ELECTROENCEPHALOGRAPHY(EEG)

Dr. Shaikh Mujeeb AhmedM.B.B.S. MD. (PHYSIOLOGY)

EEG

The electroencephalogram (EEG) is a recording of the electrical activity of the brain from the scalp.

The first recordings were made by Hans Berger in 1929

Origin of EEG waves

Electroencephalogram

EEG is the record of electrical activity of brain( superficial layer i.e. the dendrites of pyramidal cells) by placing the electrodes on the scalp.

Generation of large EEG signals by synchronous activity

Objectives of EEG practical

Familiarize with the principles of techniques involved

Count frequencies and measure the amplitudes of the record obtained.

Categories the records into appropriate rhythms – α, β, θ,and δ.

Cont…

Objectives of EEG practical

Identify and describe changes produced by provocation tests.

e.g. eye opening & closing, intermittent photic stimulation (IPS) clapping sound, induce thinking & hyperventilation.

Appreciate clinical uses of EEG

EEG Waves

Alpha wave -- 8 – 13 Hz. Beta wave -- >13 Hz. (14 – 30 Hz.) Theta wave -- 4 – 7.5 Hz. Delta waves – 1 – 3.5 Hz.

D T A B

Different types of brain waves in normal EEG

EEG Recording From Normal Adult Male

Alpha wave

rhythmic, 8-13 Hz mostly on occipital lobe 20-200 μ V normal, relaxed awake rhythm with eyes

closed

Beta wave

irregular, 14-30 Hz mostly on temporal and frontal lobe mental activity excitement

Theta wave

rhythmic, 4-7 Hz Drowsy, sleep

Delta wave

slow, < 3.5 Hz in adults normal sleep rhythm

Different types of brain waves in normal EEG

Rhythm Frequency(Hz)

Amplitude(uV)

Recording& Location

Alpha(α) 8 – 13 50 – 100 Adults, rest, eyes closed.Occipital region

Beta(β) 14 - 30 20 Adult, mental activityFrontal region

Theta(θ) 5 – 7 Above 50 Children, drowsy adult, emotional distressOccipital

Delta(δ) 2 – 4 Above 50 Children in sleep

D T A B

Requirements

EEG machine (8/16 channels). Silver cup electrodes/metallic bridge

electrodes. Electrode jelly. Rubber cap. Quiet dark comfortable room. Skin pencil & measuring tape.

Computerized EEG Machine

Electrode Positioning system

EEG Electrodes

Sliver Electrodes Electrodes Cap

Procedure of EEG recording

A standard EEG makes use of 21 electrodes linked in various ways (Montage).

Ask the subject to lie down in bed. Apply electrode according to 10/20%

system. Check the impedance of the

electrodes.

10 /20 % system of EEG electrode placement

Procedure of EEG recording

Ask the subject to close his/her eyes. Select a montage. Press run switches on to run the

paper.

Procedure of EEG recording

Press the calibration knob to check voltages & time constant.

Always observe subject for any abnormal muscle activity.

Ask the subject to open eyes for 10 sec.and ask him/her to close eyes. (do this procedure for several times in each montage)

EEG Electrodes Each electrode site is labeled with a letter

and a number. The letter refers to the area of brain

underlying the electrode e.g. F - Frontal lobe and T - Temporal lobe. Even numbers denote the right side of the

head and Odd numbers the left side of the head.

Two types of recording

Bipolar – both the electrodes are at active site Bipolar montage are parasagital montage.

Unipolar – one electrode is active and the other is indifferent kept at ear lobe.

Always watch for any abnormal muscle activity.

Ask the subject to open eyes for 10 sec. then ask them to close the eyes.

Montage

Different sets of electrode arrangement on the scalp by 10 – 20 system is known as montage.

21 electrodes are attached to give 8 or 16 channels recording.

Analysis

Electrical activity from the brain consist of primarily of rhythms.

They are named according to their frequencies (Hz) and amplitude in micro volt (μv).

Different rhythms at different ages and different conditions (level of consciousness)

Usually one dominant frequency (background rhythm)

Factor influencing EEG Age

Infancy – theta, delta wave Child – alpha formation. Adult – all four waves.

Level of consciousness (sleep) Hypocapnia(hyperventilation) slow & high

amplitude waves. Hypoglycemia Hypothermia Low glucocorticoids Slow waves

NORMAL EEG CHANGES

Desynchronization or Alpha block

Cause: Eyes opening (after closure) Thinking by the subject (mathematical calculation)

Sound (clapping)

Eye opening

Alpha rhythm changes to beta on eye opening (desynchronization / α- block)

Thinking

Beta waves are observed

Provocation test

Intermittent photic stimulation Increase rate & decrease amplitude

Hyperventilation Decrease rate & increase in

amplitude

Use of EEG Epilepsy

Generalized (grandmal) seizures. Absence (petitmal) seizures.

Localize brain tumors. Sleep disorders (Polysomnography)

Narcolepsy Sleep apnea syndrome Insomnia and parasomnia

Helpful in knowing the cortical activity, toxicity, hypoxia and encephalopathy &

Determination of brain death. Flat EEG(absence of electrical activity) on two records run

24 hrs apart.

Sleep studies The EEG is frequently used in the

investigation of sleep disorders especially sleep apnoea.

Polysomnography : EEG activity together with

heart rate, airflow, respiration, oxygen saturation and limb movement

Sleep patterns of EEG There are two different kinds of sleep:

Rapid eye movement sleep (REM-Sleep) Non-REM sleep (NREM sleep)/ slow wave

sleep NREM sleep is again divided into 4

stages (I to IV). The EEG pattern in sleep is given in the following table:

Stages of sleep EEG pattern Somatic or Behavioral changes

Alert Alpha activity on eye closedDesynchronization on eye opening

Respond to verbal commands

I (Drowsiness) Alpha dropout & appearance of vertex waves & theta.

Reduced HR & RR

II (Light sleep) Sleep spindles, vertex sharp waves & K-complexes

Reduced HR & RR

III ( Deep Sleep) Much slow background K-complexes

Reduced HR & RR

IV (very deep sleep)

Synchronous delta waves, some K-complexes

Reduced HR & RR

REM sleep(paradoxical sleep)

Desynchronization with faster frequencies

HR, BP & RR irregularMarked hypotoniaRapid eye movement 50 – 60 /min.Dreaming threshold of arousal

Changes in brain waves during different stages of sleep &

wakefulness

Changes in brain waves during different stages of sleep &

wakefulness

Sleep Spindle

K - complex

EEG & Epilepsy

EEG in different types of epilepsy

Gra

nd

mal s

eizu

re

Petitm

al s

eiz

ure

Video monitoring

Simultaneous video monitoring of the patient during the EEG recording is becoming more popular. It allows the physician to closely correlate EEG waveforms with the patients activity and may help produce a more accurate diagnosis.

EEG Artifacts Biological artifacts

Eye artifacts (including eyeball, ocular muscles and eyelid)

ECG artifacts EMG artifacts Glossokinetic artifacts (minor tongue

movements)

External artifacts Movement by the patient settling of the electrodes Poor grounding of the EEG electrodes the presence of an IV drip

ECG Artifacts

LET’S ANSWER

What is EEG and what is montage?

The electroencephalogram (EEG) is a recording of the electrical activity of the brain from the scalp.

Arrangements of electrodes by 10/20% system is known as montage.

What is the advantage of provocation test while recording EEG?

Provocation test e.g. hyperventilation, intermittent photic stimulation are done to trigger the epileptic focus.

Compare & contrast Alpha & Beta waves of EEG

Alpha In awake resting

with eyes closed Frequency 8 – 13 Hz.

Voltage – 50uV.

Beta In awake thinking

subjects. Frequency - >13Hz.

(14 – 30 Hz.) Voltage – 20uV.

What stage of sleep is indicated by slow waves of EEG(4 – 7Hz.) ?

4 – 7 Hz. (Theta) waves are seen during stage 2 & 3 (light and deep ) sleep

In the following tracing of EEG what changes do you observe after point “A”.What may be it’s possible cause?

Rhythm has changed from α toβ(at point “A”) ,it is called alpha block or desynchronization.

Causes: Eyes opening Thinking e.g. mathematical calculations. Sound (clapping)

What changes are seen in EEG of an epileptic child suffering from absence seizure (petit mal)?

In petit mal spike and wave (dome shaped) 3 cps. Pattern is seen.