1 - EEG and Seizures Flashcards
What does an EEG measure?
Electrical potential difference
Need at least 2 electrodes to compare to area of interest and indiferent electrode/2nd area of interest
10/20 System for EEG Measurement
Standardized placement for EEG electrodes, allows recording from all major divisions of cerebral cortex
Characteristic EEG Frequencies:
Beta
Frequency: 13-30 Hz
Amplitude: Small
Purpose: Mental activity; active, alert wakefullness; REM sleep
Characteristic EEG Frequencies:
Alpha
Frequency: 8 - 13 Hz
Amplitude: Small-Medium
Function: Relaxed Wakefullness; prominent over parietal and occipital lobes
Characteristic EEG Frequencies:
Theta
Frequency: 4-8 Hz
Amplitude: Medium
Function: Awake, drowsy, and non-REM sleep
*More prominent in young than adult
Characteristic EEG Frequencies:
Delta
Frequency: 0.5 - 3.5 Hz
Amplitude: Large
Function: Non-REM sleep
Where is EEG current generated?
How are they generated?
Current Sink vs Current Source
Extracellular Space
Ion flow across cell membranes–changes local extracellular potential
- Current Sink: Generated by inward ion current (leaves extracellular space negative)
- Current Source: Generated by outward ion current (makes extracellular space positive)
How to intracellular synaptic potentials compare to extracellular field potentials?
Cause?
Extracellular < Intracellular
Extracellular resistance is much lower than membrane resistance
Synchronized Activity vs Desynchronized Activity
Synchronized - High Amplitued (low frequency)
Desynchronized - Low Amplitude (high frequency)
Why does EEG reflect synaptic activity?
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***
How does EEG reflect dipole formation?
What type of cells does an EEG reflect activity of?
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)
Can an EEG distinguish between EPSPs and IPSPs?
No
Sources can be created by excitatory or inhibitory synaptic current
Signks can be created by either excitatory or inhibitory synaptic current
What is causing the desynchronization at the second arrow?
Alpha Waves desynchronize during cortical activation
(In the example, the patient is opening their eyes at arrow 2, closing again at arrow 3)
Sensory Evoked Potentials
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)
Auditory Evoked Potential (AEP)
Can be used to detect hearing problems in very young children
Can be used to assess brain stem integrity in unconscious patients
Somatosensory Evoked Potential
Peripheral nerve stimulation of upper limb or lower limb
Recording sites at peripheral, spinal, and cortical levels
Visual Evoked Potential
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
Clinical: Seizure definition
Abnormal electrical brain activity
Relatively common (8-10%)
Outward appearance depends on brain area(s) affected
Clinical: Epilepsy
Chronic bouts of abnormal brain activation = Recurrent seizures
0.5-1% of population
Set of related disorders, not a single disease
Seizure Category:
Partial Seizure - Focal
(Simple vs Complex)
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)
Seizure Category:
Generalized
Originate simultaneously in multiple areas of both hemispheres
Involve Loss of Consciousness
May cause falls, or muscle spasms
Seizure Category:
Generalized -
- Absence
- Tonic
- Clonic
- Tonic-Clonic
- Atonic
(Don’t need to know sub-types, but difference between partial and generalized)
- Absence: May appear to be staring into space and or may exhibit muscle jerking or twitching
- Tonic: Stiffening of muscles, generally back, legs, arms (frozen)
- Clonic Seizures: Repetitive jerking movements of muscles on both sides of the body (spasms)
- Tonic-Clonic Seizures (Grand Mal): Both tonic and clonic muscle contractions; stiffening of the body and repeated jerks of arms and or legs (both)
- Atonic Seizures: Loss of normal muscle tone, may cause fall
Secondary Generalization vs Generalized Seizure:
How can focal seizures spread?
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
Use of EEG for diagnosing seizure?
Location of abnormal EEG activity can distinguish between focal and generalized seizure
Focal Seizures: Indicate which cortical area contains focus
Partial (focal) vs Generalized Seizure EEG?
Partial - Only a few EEG channels
Generalized - Massive abnormal discharge
Characteristic of Absence Seizure?
EEG of spike and peak
EEG Windows During Seizure?
Interictal: Period between seizures
Ictal: During seizure; tonic (continuous) and clonic (phasic)
Post-ictal Depression: Disorientation, drowsiness, confusion
Causes of Seizure disorders?
- 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)
Seizure Disorders: Treatment
Medication
- Enhance Inhibitory Neurotransmitters (GABA) - Barbiturates, Benzodiazepines
- Reduce Synaptic Excitation
- 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
Seizure Disorders: Treatment
Surgery
- Remove epileptic focus (lesionectomy, lobectomy - temporal lobe resection most common)
- Cut pathways involved in spread of seizures - trasect part of corpus callosum, multiple subpial transections
