Normal EEG Wave PatternDr.Roopchand.PSSenior Resident(Academic)Dept. Of NeurologyTDMC Alappuzha

Normal EEG:

•Variety of wave forms.▫Frequency▫Amplitude▫Spatial distribution▫Reactivity to different stimuli

•The frequency of clinically relevant EEG between 0.3 to 70Hz

TYPES:

•EEG Waves are named on the basis of their frequency range.

•Delta – Below 3.5 HZ ( 0.1-3.5Hz)•Theta – 4 to & 7.5Hz•Alpha – 8 to 13 Hz•Beta – Above 13Hz (14-40Hz)

•The voltage of the EEG signal determines its amplitude.

•Normal range is between 10 to 100µV.•Amplitude is measured from peak to peak.

▫Called range•A specific frequency band has a defined

amplitude range.•More than 50% amplitude difference

between two homologous brain areas in two hemispheres is abnormal.

Alpha Rhythm:

•Frequency of 8-13Hz▫During wakefulness▫Over posterior regions of the head.

•Amplitude < 50µV.•Best seen with eyes closed and physical

and mental relaxation.•Attenuated by attension

▫Visual or mental effort.

•60% individuals alpha amplitude range between 20 -60µV, 28% below 20 and 6% above 60µV.

•Amplitude higher on the right.•Morphology : rounded or sinusoidal.•Spatial distribution: Posterior half of the

head.▫Occipital, Parietal, posterior temporal.

•Alpha reactivity▫Eye opening, sensory stimuli, mental

activity.

•Three types of alpha waves.▫P- persistent▫R- responsive▫M- minimal

•Origin of alpha waves: thalamus, cortex and corticothalamic reverberating circuits.

Beta Rhythm:• Any rhythmical EEG activity above 13HZ.

▫18-25Hz(common), 14-16Hz(less common), 35-40Hz(rare).

• Amplitude 10 - 20µV• Spatial distribution:

▫Frontal beta: common, very fast, no relationship to physiological rhythm.

▫Central beta: mixed with rolandic mu rhythm, blocked by tactile stimuli.

▫Posterior beta: fast alpha equivalent, blocked by eye opening.

▫Diffuse beta: fast, no relationship to physiological rhythm.

•The voltage of Beta is the first to reduce in cortical injury, subdural or epidural collection.

•Very fast activity > 35Hz seen in organic psychosis.

•Beta depression may occur transiently after focal seizures.

Theta rhythm:

•Frequency between 4 – 8Hz.•Amplitude below 15mV•Present in frontocentral area.•Augmented by emotional stress and

mental task.•Temporal theta with episodic delta rise -

abnormal

Mu rhythm:

•Rhythm of alpha frequency.•In young adults•Arch like morphology.•Blocked by motor movement.•Not affected by eye opening.•Blocked by mental arithmetic.

Lambda rhythm:

•Sharp transients occuring over the occipital region.▫During awake state.▫During visual exploration.

•50µV amplitude, 200-300msec duration.•Related to occulomotor visual integration

and arousal mechanism.

Sleep EEG:

•NREM Sleep:▫I Drowsiness▫II Light sleep▫III Deep sleep▫IV Very deep sleep

•REM sleep

Stage I NREM Sleep:

•Early drowsiness :slow activity and drop out of alpha.

•Trains of 2-3Hz and 4-7Hz waves diffusely prominent.

•Paradoxical Alpha: If patient aroused in this stage, posterior alpha activity reappears with a higher amplitude than individuals regular rhythm.

•Deep drowsiness: Vertex waves▫Small spiky discharge of positive polarity

followed by a large negative wave.▫Maximum at the vertex.▫Physiologic.

Positive Occipital Sharp Transients of Sleep (POSTS):

•Physiologic potentials of deep drowsiness.•In stage II and III NREM•Spontaneous monophasic triangular

waves in the occipital region.

NREM stage II:

•Slow and fast frequencies: back ground frequency, high intraindividual variation.

•Sleep spindles: rhythmic waves of 12-14Hz, amplitude gradually increases and then gradually decreases.

•Frontocentral in location.•With deep sleep frequency slows down to

6-10Hz.

•Vertex sharp waves:•K complexes: Seen in Stage II, III IV

NREM sleep.▫Frontal and central region

•Initial sharp component followed by a slow component.

•Sharp component is biphasic.•Slow component represented by large

waves followed by superimposed spindles representing fast component.

Stage III NREM Sleep:

•Background activity shows delta frequency (0.7-3Hz).

•Rhythmic 5-9Hz low voltage activity.•Sleep spindles- less prominent•K complexes.

Stage IV NREM Sleep:

•Prominent Delta activity.•Sleep spindles and K complexes are rare.•Arousal at this stage associated with sleep

disorders.▫Somnambulism▫Nocturnal terror▫Enuresis

REM sleep:

•Seldom recorded in routine EEG.•Low voltage polyrhythmic activity.•Ocular potentials.•Alpha bursts.•Arousal from sleep:

▫Quick process▫Single and sharp K complexes followed by

immediate change to awake pattern.

EEG in Newborn:

•Electrical activity of brain is discontinuous with long periods of quiescence – trace discontinua.▫< 34 wks GA

•Trace alterenant – semi periodic voltage attenuation.

•Hemisphere synchrony is less in preterms.

•Beta – Delta complexes : Hall mark of prematurity(24-38wks).

•Temporal theta bursts: mid temporally B/L

•Frontal sharp waves: 35-44wks, found in transitional stage of sleep, B/L synchronous.

GA 24 to 27wks 28 to 31wks 32 to 35 wks 36 to 41 wks

Trace discontinua

+ + +NREM -

Trace alternant

- - - +NREM

Continuous - - + +awake &REM

Symmetry - - +occipital +

Sleep awake diff

- - + +

Post. Basic rhythm

- - - -

Awake slow activity

High voltage bursts

Very slow Occipital Slow delta

Fast activity Small B of 16Hz

Frequent ripple

16-20Hz sparse

Sleep slow/fast

slow Slow+little fast

Irreg.slow in oc

Delta, theta

Sharp waves bursts intermittent + Minor sharp bursts

Spindles, K comp, vertex sharp, frontal theta

- - - -

Infants (2mo to 12mo):

•2-3.5Hz, 50-100µV irregular delta activity.•Stable 5hz occipital rhythm by 5 mo.•6-7Hz by 12 mo•Photic driving response @ 3-4mo (theta

range).•EEG changes of drowsiness by 5mo.•Hypnogogic hypersynchrony: prominent

thea activity(4-6Hz) over centroparietal region seen during drowsiness.

•NREM: 0.7-3Hz,100-150µV occipital waves.

•Sleep spindles appear at 2mo.•Sharp spindle configuration.•Absence of spindles at 3 to 8 mo –

abnormal•Vertex waves and K complex – 5th mo.•REM: 5% at birth, 40% at 3-5mo, 30% at

6mo.•Sharp occipital activity of 2-4Hz

12-36 moths:

•Posterior occipital rhythm: 6-7 Hz at 2nd yr, 7-8Hz at 3rd yr.

•Eye opening associated with biphasic occipital rhythm.

•18-25Hz fast activity s/o mild CP.•Generalized slow voltage(4-6HZ) activity

during drowsiness- hypnogogic theta activity.

•NREM: high voltage 1-3Hz and medium voltage 4-6Hz activity mainly in the occipital region.

•Vertex waves are seen, K complexes abundantly present.

•POSTS are poorly developed.•Arousal: high voltage 4-6Hz

superimposed with slower frequency.•Prolonged frontal sharps waves between

2-12 yrs –abnormal.

3-5 years:•Posterior basic rhythm reaches to alpha range.•Slow fused transients: Posterior slow activity

may be preceded by sharp contoured potentials.

•Rolandic Mu waves starts appearing.•Hypnogogic theta activity is not seen.•Posterior slowing of theta to delta range

appears in drowsiness.•6 and 14Hz spikes may appear and disappear

during deep drowsiness.•Well defined spindles and K complexes, POSTS

poorly developed.

6-12 years:

•Posterior alpha rhythm reaches 10H by 10 years.

•Rolandic rhythm rises and peaks by 13-15yrs.

•Anterior rhythmical theta (6-7Hz) appears by 6-12yrs and peaks by 13-15yrs.

•Hyperventilation: 1.5-4Hz slowing.•Drowsiness characterized by gradual

alpha drop out.•Vertex waves, spindles, K complexes are

seen, POSTS start to appear.

13-20 years:•Only subtle difference between children and

adolescent•Amplitude of alpha is slightly less than children.•Fast activity is seen in the fronto-central area.•Rolandic mu waves sometimes seen.•Mature occipital lamda waves.•Anterior 6-7Hz theta activity falls after 15 years.•Response to hyperventilation is less prominent

than children.• IPS reveals photic driving response in medium

and fast range of flicker(6-20Hz)

•Voltage of K complex and sleep spindles are higher.

•POSTS are abundant in stage II NREM sleep.

OLD age:•Normal healthy pattern seen till 60-70 years.•Alpha slowing: MC finding in old age.(9Hz).

▫Poor alpha blocking▫Related to decline in mental function.

• Increased fast activity: suggest well preserved mental faculty.

•Diffuse slowing: 1.5-2Hz, anterior bradyarrythmia.▫Normal or may be associated with vascular ds,

headache, dementia, ataxia.

•Diffuse slowing may be seen in hypotension.

•Focal alteration: ▫Minimal temporal slow waves : dizziness,

head ache.▫Burst of rhythmical temporal theta: CVA▫Temporal slow & sharp waves:

Vertibrobasilar insufficiency.•Wicket spikes seen during sleep(6Hz

spikes with negative component)

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