Focal epileptiform patterns principles of polarity

Focal epileptiform patterns principles of polarity

1. Focal epileptiform patterns: Principles of polarity

2. Introduction – EEG is a 2 dimensional view of 3 dimensional electrical process occurring in brain. Electrical activity on scalp can be produced by infinite number of sources or generators inside the brain . Inverse Problem – Starting with EEG signal recorded on scalp, trying to infer the location and properties of original sources of a ctivity in brain producing these signals .

3. Basic concepts in source localization EEG mainly records electrical activity from cortex, predominantly from cerebral convexities . Which is produced by extracellular post synaptic potentials at apical dendrites of pyramidal cells. Electrical activity from base of the brain and sulci contribute very little to scalp recorded electrical activity.

4. Pyramidal cells are arranged in functional vertical columns , lying perpendicular to cortical surface. Single large apical dendrite of each pyramidal cell is directed vertically towards the surface of cortex Excitatory synapses at these apical dendrites are located predominantly toward the surface of cortex

5. EPSP at the apical dendrites cause flow of positive ions in to cells resulting in local negative potential in the extracellular space . Proximally – passive flow of positive ions out of cells resulting in positive extracellular potential. Causing current flow in a small loop. – dipole formation . Leading to negative end at surface and positive end in deeper cortex.

6. EPSP with resulting negative extracellular field potential causes Upward deflection in EEG . Epileptiform abnormalities result from summation of EPSP of a large number of neurons forming a large dipole of over regions generating epileptiform activity .

7. Limitations – Source dipole is near the surface At least one recording electrode is over the source Reference electrode is outside the active region Head is a uniform conductor Pitfalls in localization in EEG Spike may be detected at an electrode located away from the actual source ,especially if source is located deep in the brain or within a sulcus An electrode located over skull defect or natural foramina may show higher amplitude of a spike even if source is away from the electrode . Potentials produced from deeper sources have lower amplitude and wider distribution . Multiple sources can either re inforce ( if present parallelly ) or cancel each other ( if present randomly). If 2 sets of dipoles are located at different depths from surface, superficial potentials produce larger and prominent surface field and obscure potentials from deeper sources.

8. THE 10-20 SYSTEM

9. Principles of polarity Difference between 2 inputs is negative – upward / negative deflection on EEG Difference - positive – downward deflection Difference -Equal potential – no deflection .

10. Epileptiform abnormalities Can be a Spike , Sharp wave or spike wave . IFSECN - (International Federation of Societies For Electroencephalography and Clinical Neurophysiology ) criteria Is a transient ( paroxysmal ) abnormality Stands out of background activity. Has a duration of 20 -200 msec. Should have a physiological field on scalp Usually disturbs the background activity – Has an after coming slow wave. Main component of Spike or Sharp wave is generally negative. Amplitude varies depending on intensity of source.

11. Localization In Bipolar Montages , Phase reversal helps in Localization . Ex- in this example , electrical field is small and involves only C4 electrode F4 is more positive than C4 – output positive – downward deflection C4 is more negative than P4 – negative output _ Upward deflection . Reversal of polarity is seen at C4 . Hence phase reversal helps in localization and the electrode across which it occurs is taken as max negativity or true location of Spike.

12. Ex-2 negative potential is involving more than a single electrode , but max at C4 , Also involving F4and P4 to some extent. F4 and P4 is more negative compared to Fp 2 and O2 . F4-P4 – positive output – negative deflection. C4-p4 – negative output -_ upward deflection. Phase reversal at C4 But not a true phase reversal as both F4 and P4 are also negative , but because of the manner in which electrodes are placed – Instrumental phase reversal.

13. End of Chain Phenomenon – if the maximum negative potential involves the electrode , which is at the end of a chain in bipolar montage , it cannot produce a phase reversal . When maximum potential is at O1, O2 or fronto polar (Fp1, Fp2 ).

14. In referential montages - the input 2 of each channel is usually considered to have Zero potential . Hence the channel will demonstrate the true potential at Input 1 . Ex- negative potential is only at C4- causing upward deflection at C4 alone with amplitude corresponding to magnitude of negative potential Some times phase reversal can be seen in referential montage. It can be a true phase reversal as in case of temporal lobe epilepsy or in contamination of reference

15. Ex-2 Negative potential is also at F4 and P4 - produce negative deflection but of lower amplitude with maximum at C4 . Localization is done by amplitude of the spike with electrode showing max amplitude is considered to overlie the source .

16. Principles of polarity and localization in EEG If input 1 is negative as compared to input 2, it produces upward (negative) deflection in EEG If input 1 is positive as compared to input 2, it produces downward (positive) deflection in EEG Localization in bipolar montage The electrode where the negative phase reversal occurs has maximum negative potential If there is no phase reversal, the maximum potential is either at the first or the last electrode of the chain (end of the chain phenomenon) Localization in referential montage The maximum electrical field is under the electrode showing the maximum amplitude If there is a phase reversal, it indicates either true phase reversal or the contamination of the reference

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18. Focal epileptiform patterns in Adult Epilepsies International league against epilepsy (ILAE ) -defined focal epileptic seizures as originating within networks limited to one hemisphere that may either be discretely localized or more widely distributed. Clinically -features consistent with the initial activation of one part of one hemisphere. Nearly one-third of patients with focal epilepsies may be resistant to antiepileptic drugs, and may have to be considered for surgical treatment .

19. The IED are classified morphologically in to Spike , sharp waves , spike / sharp and slow wave complexes and multiple spike complexes . Diagnostic characteristics of inter ictal e pileptiform discharges • Paroxysmal with spiky configuration standing out from the background . • Duration al 70-200 m sec for a sharp wave and 20-70 m sec for a spike . • Abrupt change in polarity from preceding wave forms. • Surface negative polarity . • A physiological field .

20. Spike wave - 20 -70 msec . sharp wave - transient, duration of 70-200 milliseconds Spike / Sharp and wave complex – spike / sharp followed by a dome shaped wave of > 200 msec. Poly spike - two or more spikes occurring one after another with a frequency of >10 Hz.

21. Temporal lobe Epilepsy Either medial temporal lobe or lateral ( neocortical ) temporal structures.

22. Clinical characteristics The classical semiology of MTLE comprises four As: Aura Arrest Automatisms Amnesia I psilateral upper extremity automatisms .associated with dystonic posturing of the contralateral upper extremity- lateralizes the origin . Medial temporal lobe structures - amygdala, hippocampus , para hippocampal gyrus and entorhinal cortex.

23. Anatomical and physiological barriers – The 10-20 system of scalp electrodes placement does not adequately cover the basal and anterior portions of the temporal lobe. The F7 / F8 a nd T1/T2 are used to improve the ascertainment of the anterior temporal lEDs . Sphenoidal and nasopharyngeal electrodes were thus developed for better localization.

24. Anterior temporal (T1/ T2) -Silverman's electrodes) placed 1 cm above at the junction of anterior two-third and posterior one-third of a line connecting the outer canthus of the eye and external auditory meatus.

25. Amygdala generates a very restricted local field potential and hippocampus because of double 'C' conformation , an electrically closed loop is formed where the electrical activity gets cancelled . As a result , fronto -temporal and temporal electrodes detect the IEDs originating from the hippocampus only if they are propagated to these regions and otherwise remain entirely undetected by scalp EEG. Surface IEDs in TLE on the scalp are , in fact, epileptiform expression of the parahippocampal gyrus , entorhinal cortex, temporal neocortex rather than the hippocampus itself. The scalp EEG depicts only a small fraction of lEDs originating from mesial temporal structures

26. The classical lEDs of MTLE are the spikes and the sharp wave complexes with a maximal voltage over the anterior temporal region Exhibit phase reversals at F7 / F8 . maximum electronegativity of the temporal spikes actually is seen anterior and inferior to the standard 10-20 electrode positions. This explains the need for additional nonstandard scalp electrodes have a maximum negativity at the anterior temporal (F7 / F8, T1/T2, and sphenoidal ) electrodes . Type 1 spikes- are sharp contoured and have a steep gradient Mid-temporal (T3/T4) IEDs may also occur in MTLE, but predominant mid temporal IEDs may indicate a larger extra-mesial temporal generator.

27. sphenoidal electrodes are used in the following situations 1)When the routine recording reveals bilateral independent temporal IEDs without lateralizing the significance (not more than 75% of IEDs are lateralized) 2 ) When the routine recording reveals an ictal onset that is not clearly lateralized to the temporal lobe showing the MRI abnonnality 3) bilateral hippocampal atrophy on MRI 4) suspected M TLE with normal MRI

28. Temporal lobe Ictal EEG patterns Temporal ictal patterns were divided in to 3 types based on ictal source and the characteristics.

29. Type -1 A- strongly suggest mesial temporal seizure origin. Type -2 favors neocortical onset Type 3 -( no well defined ictal rhythm)- may be due to insufficient net voltage when onset zone is confined to hippocampus or asynchrony between contributing structures or signal distortion at scalp.

30. Type I A pattern showing rhythmic 5 Hz sharp contoured theta rhythm over the left temporal region

31. Type II A pattern showing polymorphic delto activity over the left temporal region

32. Type 2 C pattern characterized by polymorphic delta activity (Type II A) followed by rhythmic fast spike activity

33. MRI reve als a l esio n ( l ow grade neoplasm) over the r i ght anter io - mesial temporal lobe CASE

34. Bilateral independent anterior temporal IEDs, 90% of them were distributed over the right temporal region Despite bilateral independent IEDs, IED significantly(> 75%) lateralized lo the side of MRI demonstrable lesion along with the ictal EEG provided sufficient non-invasive data to reliably hypothesize the epileptogenic zone

35. Rhythmic beta to theta rhythm over the right anterior temporal region

36. Temporal intermittent rhythmic delta activity (TIRDA) – short runs of rhythmic, saw toothed or 1-4 Hz delta activity of 50- 100 μVin amplitude and more than10 seconds in duration occurring predominantly over the anterior temporal regions. TIRDA has a high correlation with MTLE with HS. TlRDA generally occurs unilaterally , however, during drowsiness and sleep, the bilateral TlRDA may be seen often asynchronously . In contrast to MTLE with HS, the temporal intermittent polymorphic delta activity ( TIPDA ) is more frequent in patients with LTLE and TLE due to lesions such as low-grade neoplasms

37. Temporal 'plus' epilepsies – are characterized by the seizures involving complex epileptogenic networks that involve temporal lobe and neighboring structures such as the orbito -frontal cortex, insula, frontal and parietal operculum and temporo - paricto -occipital junction. In temporal plus epilepsy the interictal discharges are often bilateral or pre-central and the ictal EEG may show early involvement of frontal, tempo-parietal or central regions.

38. Features that are atypical of mesial temporal lobe epilepsy Clinical features Sensory auras Complex visual auras Auditory auras Hyperkinetic seizures Simple motor seizures EEG and imaging findings No interic1al epileptiform discharges (IEDs) Extra temporol lEDs Non laterizing ictal EEG patterns Normal MRI Contra/ extratemporal PET hypometabolism

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