EEG has relatively low sensitivity (25–56%) for diagnosing epilepsy but high specificity (78–98%). Epileptiform discharges are seen in 0.5% of healthy adults on routine EEG but 10-30% in those with brain pathologies. Factors like the location of the epileptogenic zone, seizure frequency, and timing of the EEG affect whether a patient shows interictal epileptiform discharges. Intraoperative electrocorticography directly records cortical potentials during epilepsy surgery to localize the irritative zone, map brain functions, and predict surgical outcomes. It has higher spatial resolution than scalp EEG but recordings are limited by anesthesia effects and duration.

1. DR.ASHUTOSH RATH
GMCH, GUWAHATI

2. EEG IN EPILEPSY
Sensitivity and specificity
• Epileptiform activity is specific, but not sensitive, for diagnosis of epilepsy as
the cause of a transient loss of consciousness or other paroxysmal event that
is clinically likely to be epilepsy.
• EEG has relatively low sensitivity (25–56%).
• Specificity is better, but again variable at 78–98%.
• correlation between different EEG patterns and epilepsy is highly variable

3. How often and in which circumstances do non-epileptic subjects
show IED in the EEG?
• In healthy adults with no declared history of seizures, the incidence of
epileptiform discharge in routine EEG was 0.5%.
• 2–4% in healthy children and in nonepileptic patients referred to
hospital EEG clinics.
• incidence increases substantially to 10–30% in cerebral pathologies
such as tumour, prior head injury etc.

4. Factors influencing whether patients with epilepsy will
show IED in the EEG
A.Location of epileptogenic zone
B.Frequency of seizures
C.Timing of EEG

5. How to improve the yield of interictal EEG ?
About 50% of patients with epilepsy show IED in the first EEG test
1)Repeating EEG
2)Having both sleep and awake EEG
3)Sleep deprivation
4)Hyperventilation
5)Photic stimulation

6. Does abnormal EEG predict seizure recurrence?
In a systematic review, the pooled risk of recurrence at two
years was
• 27% if the EEG was normal
• 37% if there were non-specific abnormalities, and
• 58% if epileptiform activity was present.

8. HOW CAN WE IMPROVE ?

9. HOW FAR HAVE WE ADVANCED: EEG
INTRAOPERATIVE ELECTROCORTICOGRAPHY

10. Introduction
WHAT IS INTRAOPERATIVE ECOG?
Neuronal activity in the brain gives rise to transmembrane currents
that generate potentials.
The registry of these potentials
• when recorded from the scalp - EEG
• when recorded directly from the surface of the cortex –ECOG
INTRAOPERATIVE ELECTROCORTICOGRAPHY (ECOG)
The recording of electrophysiological activity from electrodes placed
directly on the exposed surface of a brain during a surgery.

13. A BRIEF HISTORY
• The first use of intraoperative ECoG recordings was
performed by FOERSTER and ALTERNBERGER in 1935.
• HERBERT JASPER and WILDER PENFIELD further
developed this technique for localisation of epileptogenic
focus during surgical treatment of epilepsy.

14. Wilder Penfield (1891-1976) on the right and Herbert Jasper (1906-1999) on the left (adopted by public
domain at http://baillement.com/lettres/penfield.html).

15. WHY DO WE NEED ECOG?
• The guiding principle in epilepsy surgery is to remove the
“epileptogenic zone” .
• epileptogenic zone includes the ictal onset zone plus the
surrounding “irritative zone”, where frequent interictal
discharges are seen (Rasmussen, 1983; Lüders and Awad, 1992;
Lüders et al., 1993).
• Surgical treatment of lesional epilepsy requires that the
epileptogenic tissue be removed or disconnected.

18. IS’nt MRI SUFFICIENT FOR THAT PURPOSE?
• Demonstration of a structural lesion in the magnetic
resonance imaging (MRI) does not prove that it is the cause of
epilepsy
• EEG evidence is required to establish the epileptogenesis of
the lesion
WHY NOT THE TRUSTED OLD SCALP EEG?
 High-resistance tissues (skull, meninges, skin) between the
current source and the recording electrodes causes
attenuation.
 The spatial resolution of the EEG is very low (around 5–9 cm).

20. • Scalp EEG is limited by imprecise localisation and
determination of the extent of epileptogenic zones.
• ECoG records the cortical potentials directly from the surface or
by depth electrodes bypassing the signal-distorting tissues.
USE OF ECOG
(1)To localise the irritative zone,
(2) To map out cortical functions, and
(3) To predict the success of surgery for epilepsy

21. Technique
• Intraoperative ECoG requires a craniotomy
to access the brain surface.
• There are two kinds of electrode systems
• The use of either system depends on the
individual surgeon’s preference at the time
of the operation.

22. PRESET GRID OR STRIPS
• arrays of evenly-spaced
electrodes are imbedded in
“strips” or “grids” of silicone
plastic.
• Grids and strips come in
standard pre-set sizes but can
also be trimmed to
accommodate the size and
shape of exposed cortical
surface.
• The diameter of each contact
electrode is typically 5 mm
and the distance between
electrodes is typically 1 cm.

23. DISADVANTAGES:
1. the brain surface between the electrodes is not accessible due
to the plastic backbone.
2. fixed uneditable distance between the electrodes.
MODIFICATIONS
Densely spaced micro-electrodes to increase resolution
Making holes in the plastic backbone to increase accessibility to
the underlying brain, or
using heat-sensitive moldable electrodes

24. RIGID WIRE ELECTRODES SYSTEM
• It uses a collection of individual rigid wire electrodes held in place over exposed
cortical surface and secured in a circular-shaped metal frame that is attached to
the skull through a small burr hole.
• The tip of the electrode that touches the brain surface is covered with electric
conductive material, such as carbon. There is typically a spring or other flexible
connection between the tip of the electrode and the attachment to the metal
frame, so that the electrode can move with the brain surface during normal
respiratory cycle without piercing into the brain.

25. Advantages over the preset electrode system
1. Because there is no intervening material between electrodes, the brain surface
between the electrodes can be directly stimulated or surgically manipulated while the
electrodes are on the surface of the brain
2. the location of each electrode can be determined by the surgeon

26. Duration of recording
• The monitoring time runs from 5 to 30 minutes.
• longer the recording time more is the sensitivity.
• Longer recordings allow more time for anaesthetic effects
to dissipate.

27. APPLICATIONS AND INTERPRETATION
• The ECoG recording shows epileptiform potentials, which
are sharp, transient and are different from the
background activity.
• Not always possible to capture a spontaneous seizure,
• spontaneous Interictal Epileptiform Activities (IEAS) are
frequently seen.
• These recordings arise from the irritative zone during the
interictal period.

28. IEA’s
• They may be spikes, polyspikes, sharp waves,
spikes-and-waves, sharp-and-slow wave complexes and/or
any combination.
• The amplitude of the IEAs correlates directly with the
proximity to the epileptogenic focus.
• The IEAs are rarely seen in asymptomatic individuals.
• High positive predictive value for the diagnosis of epilepsy.

30. ADVANTAGES OF ECOG
1. Flexible placement of recording and stimulation electrodes.
2. Recordings can be performed before and after each stage of
resection to assess the presence or absence of epileptiform
activity.
3. It allows direct electrical stimulation of the brain so that the
regions involved in functions that must be spared by the
resection (e.g. Eloquent cortex) can be delineated with a high
degree of confidence.

31. LIMITATIONS OF ECOG
1. The limited sampling time
2. Spontaneous epileptiform activity consists exclusively of interictal spikes
and sharp waves, and seizures are rarely recorded
3. It is impossible to distinguish primary epileptiform discharges from
secondarily propagated discharges arising at a distant epileptogenic site
4. Both the background activity and epileptiform discharges may be altered by
the anaesthetics, narcotic analgesics and by the surgery itself.
5. “Bottom of the sulcus “ problem.

33. EFFECT OF IV ANESTHETIC AGENTS ON
ECOG

34. Stereotaxic Depth Electrodes
• fine, flexible plastic electrodes attached to wires that carry currents from deep and superficial
brain structures.
• current is recorded through contact points mounted in the walls of the electrodes.
• helpful in determining the side of origin in temporal lobe epilepsy &
• in frontal lobe epilepsy in which the spread of abnormal discharges from one frontal lobe to the
other is so rapid that the site or side of origin is difficult to ascertain.
• Targets for the deepest contact points commonly include
 the cingulate gyrus
 the subfrontal region
 the amygdala
 several sites within the hippocampus

35. Technique
• A STEREOTACTIC HEADFRAME is affixed to
the patient's skull
• Target sites for electrodes are selected
using the stereotactic imaging studies.
• This technique is extremely precise in
localization.

36. • Some centers use arteriography to
visualize blood vessels of the brain
along with stereotactic headframe,
to avoid injury to critical vascular
structures.
• Recently, computer-assisted
"FRAMELESS STEREOTAXY" has
been used to place electrodes,
avoiding the need for stereotactic
headframe placement.

37. • Once the electrodes are in place, they may be left there for a week or
two, with the wires tunneled through the skin and connected to the eeg.
• During that time, continuous telemetry is performed to record the
onset of seizure activity.
• Usually, antiepileptic medications are tapered to facilitate the capture
of seizure activity.
• they are not used in the acute ECoG settings due to potential spikes
generated from the acute piercing injury to the brain from the
placement of the electrode.
• the depth electrode-defined irritative zone has essentially no value in
defining the epileptogenic zone.

38. RISK OF IMPLANTED ELECTRODES
1. Infection
• Is considered a major risk of implanted EEG electrodes.
• Infection rate of about 2-3%.
• Meticulous surgical technique, to prevent cerebrospinal fluid leakage, keeps
the risk of infection low
2. Hemorrhage.
• incidence of significant ICH is 1% or less.
3. Direct brain injury due to the passing of depth electrodes has not been
demonstrated because the electrodes are so fine that they normally dissect
through neural tissue without direct brain injury.