Normal EEG Patterns

Dr Lim Shih Hui

Senior Consultant Neurologist

Singapore General Hospital

EEG Interpretation

Normal Lack of Abnormality

Abnormal Non-epileptiform Patterns

Epileptiform Patterns

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 hemispheres

is significant. 10% of adult have little or no alpha

Normal Alpha Rhythm

Normal Alpha Rhythm

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 indicates an abnormality on the side that fails to attenuate

Normal Alpha Reactivity

Normal Alpha Reactivity

Eyes Closed

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.

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: 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

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 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

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

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.

Temporal Slowing Of The Elderly

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)

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

Alpha Activity may be occurrence or persistence over the temporal

regions 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

Other Activities During Stage I Sleep

Vertex Sharp Transients Positive Occipital Sharp

Transients of Sleep (POSTs)

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 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)

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

Sleep Spindles K Complex

Sleep Spindles 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 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

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, 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 The findings accepted as unequivocal

evidence of abnormality: epileptiform discharges clear-cut focal or lateralized slowing or

asymmetry of activity

Contraindications: significant cardiac or cerebrovascular disease,

or respiratory dysfunction.

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

Photoparoxysmal Response

Photic stimulation may elicit posterior dominant or generalized epileptiform discharges in patients suspected of having photosensitive seizure disorders

Photo-paroxysmal response: 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 / self-sustained

Photoparoxysmal Response

Photomyoclonic Response

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

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-temporal

theta discharge of drowsiness) Subclinical rhythmic EEG discharge of adults Phantom spike and wave