Download Normal EEGs

Normal EEG Patterns
Dr Lim Shih Hui
Senior Consultant Neurologist
Singapore General Hospital
EEG Interpretation
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Normal
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Lack of Abnormality
Abnormal
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Non-epileptiform Patterns
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Epileptiform Patterns
Alpha Rhythm
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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 hemispheres
is significant.
10% of adult have little or no alpha
Normal Alpha Rhythm
Normal Alpha Rhythm
Alpha Rhythm: Reactivity
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Should attenuate bilaterally with
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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 indicates
an abnormality on the side that fails to attenuate
Normal Alpha Reactivity
Eyes Closed
Normal Alpha Reactivity
Beta Activity
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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.
Beta Activity
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Frequency of over 13 Hz; if >30-35 Hz  gamma activity or exceedingly
fast activity by Gibbs.
Average voltage is 10-20 microvolts
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Two main types in adults:
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The precentral type: predominantly over the anterior and
central 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.
Generalized Beta Activity
Beta Activity
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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.
Theta Activity
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The term theta was coined by Gray Walter in 1944
when it was believed that 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 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 be
present over the temporal regions in older adults.
Theta Activity
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The term theta was coined by Gray Walter in 1944 when it was believed
that 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 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 be
present over the temporal regions in older adults.
Theta Activity
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The term theta was coined by Gray Walter in 1944 when it was believed
that 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 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 alphatheta waves can be present over the temporal
regions in older adults.
Temporal Slowing Of The Elderly
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Occur chiefly over the age of 60 years
Confined to the temporal regions and are usually maximal
anteriorly
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 by
drowsiness and hyperventilation
Occur sporadically as single or double waves but not in longer
rhythmic trains
Present for only a small portion of the tracing (up to 1%) of the
recording time when the patient is in a fully alert state
EEG of Drowsiness
(Stage I Sleep)
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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 posterior
regions
Drowsy
Drowsy
Drowsy
EEG of Drowsiness
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Alpha Activity
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Mu activity
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may be occurrence or persistence over the temporal
regions after a disappearance of the occipital alpha
may be asymmetric
may persist
Beta activity
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over the fronto-central regions
may become more prominent during drowsiness
20-30 Hz; occasional bursts of 30-40 Hz activity
Other Activities During
Stage I Sleep
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Vertex Sharp Transients
Positive Occipital Sharp
Transients of Sleep (POSTs)
Vertex Sharp Transient V-Wave
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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 an
asymmetric fashion
Should be careful not to mistake V-waves for abnormal
epileptiform 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
Vertex Sharp Transients
Post Occipital Sharp
Transients of Sleep (POSTs)
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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
POSTs
Stage II Sleep
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Sleep Spindles
K Complex
Sleep Spindles
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In adults, a frequency of 13-14 Hz
occur in a symmetric and synchronous fashion over
the two hemispheres
Usually these occur at intervals between 5-15
seconds,
Spindle trains ranging from 0.5-1.5 seconds in
duration
More prolonged trains or continuous spindle activity
may be seen in some patients on medication,
particularly benzodiazepams
Sleep Spindles
Sleep Spindles
Sleep Spindles
K-Complex
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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
K-Complex
Hyperventilation
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Often produces little change in the EEG in adult
If there is a change, usually consists of
generalized slowing.
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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, the
vigor of hyperventilation, blood sugar levels, and
posture
Intermittent Slow During HV
Intermittent Rhythmic Slow During HV
Persistence slowing following
cessation of hyperventilation:
Check if patient is still continuing
to hyperventilate or if patient is
hypoglycemic
Hyperventilation
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The findings accepted as unequivocal
evidence of abnormality:
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epileptiform discharges
clear-cut focal or lateralized slowing or
asymmetry of activity
Contraindications:
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significant cardiac or cerebrovascular disease,
or respiratory dysfunction.
Photic Stimulation
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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
Photoparoxysmal Response
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Photic stimulation may elicit posterior dominant or
generalized epileptiform discharges in patients
suspected of having photosensitive seizure
disorders
Photo-paroxysmal response:
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complex waveform
repeat at a frequency which is independent of the flash
rate
field extends beyond the usual posteriorly-situated
photic driving region and may be frontally dominant
Time-locked with stimulus or not time-locked / selfsustained
Photoparoxysmal Response
Photomyoclonic Response
Physiologic Activities That Can Be
Confused With Epileptiform Activities
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Vertex transients of light sleep
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Hypnagogic hypersynchrony
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Positive occipital sharp transients of sleep
(POST)
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Mu rhythm
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Lambda waves
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Breach rhythms
Benign Variants Of Unknown
Clinical Significance
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Benign epileptiform transients of sleep (small
sharp spikes)
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6- and 14-Hz positive spikes
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Wicket spikes
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Psychomotor variants (rhythmic mid-temporal
theta discharge of drowsiness)
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Subclinical rhythmic EEG discharge of adults
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Phantom spike and wave