1 - EEG and Seizures

Question 1

What does an EEG measure?

Answer 1

Electrical potential difference

Need at least 2 electrodes to compare to area of interest and indiferent electrode/2nd area of interest

Question 2

10/20 System for EEG Measurement

Answer 2

Standardized placement for EEG electrodes, allows recording from all major divisions of cerebral cortex

Question 3

Characteristic EEG Frequencies:

Beta

Answer 3

Frequency: 13-30 Hz

Amplitude: Small

Purpose: Mental activity; active, alert wakefullness; REM sleep

Question 4

Characteristic EEG Frequencies:

Alpha

Answer 4

Frequency: 8 - 13 Hz

Amplitude: Small-Medium

Function: Relaxed Wakefullness; prominent over parietal and occipital lobes

Question 5

Characteristic EEG Frequencies:

Theta

Answer 5

Frequency: 4-8 Hz

Amplitude: Medium

Function: Awake, drowsy, and non-REM sleep

*More prominent in young than adult

Question 6

Characteristic EEG Frequencies:

Delta

Answer 6

Frequency: 0.5 - 3.5 Hz

Amplitude: Large

Function: Non-REM sleep

Question 7

Where is EEG current generated?

How are they generated?

Current Sink vs Current Source

Answer 7

Extracellular Space

Ion flow across cell membranes–changes local extracellular potential

1. Current Sink: Generated by inward ion current (leaves extracellular space negative)
2. Current Source: Generated by outward ion current (makes extracellular space positive)

Question 8

How to intracellular synaptic potentials compare to extracellular field potentials?

Cause?

Answer 8

Extracellular < Intracellular

Extracellular resistance is much lower than membrane resistance

Question 9

Synchronized Activity vs Desynchronized Activity

Answer 9

Synchronized - High Amplitued (low frequency)

Desynchronized - Low Amplitude (high frequency)

Question 10

Why does EEG reflect synaptic activity?

Answer 10

Action Potentials are short duration, so synchronization is less likely

Synaptic Potentials are longer duration, greater chance of overlap and summation (synchronization)

***Synaptic Potentials compose most of the EEG signal***

Question 11

How does EEG reflect dipole formation?

What type of cells does an EEG reflect activity of?

Answer 11

Dipole = area of concentrated + and - charge in extracellular space

Large Dipole (greater charge separation over distance) = large EEG

EEG primarily reflects population activity in cortical pyramidal neurons (most numerous, long apical dendrites allow formation of long dipol)

Question 12

Can an EEG distinguish between EPSPs and IPSPs?

Answer 12

No

Sources can be created by excitatory or inhibitory synaptic current

Signks can be created by either excitatory or inhibitory synaptic current

Question 13

What is causing the desynchronization at the second arrow?

Answer 13

Alpha Waves desynchronize during cortical activation

(In the example, the patient is opening their eyes at arrow 2, closing again at arrow 3)

Question 14

Sensory Evoked Potentials

Answer 14

Specific change in EEG activity due to sensory stimulation

These are small, must be averaged across many trials

Can be used to measure central conduction velocity (imp. for Multiple Sclerosis)

Question 15

Auditory Evoked Potential (AEP)

Answer 15

Can be used to detect hearing problems in very young children

Can be used to assess brain stem integrity in unconscious patients

Question 16

Somatosensory Evoked Potential

Answer 16

Peripheral nerve stimulation of upper limb or lower limb

Recording sites at peripheral, spinal, and cortical levels

Question 17

Visual Evoked Potential

Answer 17

Stimulates retina (patterened visual stimulus)

Measure visual evoked potential over occipital cortex

Can compare L/R eyes conduction velocity; to determine site/side of lesion

Question 18

Clinical: Seizure definition

Answer 18

Abnormal electrical brain activity

Relatively common (8-10%)

Outward appearance depends on brain area(s) affected

Question 19

Clinical: Epilepsy

Answer 19

Chronic bouts of abnormal brain activation = Recurrent seizures

0.5-1% of population

Set of related disorders, not a single disease

Question 20

Seizure Category:

Partial Seizure - Focal

(Simple vs Complex)

Answer 20

Partial Seizure: Focal - Originate in one area of the brain; classified by brain area involved (ex. partial temporal lobe seizure)

Simple Partial Seizure: Remain conscious; may experience unusual feelings or senses; hallucinations

Complex Partial Seizure: Alteration or Loss of Consciousness; may show repetitious behavious such as blinks, twitches, walking is a circle (automatisms)

Question 21

Seizure Category:

Generalized

Answer 21

Originate simultaneously in multiple areas of both hemispheres

Involve Loss of Consciousness

May cause falls, or muscle spasms

Question 22

Seizure Category:

Generalized -

1. Absence
2. Tonic
3. Clonic
4. Tonic-Clonic
5. Atonic

(Don’t need to know sub-types, but difference between partial and generalized)

Answer 22

1. Absence: May appear to be staring into space and or may exhibit muscle jerking or twitching
2. Tonic: Stiffening of muscles, generally back, legs, arms (frozen)
3. Clonic Seizures: Repetitive jerking movements of muscles on both sides of the body (spasms)
4. Tonic-Clonic Seizures (Grand Mal): Both tonic and clonic muscle contractions; stiffening of the body and repeated jerks of arms and or legs (both)
5. Atonic Seizures: Loss of normal muscle tone, may cause fall

Question 23

Secondary Generalization vs Generalized Seizure:

How can focal seizures spread?

Answer 23

Focal seizures can spread (from specific area) via Cortico-Cortical Pathway, Cortico-Thalamo-cortical connection

Secondary Generalization: Spread of seizure from focal origination point

Generalizaed Seizure: Originate simulataneously at widespread locations

Question 24

Use of EEG for diagnosing seizure?

Answer 24

Location of abnormal EEG activity can distinguish between focal and generalized seizure

Focal Seizures: Indicate which cortical area contains focus

Question 25

Partial (focal) vs Generalized Seizure EEG?

Answer 25

Partial - Only a few EEG channels

Generalized - Massive abnormal discharge

Question 26

Characteristic of Absence Seizure?

Answer 26

EEG of spike and peak

Question 27

EEG Windows During Seizure?

Answer 27

Interictal: Period between seizures

Ictal: During seizure; tonic (continuous) and clonic (phasic)

Post-ictal Depression: Disorientation, drowsiness, confusion

Question 28

Causes of Seizure disorders?

Answer 28

• Trauma, Stroke, Tumors
• Congenital developmental irregularities
• Genetic disorders, channelopathies affecting voltage-gatd ion channels or NT receptors
• Imbalance of synaptic excitation and inhibition (brain contains many reverberating circuits)

Question 29

Seizure Disorders: Treatment

Medication

Answer 29

1. Enhance Inhibitory Neurotransmitters (GABA) - Barbiturates, Benzodiazepines
2. Reduce Synaptic Excitation
3. Reduce ability of neurons to generate repetitive trains of action potentials - reduce activation/enhance inactivation of voltage gated Na channels, T-type voltage gated Ca2+ channels

Question 30

Seizure Disorders: Treatment

Surgery

Answer 30

1. Remove epileptic focus (lesionectomy, lobectomy - temporal lobe resection most common)
2. Cut pathways involved in spread of seizures - trasect part of corpus callosum, multiple subpial transections