Rationale
The epileptic cortex has been described as consisting of epileptogenic, ictal symptomatogenic, pacemaker and irritative zones. Invasive subdural grid recording provides precise localization of these epileptic zones to better understand seizures. We applied computerized topographic maps of spikes on grids to define the epileptic zones for surgery in children with extra-temporal lobe epilepsies.

Methods
13 patients underwent invasive subdural grids to record ictal onset to analyze the epileptogenic zone, between 1996 and 1998. Digital EEG was recorded at a sampling rate of 200 Hz, using automatic spike and seizure, and manual event detections. A photograph of intraoperative subdural grid was scanned and converted to an 8 bit dithered bitmap. A voltage topographic mapping program(Prism, Persyst, AZ) was used to plot the squared interpolation amplitude of spikes on electrode placements in the photograph of the grid and cortical surface.

Results
Epileptogenic zones were defined in 10 patients, as leading spikes were plotted on the subdural grid topography. Ictal symptomatogenic zones were identified in 2 patients without leading spikes. 12 of the 13 patients underwent surgery, including cortical excision plus multiple subpial transection (MST) in 10, and MST alone in 2. Seizure outcomes consisted of seizure free (5), improvement of seizure control (7) for mean follow up period of 11 months.

Conclusion
The epileptic zones are to be clarified for epilepsy surgery to control seizures. The computerized voltage topographic maps on bitmap templates of subdural grids demonstrate the irritative zone from interictal spikes, the epileptogenic zone from leading spikes, and the ictal symptomatogenic and pacemaker zones from intraictal spikes. The compromised differentiation of epileptic cortex by this method resulted in favorable seizure outcome in children with the extra-temporal lobe epilepsies.

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Epileptic regions consisting of epileptogenic, ictal, symptomatogenic, pacemaker, and irritative zones have been described for better understanding intractable seizure disorders for epilepsy surgery to control seizures. Invasive subdural grid recording provides precise localization of these zones and additional eloquent cortex for the neurosurgical plan. Digital EEG data acquisition and analysis have been developed that improve the automatic, objective, and accurate approach to treatment in this area of clinical neurophysiology (Duffy 79, Nagata 84, Ebersole 91).

Two dimensional voltage topographic mapping of brain potentials has its roots in the toposcopic discharges of Walter and Shipton (Walter and Shipton 51). Although brain maps were believed not to provide any new information as compared to the classic EEG traces (Lopes da Silva 90), given the extensive amount of data collected, including interictal epileptiform discharges and different ictal onsets from pediatric seizure disorders, brain mapping greatly simplifies the task of visualizing epileptic phenomenon.

We applied computerized topographic maps of spikes in interictal and ictal periods using extensive subdural grids and images of the brain surface to define epileptic zones for surgery in children with extra-temporal lobe epilepsies.

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Anatomical View
Color contoured voltage topography of spike discharges creates a steady state image of epileptiform discharges on the cortical surface. The amplitude map is overlaid on the bitmap image of the brain surface and individual electrodes (captured intraoperatively) and is used to visualize various epileptic zones from gyrus to gyrus.

Propagation of Interictal Discharges
Analysis of the amplitude before and after the peak in interictal spike discharges illustrates neuronal propagation in the irritative zone. Among the interictal spikes, stable discharges, local (intra-gyral), regional (intra-lobar) and hemispheric (inter-lobar) spreading discharges of neuronal propagation are demonstrated using this amplitude value on the actual image of the brain surface in the time-sequenced maps.

Visual Interpretation
Using extensive subdural grids (more than 40 contact electrodes) in children with extratemporal lobe epilepsy, amplitude plots among the selected electrodes are created automatically and instantaneously. In addition, the color contoured maps of epileptiform discharges in ictal leading spikes are classified based on the various ictal onset patterns. In some children with extratemporal epilepsy, ictal propagation varied. For epilepsy surgery to control intractable and catastrophic seizure disorders, delineation of entire ictal onset zones including epileptogenic zones is necessary.

Accurate Surgical Plan
The epileptic regions are overlaid on the anatomical digitized images including functional mapping of primary motor, sensory, and language cortex. This facilitates the neurosurgical plan of cortical excision and/or multiple subpial transection around the ictal onset, ictal symptomatogenic, pacemaker, irritative zone, and eloquent cortex. The accurate mapping prior to surgery provides for maximum cortical excision in the epileptic zone while avoiding postoperative dysfunction.

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Seizure Pattern	Irritative Zone	Surgery	Seizure Outcome	Follow-up (months)	Epileptogenic Zone	MEG Spike Sources
leading spikes >low amplitude fast waves	Rt F, C	cortical excision, MST	I	29	identified	clusters
polyspikes and slow waves >low amplitude fast waves	Rt P	MST	III	25	identified /MST alone	cluster
multiple type, rhythmic 4-5 Hz	Lt hemisphere	cortical excision, MST	II	23	partially identified	cluster
rhythmic spike waves >generated	Lt F, T	temporal lobectomy, MST	II	24	partially identified	clusters
paroxysmal G >Rt P, T, low amplitude fast waves	Rt F, C, T, Lt F	cortical excision, MST	II	died	partially identified	cluster
leading sharp waves >building up	Lt O	cortical excision, MST	I	21	identified	NA
building up sharp and slow waves	Bilateral C	MST	II	18	identified /MST alone	clusters
leading spikes >paroxysmal G >low amplitude fast waves	Rt F, P, T, Lt F	cortical excision, MST	IV	17	not identified	scatter
leading spikes >paroxysmal G >low amplitude fast waves	Rt F, T, P	cortical excision, MST	I	13	identified	cluster
leading spikes >building up	Rt hemisphere	cortical excision, MST	II	23	identified /MST	NA
low amplitude fast waves	Lt F, P	cortical excision, MST	I	13	identified	scatter
leasing spike and slow >brief low amplitude attenuation >building up	Lt C	cortical excision, MST	I	8	identified	cluster
low amplitude fast waves	Lt C, P	cortical excision, MST	I	7	identified	cluster

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Case#9.
Different ictal onset zone (right anterior frontal) and 5 ms period propagation of leading spikes during seizure #4 in Group A (M, motor and S, sensory cortex). Click on the preview image below to view the full-size image:


Case#9.
Different ictal onset zone (right posterior frontal) and 5 ms period propagation of leading spikes during seizure #2 in group B, (M, motor and S, sensory cortex).Click on the preview image below to view the full-size image:



Case#9.
Interictal MEG spike sources, summary of IVEEG includes ictal onset zone (lobectomy, cortical excision, and MST) and functional mapping (motor of face, hand, and sensory cortex by left median nerve EP), on subdural grid on skull x-ray. Click on the preview image below to view the full-size image:



Case#10.
EEG tracing of 107 subdural grid electrodes at the beginning of seizure #4, (Group I) including three leading spikes (cursors). Click on the preview image below to view the full-size image:



Case#10.
Topographic mapping of the three leading spikes on seizure #4. Click on the preview image below to view the full-size image:



Case #10.
Summary of ictal onset pattern after 4 day IVEEG, (group I, 5 Szs, GTC, arms extension; II, 5 Szs, proceeding wiggling, GTC; III, 2 Szs, proceeding wiggling, GTC; IV, 1 undetermined, staring, GTC), motor function, and sensory evoked response by left median nerve stimulation, and previous surgery. Click on the preview image below to view the full-size image:



Case#11.
Different interictal discharge propagation on the left frontal region, 10 ms period spike propagation. Click on the preview image below to view the full-size image:



Case #11.
Cortical mapping after 5 day IVEEG, epileptic regions (ictal onset, ictal symptomatogenic, irritative zone) and functional mapping (motor of hand and tongue, sensory, and language cortex). Click on the preview image below to view the full-size image:



Case #13.
Interictal discharges, 5 ms period propagation on left parietal region. Click on the preview image below to view the full-size image:



Case #13.
Cortical mapping from IVEEG (ictal onset zone, pacemaker zone, irritative zone, motor, sensory, and language cortex) and interictal MEG spike sources (open triangle) and sensory evoked field on MSI. Click on the preview image below to view the full-size image:

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The epileptic zones are to be clarified for epilepsy surgery to control seizures. The computerized voltage topographic maps using bitmap templates of subdural grids and brain surface demonstrate the irritative zone from interictal discharges, ictal onset zone from leading spikes of the electrographical seizure, ictal symptomatogenic, and pacemaker zones from interictal spikes. The comprised differentiation of epileptic cortex by this method resulted in favorable seizure outcome in children with extratemporal lobe epilepsies.

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1. Duffy FH, Burchfiel JL, Lombroso CT. Brain Electrical Activity Mapping (BEAM): A method of extending the clinical utility of EEG and evoked potential data. Ann Neurol 1979;5:309-321

2. Ebersole JS, Wade PB. Spike Voltage Topography Identifies Two Types of Frontotemporal Epileptic Foci. Neurology 1991;41:1425-1433

3. Lopes da Silva FH. A Critical Review of Clinical Applications of Topographic Mapping of Brain Potentials. Journal of Clinical Neurophysiology, 1970;7(4):535-551

4. Nagata K. Yunoki K, Araki G, et al. Topographic Electroencephalographic Study of Transient Ischemic Attacks. Electroenceph Clin Neurophysiol 1984;58:291-301

5. Walter WG, Shipton HW. A New Toposcopic Display System. Electroenceph Clin Neurophysiol, 1951;3:281-292

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