Localization of Seizure Foci Dipole Model.
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1. Advanced EEG Interpretation “Pitfalls and Caveats” Michael Duchowny, M.D.
Director, Comprehensive Epilepsy Program
Miami Children’s Hospital
Professor of Neurology
Miller School of Medicine
University of Miami
2. Localization of Seizure Foci Dipole Model “Sink-source” model of current flow from the neuron through extracellular space.
Field potential measures the net sum of the individual sources and sinks of a large population of neurons.
“Dipole layer” cortical layer creates a layer with superficial sinks and deep sources.
5. Localization of Seizure Foci Volume Conduction Volume conduction represents passive (energy-independent) spread of current from a generator through the surrounding media.
Conduction is virtually instantaneous- spikes at different electrodes are recorded instantaneously.
Discharges attenuate with distance but preserve polarity and morphology
6. Localization of Seizure Foci Cortico-cortical Propagation Cortico-cortical propagation is an active (energy-dependent) process.
Propagated potentials do not attenuate, but are comparatively slower, leading to measurable time differences.
Propagated spikes may differ in morphology and polarity from “primary” spikes.
7. Recording Techniques Scalp Electrodes Perimeter electrodes- necessary to determine maximal voltage.
Sensitivity- determined by the depth, extent and orientation of the generator.
Specificity- determined by the surface anatomy ( e.g. skull configuration) and number of electrodes.
8. Recording Techniques Montage Design
NO PERFECT MONTAGE!
Reference contamination- reference located within the spike field.
Reference-active electrode distance- may lead to attenuation.
9. Recording Techniques Source Derivation Montage Compares activity at each electrode to the average of neighboring electrodes.
Mathematically cancels currents from remote generators flowing tangentially.
Narrows the bell-shaped distribution curve, and thus “highlights” discrete foci.
10. Recording Techniques Subtraction Montage Simple montage specifically designed to identify asynchrony of two apparently synchronous spikes of equal amplitude.
Referential derivation- reveals polarity, voltage and morphology of the discharges.
Subtraction derivation- compares the regions to identify asynchrony.
11. Reference Selection No ideal reference- even distant electrodes should never be assumed to be “inactive”
ring electrode around the neck
Digital EEG with off-line formatting capability- decidedly advantageous
Intracranial EEG recording- subgaleal electrode placed at a distance serves as an adequate reference
12. Interpretation of Patterns Spikes WHAT TO MEASURE?
Initial or most prominent deflection?
Peak-to-peak or peak-to-baseline measurement?
Morphologic diversity- which spikes are significant?
13. Interpretation of Patterns Ictal Discharges Artifacts
14. Interpretation of Patterns Background Abnormalities Focal background slowing, may be more widespread, bilateral, or project to non-epileptogenic regions.
Fast activity- Focal attenuation often correlates with the seizure focus.