophasic, surface-positive transients

Occurring singly or in trains of 4-5 Hz over the occipital head

regions May have a similar appearance to the lambda waves during

the awake record but are of higher voltage and longer

duration

Usually bilaterally synchronous but may be asymmetric over

the two sides

Predominantly seen during drowsiness and light sleep


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POSTs


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Stage II Sleep

Sleep Spindles K Complex


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Sleep Spindles In adults, a frequency of 13-14 Hz

occur in a symmetric and synchronous fashion overthe two hemispheres

Usually these occur at intervals between 5-15seconds,

Spindle trains ranging from 0.5-1.5 seconds induration

More prolonged trains or continuous spindle activitymay be seen in some patients on medication,particularly benzodiazepams


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Sleep Spindles


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Sleep Spindles


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Sleep Spindles


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K-Complex

A broad diphasic or polyphasic waveform

(>500 msec)

Frequently associated with spindle activity

K-complexes can occur in response to

afferent stimulation and may be linked to an

arousal response


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K-Complex


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Hyperventilation

Often produces little change in the EEG in adult

If there is a change, usually consists of

generalized slowing. either gradual or abrupt onset in theta or delta range

may continue as series of rhythmic slow waves or

consist of repeated bursts of slow waves at irregular

intervals

Degree of response depends on the age, thevigor of hyperventilation, blood sugar levels, and

posture


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Intermittent Slow During HV


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Intermittent Rhythmic Slow During HV


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Persistence slowing following

cessation of hyperventilation:

Check if patient is still continuing

to hyperventilate or if patient is

hypoglycemic


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Hyperventilation

The findings accepted as unequivocal

evidence of abnormality:

epileptiform discharges

clear-cut focal or lateralized slowing orasymmetry of activity

Contraindications:

significant cardiac or cerebrovascular disease,

or respiratory dysfunction.


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Photic Stimulation

Flash rate eliciting maximum driving response increases in

rough parallel with age (Niedermeyer, 1982)

Driving response may normally have a notched appearance

resembling a spike-wave discharge. It can be distinguished from spike-waves by its time-locked

appearance with the flash rate and its failure to persist after

the stimulation stops.

Asymmetries of photic driving probably have less clinical

value and can only be interpreted in association with other

significant asymmetries


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Photoparoxysmal Response

Photic stimulation may elicit posterior dominant orgeneralized epileptiform discharges in patientssuspected of having photosensitive seizuredisorders

Photo-paroxysmal response: complex waveform

repeat at a frequency which is independent of the flashrate

field extends beyond the usual posteriorly-situatedphotic driving region and may be frontally dominant

Time-locked with stimulus or not time-locked / self-sustained


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Photoparoxysmal Response


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Photomyoclonic Response


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Physiologic Activities That Can Be

Confused With Epileptiform Activities

Vertex transients of light sleep

Hypnagogic hypersynchrony

Positive occipital sharp transients of sleep(POST)

Mu rhythm

Lambda waves Breach rhythms


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Benign Variants Of Unknown

Clinical Significance Benign epileptiform transients of sleep (small

sharp spikes)

6- and 14-Hz positive spikes

Wicket spikes

Psychomotor variants (rhythmic mid-temporaltheta discharge of drowsiness)

Subclinical rhythmic EEG discharge of adults

Phantom spike and wave

EEG Lecture Normal EEGs

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