Electroenchephalography

Dr. Nabajyoti Kalita

History

• 1874- Caton recorded spontaneous electrical activity from live exposed cortices of cats, rabbits and monkeys

• 1891- Adolph Beck demonstrated that the dog’s visual cortex produced large electrical potentials when the eyes were rhythmically illuminated

• 1924-Hans Berger finally succeeded in recording the human EEG. he also coined the term Electroencephalogram

• In his classic series of 23 papers Berger described many aspects of human EEG:

1) that brain electrical activity came from neurons and not blood vessels or connective tissue

2) that waking alpha waves were blocked by eye opening

3) the characteristics of EEG activity change with age, sensory stimulation, state of consciousness

• 1934- Lord Adrian publicly confirmed Berger’s work and the field of EEG was born

• Despite the fact that EEG originated in psychiatry, the strongest initial impetus for its use came from neurology, particularly the study of epilepsy

• But EEG didn’t flourish in psychiatry because of 2 factors:

1) lack of specificity of EEG abnormalities to known psychiatric syndromes

2) continuing discovery of EEG abnormalities correlating with epilepsy, tumors, encephalopathies, stroke syndromes, and coma

• The difference in electrical potential measured between any two EEG electrodes fluctuates or oscillates rapidly

• The earliest EEG recordings involved only one pair of electrodes, or one channel of recording

• Later 4,8,10,12,16 and even 64 channel recording machines came into existence

Scalp-recorded EEG signals are the result of summated field potentials generated by EPSPs and IPSPs in vertically oriented pyramidal cells of the cortex

An EPSP in a dendrite produces electrical negativity in the immediately surrounding areaThe reverse occurs with an IPSP, generating an electrical positivity nearby

The summation of EPSPs and IPSPs is enhanced

Limitations of scalp eeg

• Although approximately one-third of the outer convexity of the cortex may be within reach, much cortical area consists of mesial, inferior, and deep buried cortical tissue which is not in the reach of S-electrodes

• Furthermore, substantial impedance to electrical conduction from skin, skull, dura, and brain tissue exists

• Weak electrical signals, even those close to the surface,may escape detection

• Because of the limitations of scalp EEG, a normal EEG can never constitute positive proof of absence of brain dysfunction(low specificity)

Electrode Placement

• In 1947,it was decided at an international EEG congress held in London to standardize the system of electrode placement

• 10-20 international system of electrode placement was developed which became standard worldwide since 1958

special electrodes

• Sphenoidal electrodes-• Nasopharyngeal electrodes

montage selection

• In EEG , the way electrode pairs are arranged for a recording is called a montage

• TWO MAIN TYPES OF MONTAGES: 1)Referential 2)Bipolar

• The majority of abnormal cerebral activities tend to appear at the surface as negative potentials

• A given channel of EEG activity is derived from two inputs

• By convention, the first electrode of a pair constitutes input 1, whereas the second electrode provides input 2

• The direction of the pen deflection is based on whether input 1 is “more negative” or “less negative than the input 2

• If the input 1 is closer to the source of a negative field and, hence, more “negative” than the input 2, then there is an upward pen deflection

• Conversely, if the input 1 is more distant from the source of the field than the input 2 and, hence, less negative than the input 2 then the pen deflection is downwards

sensitivity

• The amplification used in EEG recording is adjustable and can be increased or decreased according to the need

• accepted standard sensitivity across laboratories is 7 µV-7.5µV for each millimeter of pen deflection

frequency filter settings• Nearly all of the EEG activity that is analyzed for clinical or

research purposes falls within the frequency range of .5 to 40.0 or 50.0 Hz.

• Extreme lowering of lower range will include unwanted slow waves generated by skin(artifact)while raising it to 5.0hz will exclude some genuine slow waves of brain origin(fig.)

• Similarly a too high upper limit will include the unwanted muscle potential while lowering it down will lead to exclusion of genuine fast spike discharges of brain origin(fig.)

Special activations

• Certain activating procedures tend to increase the probability that abnormal discharges, particularly spike or spike-wave seizure discharges, will occur

• Medication Activation-alpha-chloralose.• Hyperventilation- for 1 to 4 minutes,: increase in

generalized medium- to high-voltage synchronous slow waves in the delta range, which then quickly subside when over breathing stops

• Especially effective in eliciting the classic diffuse three-per-second spike-and-wave complex of Absence seizures.

• Photic stimulation-involves placing an intense strobe light approximately 12 inches in front of the subject's closed eyes and flashing at frequencies that can range from 1 to 50 Hz.

• Sleep deprivation- CNS stress produced by 24 hours of sleep deprivation alone can lead to the activation of paroxysmal EEG discharges in some cases

normal intrinsic frequencies

Alpha waves-• Highly rhythmic,8 to 13 Hz

- dominant brain wave frequency of the normal eyes-closed wake EEG

- most prominent over the posterior cortex, particularly the parietal, posterior temporal, and occipital cortex

- occipital region being best suited to show this activity- alpha can be blocked or attenuated by engaging in visual imagery,

numeric calculation, or almost anything requiring significant concentration(fig.)

• Beta waves- frequency >13hz- Particularly over frontal-central regions- The voltage of beta activity is almost always lower

than that of activity in the other frequency bands• Theta waves- frequency of 4.0-7.5 Hz- prominent in the drowsy and sleep tracings- Excessive theta in wake,suggests a focal

pathological process

o Delta waves-frequency 3.5 Hz or less- a prominent feature of deeper stages of sleep- The presence of significant generalized or

focal delta in the wake EEG is strongly indicative of a pathophysiological process

o Gamma waves-high frequency oscillations >30 Hz

- Importance in neurobehavioral disorders

• Changes with age- appearance of the EEG tracing changes dramatically from birth to advanced age

• preponderance of irregular delta activity in the tracing of the infants

• EEG activity gradually increases in frequency and becomes more rhythmic with increasing age

Artifacts

• Artifacts are electric potentials of nonbrain origin that are in the frequency and voltage range of EEG signals and that are detected by scalp electrodes

• Common artifacts include eye blinks, vertical or lateral eye movements,muscle potentials from jaw clenching, perspiration artifacts (galvanic skin response), and head movement

quantitative analysis of eeg

• That is,it involves calculating the amount of EEG in each frequency band such as alpha,theta,delta

• The amount of EEG in each frequency band is then displayed on a map of the scalp,this is known as topographic mapping

• Voltages or other features are displayed with contour lines or color coding

Eeg findings in organic pathophysiology

• Seizures- The hallmark EEG finding for a seizure disorder is the generalized, hemispheric, or focal spike or spike-wave discharge, or both

- Absence seizure is of utmost importance for a psychiatrist as they are often mistaken for inattention in children or other functional or organic syndromes

- EEG reveals classic diffuse three-per-second,spike-and-wave discharges with a multiple spike component(fig.)

• Structural Lesions-Structural and space-occupying lesions are typically associated with focal slowing in the EEG

• focal slowing may be accompanied by focal spike activity as well(eg.cerebral abscess)

- If SOL is located deeper into subcortical regions,it may remain invisible in S-EEG

- An increase in the focal slowing on the repeat exam might suggest a growing or expanding lesion

- 90% of cortical brain tumors can be detected by routine EEG

• Closed Head Injuries-Focal slowing is the expected EEG sequela

- may appear over the site of the trauma or over a contrecoup location

• Hepatic encephalopathy- classic Triphasic waves characterized by frontally dominant or diffuse 1.5- to 3.0-per-second high-voltage slow waves

• Alcohol-increase in the alpha activity typically accompany alcohol consumption

• Beta activity substantially increased in withdrawal

• Delirium tremens- Excessive fast activity dominate the EEG tracing

• Delirium from other causes is associated with

generalized slowing

Eeg findings in psychiatric disorders• Q-EEG- recent studies strongly support promise as a

diagnostic and prognostic test for early Alzheimer’s disease

• Dementia- rarely have normal EEG

- Normal EEG can diagnose Pseudodementia

- Useful in following progression of Alzheimer’s disease

- FTD- may have normal routine EEG

• Attention-Deficit/Hyperactivity Disorder- - 1/3rd of ADHD patients have abnormal routine EEG- Significant proportion exihibit spike or spike-wave

discharges- Q-EEG- increased theta activity in frontal lobe regions- Decrease in beta activity

• Autistic Spectrum Disorders- - 5 per sec temporal-parietal sharp wave

discharges(epileptiform activity)

- Slowing in few patients

- Approximately one-third of children with autism spectrum disorders develop epilepsy

• Mania-Q-EEG-less alpha power and high beta activity

- Left hemisphere involved more• Depression-S-EEG- can rule out underlying

medical cause of depression(tumor,cva etc)- Q-EEG-increase in alpha and/or beta waves- Alpha activity mainly in left frontal region

• Obsessive-Compulsive Disorder-- S-EEG-widespread increase of slow waves- Also,epileptiform activities over left temporal

lobe- Q-EEG -involement of ant.regions of the

scalp(frontal dysfunction)(fig.)

Schizophrenia

Q-EEG- delta activity significantly increased in both eyes open and closed conditions,most significantly over the ant.cortical regions

- Similarly theta activity increased significantly over the posterior cortical regions

- increase in slow alpha amplitude over anterior sites- increase in slow beta amplitude over anterior sites in

eyes open condition- reduction of gamma oscillations in patients with

schizophrenia

Reference

• Kaplan & Sadock’S Comprehensive Textbook of Psychiatry

• Electrodiagnosis in clinical Neurology

• Various Internet Sources