ELECTRICAL ACTIVITY OF BRAIN AND ROLE OF

EEG IN DIAGNOSING EPILEPSY

Dr. Mayank Agarwalma.gsvm@gmail.com

Contents • Neurophysiology of brain waves genesis• Brief about EEG recording method• Normal brain waves• Abnormal brain waves• Seizures and Epilepsy• Evoked cortical potential• Conclusion

EEG• The electroencephalogram (EEG) is a recording of

the electrical activity of the brain from the scalp. • The first recordings were made by Richard Caton in

1875 from the surface of dog and rabbit brain• The human EEG was first described by Hans Berger

in 1929

The first published human EEG rhythm. (Source: Berger, 1929).

• The electrical activity in neurons is generated primarily by two sources:

1. Action potential2. Electrical activity in neurons occurring at the synapse : • depolarization results in excitatory postsynaptic potential

(EPSP), and • hyperpolarisation results in inhibitory postsynaptic

potential (IPSP)The relatively slow time course of the EPSPs and IPSPs is more

compatible with the generation of the EEG rather than the fast action potentials.

Origin of EEG waves

The generation of very small electrical fields by synaptic currents in pyramidal cells.

Only if thousands of cells contribute their small voltage is the signal large enough to reach the scalp surface.

(Notice the EEG convention of plotting the signals with negativity upward.)

Diagrammatic representation of microscopic structure of the cerebral cortex

The electrical fields produced from the perpendicular arrangement of the pyramidal neurons resembles that generated by a single dipole.

Like a battery, there is a positive (+) charge, at one end and a negative (-) charge at the other end creating a potential difference or voltage

Generation of Synchronous Rhythms

The activity of a large set of neurons will produce synchronized oscillations in one of two fundamental ways:

1.They may all take their cues from a central clock, or pacemaker.

2. They may share or distribute the timing function among themselves by mutually exciting or inhibiting one another

• Thalamus acts as a pacemaker, its various nuclei exert a control upon the general electrocortical activity via thalamocortical projections.

• The thalamus is not essential for the production of brain waves but plays a large part in maintaining the normal rhythms seen in the EEG.

• Activation (such as eye opening) inputs from the reticular formation in the brainstem abolishes the rhythmic discharges in the thalamus. When you are awake, the thalamus allows sensory information to pass through it and be relayed up to the cortex. When you are asleep, thalamic neurons enter a self-generated rhythmic state that prevents organized sensory information from being relayed to the cortex.

The generation of large EEG signals by synchronous activity.

Requirements for recording EEG• EEG machine (8/16 channels).• Silver cup electrodes/metallic bridge electrodes.• Electrode jelly.• Rubber cap.• Quiet dark comfortable room.• Skin pencil & measuring tape.

Electrode Positioning systemThe majority of laboratories use the 10 to 20 system of electrode placement recommended by the International Federation of Societies for Electroencephalography and Clinical Neurophysiology

10 /20 % system of EEG electrode placement

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.

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.

Two types of recordings• Bipolar – both the electrodes are at active site• Unipolar – one electrode is active and the other is

indifferent kept at ear lobule or mastoid process.

NormalEEG Waves

• DTAB

Alpha wave• Also known as Berger Rhythm• Rhythmical waves occurring in almost all healthy adults

in relaxed and awake state with eyes closed• 8-13 Hz• Average amplitude is about 50μV.• mostly on occipital lobe >>> parietal and frontal region• Disappear during sleep• Frequency decreases in hypoglycemia, hypothermia,

hypercapnia and hypocortisolemia

Alpha block• Alpha block or alpha attenuation refers to a phenomenon in

which α wave attenuates and are replaced by the fast, irregular waves of low amplitude. • Alpha block occurs when:1.The persons open their eyes,2.When the individuals engage in conscious mental activity, such

as doing mathematical calculations and3.When any form of stimulation is applied • The term aroused or alerting response is also used to denote α

block, since it is correlated with arousal or alerting response.

Mu Rhythms • In addition to the alpha rhythms of the visual

cortex (area 18), rhythmic activities with about the same frequency range (8–13 Hz) have been shown to occur in other cortical areas, namely in the somatosensory cortex (areas 1, 2 and 3).

• These activities are known as “rolandic mu rhythms”, or “wicket rhythms” , and have a typical reactivity, since they appear when the subject is at rest and are blocked by movement.

Beta wave• irregular• 14-30 Hz• 5-20 μV• mostly on parietal and frontal region• Seen under following conditions:i. mental activity or excitementii.Arousal response (or α block),iii.Barbiturates induce β activityiv.Infants have fast β-like activity in EEG

Theta wave• 4-7 Hz• 50-100 μV• Seen in:1.parietal and temporal region in children2.Emotional stress in adults particularly during

disappointment and frustration.3.Many brain disorders, often in degenerative brain

states4.10-30% normal adult individuals in alert state

Delta wave• Slow waves• < 3.5 Hz• 100-200 μV• Seen in 1.very deep sleep2.Infancy3.Serious organic brain disease• Represents a highly synchronous inherent activity of

cortical neurons independent of thalamic inputs

• While analysis of an EEG cannot tell us what a person is thinking, it can help us know if a person is thinking.

• In general, high-frequency, low-amplitude rhythms are associated with alertness and waking, or the dreaming stages of sleep.

• Low-frequency, high-amplitude rhythms are associated with non-dreaming sleep states, certain drugged states, or the pathological condition of coma

• Gamma rhythms are relatively fast, ranging from about 30–90 Hz, and signal an activated or attentive cortex.

• Additional rhythms include spindles, brief 8–14 Hz waves associated with sleep, and ripples, brief bouts of 80–200 Hz oscillations

ABNORMAL EEG WAVEFORMS• Epilepsy

• Sleep disorders (Polysomnography)• Narcolepsy• Sleep apnea syndrome• Insomnia and parasomnia

• Consciousness dysfunction• coma • Syncope• Stupor

• Brain death• Flat EEG(absence of electrical activity) on two records run 24 hrs apart.

• Organic brain dysfunction

Epileptiform

EEG discharges• In

general,epileptiform wave may be

1.A spike (duration of 20-70 ms)

2.A sharp wave (duration of 70-200 ms)

3.A spike and wave complex

Provocation test• As EEG is recorded during interictal period, artificial

triggering of epileptiform waves can be done by:

1. Intermittent photic stimulation (flashes of bright light having frequency of 1-30/s ) done for 10 seconds

• Increase rate & decrease amplitude

2. Hyperventilation for 3-5 minutes• Decrease rate & increase in amplitude

Seizures and Epilepsy• Seizures (from the Latin sacire, “to take possession

of”) are temporary disruptions of brain function caused by uncontrolled excessive neuronal activity. These temporary symptomatic seizures usually do not persist if the underlying disorder is corrected.

• In contrast to symptomatic seizures, epilepsy is a chronic condition of recurrent seizures that can also vary from brief and nearly undetectable symptoms to periods of vigorous shaking and convulsions

• The International League against Epilepsy (ILAE) Commission on Classification and Terminology, 2005–2009 has provided an updated approach to classification of seizures, based on the clinical features of seizures and associated electroencephalographic findings

FOCAL SEIZURES •Focal seizures (note that the term partial seizures is no longer used) arise from a neuronal network either discretely localized within one cerebral hemisphere or more broadly distributed but still within the hemisphere.

•Most often, focal epilepsy results from some localized organic lesion or functional abnormality, such as 1.scar tissue in the brain that pulls on the adjacent neuronal tissue, 2.a tumor that compresses an area of the brain, 3.a destroyed area of brain tissue, or 4.congenitally deranged local circuitry.

• With the new classification system, the subcategories of “simple focal seizures” and “complex focal seizures” have been eliminated.

• Instead, depending on the presence of cognitive impairment, they can be described as focal seizures with or without dyscognitive features.

• Focal seizures with secondary generalization has been renamed to evolution of focal seizures to generalized seizures

• The routine interictal EEG in patients with focal seizures is often normal or may show brief discharges termed epileptiform spikes, or sharp waves.

• Because focal seizures can arise from the medial temporal lobe or inferior frontal lobe (i.e., regions distant from the scalp), the EEG recorded during the seizure may be nonlocalizing.

Generalised epilepsy• Typical Absence seizures (Previously known as Petit Mal seizures)

The electrophysiologic hallmark of typical absence seizures is a generalized, symmetric, 3-Hz spike-and-wave discharge that begins and ends suddenly, superimposed on a normal EEG background.

•Atypical Absence SeizureThe EEG shows a generalized, slow spike-and wave pattern with a

frequency of ≤2.5 per second, as well as other abnormal activity

•Atonic seizureThe EEG shows brief, generalized spike-and-wave

discharges followed immediately by diffuse slow waves that correlate with the loss of muscle tone

•Myoclonic seizureThe EEG may show bilaterally synchronous spike-and-

wave discharges

•Generalised tonic clonic seizure (previously known as grand mal seizure)

The EEG during the tonic phase of the seizure shows a progressive increase in generalized low-voltage fast activity, followed by generalized high-amplitude, polyspike discharges.

In the clonic phase, the high-amplitude activity is typically interrupted by slow waves to create a spike-and-wave pattern.

The postictal EEG shows diffuse slowing that gradually recovers as the patient awakens.

EVOKED CORTICAL POTENTIALS

Evoked potential refers to the surface electrical activityrecorded from the surface of the scalp in response to a specific and adequate stimulus

TYPES OF EVOKED POTENTIALS

Depending on the type of stimulus, the evoked potential can be:1.Visual evoked potential (VEP),2.Brain stem auditory evoked potential (BAEP),3.Somatosensory evoked potential.

Depending upon the latency of response, evoked potential can be classified into:

1. Stimulus-related potentials having latency <250 ms. Short latency (<10ms), mid latency (10-50 ms) and long latency (>50 ms)

2. Event-related potentials having latency >250ms.

• Some of the stimulus related potentials originate in the brainstem, some in the thalamus, and the rest in the cerebral cortex. Their presence indicates that the sensory pathway is intact.

• ERP provide information about cortical processing

• Evoked potentials are useful for assessing the prognosis of a patient in coma

• If evoked potentials with a latency of less than 50ms are absent, the patient usually does not survive. As short latency responses correspond to processing in the brainstem and thalamus. If these structures are not functioning, chances of recovery are bleak.

• On the other hand, if ERP are present, the outcome is usually good. As ERP correspond to cortical processing. If the cortical processing is going on, it means that the damage is minimal and reversible.

• Event-related evoked potentials are related to cognitive behaviour. Therefore, use of ERPs in the clinical assessment of dementia and delirium is fairly well established by now

Conclusion • Recording an EEG is relatively simple. The method is

usually non-invasive and is painless.

• An alternative way to record the rhythms of the cerebral cortex is with magnetoencephalography (MEG)

• The electrical activity of the brain, recorded from the pial surface of the brain cortex after opening the skull (e.g. during brain surgery) is termed as electrocorticogram (ECoG)

EEG is a recording of summated synaptic potentials

which are generated by the pyramidal cells in the cortex.

These in turn are modulated by rhythmic discharges from the thalamus,

which is affected by input from the reticular formation.

Different types of brain waves in normal EEGRhythm Frequenc

y(Hz)

Amplitude(uV)

Recording& Location

Alpha(α) 8 – 13 30-70 Adults in rest with eyes closed. Occipital region

Beta(β) 14 - 30 5-20 Adult involved in mental activity. ParietoFrontal region

Theta(θ) 4 – 7 50-100 Parietotemporal region in Children, drowsy adult, emotional distress

Delta(δ) <3.5 100-200 Very deep sleep, infancy

D T A B

References • Guyton 13ed• Indu khurana 1ed• Sabyasachi sircar 2ed• Bijlani 4ed• Harrison’s 19ed• Netter’s internal medicine 2ed• Neuroscience Exploring the Brain 4ed• The Generation of Brain Waves : Karen A. Baldi