7.1 Intracranial Recording Modalities (Grids, Strips, SEEG)
Key Takeaways
- Phase II intracranial monitoring is considered when noninvasive data do not adequately define the seizure network or its relationship to essential function; modality and targets are chosen by the multidisciplinary surgical team.
- Subdural grids and strips sample broad accessible cortical surfaces and support mapping, but require surgery and have limited access to sulcal/deep generators plus risks such as hemorrhage, infection, edema, and CSF leak.
- SEEG samples selected three-dimensional cortical and subcortical networks through stereotactic depth trajectories; it is less invasive than a large craniotomy but remains selective sampling with procedure-specific risks.
- Reference, ground, impedance testing, stimulation limits, and electrical-safety requirements follow the approved implant, recording system, manufacturer instructions, biomedical testing, and facility protocol.
- Contact numbering and orientation are not universal: every SEEG depth, strip, and grid label must be reconciled with the patient-specific operative and imaging map before acquisition or stimulation.
7.1 Intracranial Recording Modalities (Grids, Strips, SEEG)
Invasive intracranial electroencephalography (iEEG), designated clinically as Phase II Presurgical Evaluation, represents the definitive diagnostic gold standard for delineating the Epileptogenic Zone (EZ) and mapping adjacent eloquent cortex in patients with medically refractory focal epilepsy. While non-invasive Phase I evaluations (continuous scalp video-EEG, high-resolution 3T epilepsy-protocol MRI, 18F-FDG PET, ictal/interictal SPECT with SISCOM co-registration, magnetoencephalography [MEG], and neuropsychological assessments) establish a pre-surgical hypothesis, they frequently lack the spatial resolution required to resolve complex epileptic networks, isolate deep-seated sulcal or insular foci, or establish safe millimetric surgical resection margins.
For the Certified Long Term Monitoring Technologist (CLTM), managing invasive intracranial studies requires advanced mastery of electrode structural geometries, stereotactic implantation mechanics, differential amplification principles, high-amplitude signal processing, intracranial impedance testing, reference and ground configuration, and rigorous electrical safety protocols to prevent microshock.
1. Clinical Indications for Phase II Invasive Monitoring
Phase II invasive intracranial monitoring is not an exploratory screening test; it is an invasive, hypothesis-driven surgical diagnostic procedure indicated only when Phase I non-invasive evaluations fail to define a safe, curative surgical resection boundary.
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| INDICATIONS FOR PHASE II INVASIVE INTRACRANIAL MONITORING |
| |
| [1] DISCORDANT PHASE I NON-INVASIVE DATA |
| - Scalp ictal EEG onset contradicts structural MRI lesion (e.g., |
| left temporal scalp seizure onset in a patient with a right |
| frontal cavernoma). |
| - PET hypometabolism or SISCOM hyperperfusion points to a different |
| lobe than the scalp ictal video-EEG onset zone. |
| |
| [2] MRI-NEGATIVE (NON-LESIONAL) FOCAL EPILEPSY |
| - High-resolution 3T/7T MRI demonstrates normal brain parenchyma, but |
| scalp video-EEG, PET, and MEG demonstrate regional focal epilepsy. |
| - Requires stereotactic network sampling (SEEG) to identify occult |
| microscopic focal cortical dysplasias (FCD Type I/IIa). |
| |
| [3] LESION PROXIMITY TO ELOQUENT CORTEX |
| - The epileptogenic lesion or suspected onset zone directly abuts or |
| overlaps primary motor (M1), primary somatosensory (S1), or |
| language cortex (Broca's / Wernicke's areas). |
| - Requires subdural grids/strips or SEEG for extraoperative cortical |
| stimulation mapping to define safe surgical margins (>10 mm). |
| |
| [4] SUSPECTED DUAL PATHOLOGY |
| - Patient has two distinct structural abnormalities (e.g., ipsilateral|
| hippocampal sclerosis PLUS temporal neocortical FCD or cavernoma), |
| requiring recording to determine if seizures originate from the |
| mesial temporal structures, the neocortex, or both simultaneously. |
| |
| [5] BILATERAL OR MULTI-LOBAR SEIZURE ONSETS |
| - Clinical semiology and scalp EEG suggest bilateral independent |
| temporal lobe onsets; invasive depth electrodes are required to |
| determine the predominant side of lateralization. |
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[!IMPORTANT] Hypothesis-Driven Implantation Mandate: An invasive intracranial study must never be performed without a clear, pre-implantation anatomical-electro-clinical hypothesis. The multidisciplinary epilepsy surgery team must explicitly define: (1) the hypothesized Seizure Onset Zone (SOZ), (2) the hypothesized early propagation pathway, and (3) the specific eloquent functional boundaries that must be preserved.
2. Structural & Geometric Specifications of Intracranial Modalities
Intracranial electrodes are manufactured from biologically inert, non-ferromagnetic, highly conductive materials—predominantly platinum-iridium (Pt/Ir 90/10) alloy or pure gold contacts embedded in medical-grade silastic (polydimethylsiloxane) or polyurethane carriers—providing biocompatibility, low polarization artifact, and MRI conditional safety.
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| INTRACRANIAL ELECTRODE MODALITY TAXONOMY |
| |
| MODALITY PHYSICAL GEOMETRY SURGICAL ACCESS |
| ================== ============================ ====================== |
| Subdural Grids 2D planar matrix of disc Open craniotomy |
| contacts (8x8, 4x8, 2x8) Large dural exposure |
| 10 mm standard spacing |
| |
| Subdural Strips 1D linear array of disc Burr holes (14 mm) or |
| contacts (1x4, 1x6, 1x8) craniotomy margin |
| 10 mm standard spacing |
| |
| Stereo-EEG (SEEG) 3D cylindrical multicontact Percutaneous twist drill|
| Depth Electrodes leads (4-18 contacts/lead) holes (2 mm diameter) |
| 0.8 mm shaft diameter Robotic stereotaxy |
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Subdural Grids
- Physical Dimensions: Subdural grids consist of circular platinum-iridium disc electrodes with an overall diameter of 4.0 mm and an exposed recording diameter of 2.3 mm embedded within a flexible, transparent silastic sheet. Standard clinical grids have a 10 mm (1.0 cm) center-to-center inter-electrode spacing.
- High-Density Micro-Grids: Used for ultra-high-resolution cortical mapping and research, featuring micro-contacts (1.0 mm diameter) with 3.0 to 4.0 mm center-to-center spacing.
- Geometrical Configurations: Common matrices include $8\times 8$ (64 contacts), $4\times 8$ (32 contacts), $2\times 8$ (16 contacts), or customized L-shaped and temporal-basal designs.
- Clinical Advantages: Provides continuous 2D planar neocortical spatial coverage across gyral convexities. Grids represent the historical gold standard for extraoperative Cortical Electrical Stimulation Mapping (CESM) of continuous language/motor boundaries and Somatosensory Evoked Potential (SEP) phase reversal localization of the central sulcus.
- Limitations & Morbidity: Requires a wide open craniotomy with significant dural opening. Grids cannot record from deep sulcal fundi, the insular cortex buried deep in the Sylvian fissure, or mesial temporal structures (hippocampus/amygdala). Grids carry higher surgical risks, including mass effect, cerebral edema, acute subdural hematoma, CSF leaks, and intracranial infection/meningitis.
Subdural Strips
- Physical Dimensions: Linear silastic arrays containing 4, 6, 8, or 12 disc contacts arranged in a single row ($1\times 4, 1\times 6, 1\times 8, 1\times 12$) with standard 10 mm center-to-center spacing.
- Surgical Placement: Strips can be inserted through small burr holes (14 mm) or around craniotomy margins and gently guided along the subdural space over cerebral convexities, subtemporal surfaces (inferior temporal and parahippocampal gyri), interhemispheric fissures (mesial frontal and cingulate cortex), or suboccipital regions.
- Clinical Utility: Useful for lateralizing bilateral temporal lobe epilepsy or tracking regional seizure propagation when a full open craniotomy grid is unnecessary.
Stereo-EEG (SEEG) & Depth Electrodes
- Physical Dimensions: Flexible, semi-rigid polyurethane shafts with an outer diameter of 0.8 mm containing 4 to 18 cylindrical platinum-iridium recording contacts. Each cylindrical contact is 2.0 mm in length, separated by an insulating inter-contact distance of 1.5 mm, 3.0 mm, or 5.0 mm.
- Surgical Implantation: Electrodes are inserted percutaneously through miniature 2.0 mm twist-drill skull apertures using rigid stereotactic frames (Leksell, CRW) or, modernly, robotic stereotactic systems (ROSA, Neuromate) coupled with high-resolution frameless neuronavigation.
- Vascular Safety Trajectory Planning: High-resolution 3D volumetric T1 MRI is fused with stereotactic CT Angiography (CTA) or MR Venography (MRV). Trajectories are planned to ensure a minimum 2.0 to 3.0 mm safety margin from all pial, cortical, and sulcal blood vessels to eliminate the risk of intracranial hemorrhage.
- 3D Network Sampling: Unlike subdural grids that sit exclusively on the gyral crown (surface 2D), SEEG samples from the gyral crown, both sulcal banks, deep sulcal fundi, subcortical white matter tracts, the insular cortex, and deep mesial temporal structures (amygdala, hippocampal head, body, and tail). Implanting 10 to 20 SEEG leads (yielding 120 to 256 recording channels) provides a comprehensive 3D dynamic network model of seizure generation and propagation with minimal surgical morbidity (<1%).
3. Comprehensive Modality Comparison: Subdural Grids vs. SEEG
| Feature | Subdural Grids & Strips | Stereo-EEG (SEEG) Depth Leads |
|---|---|---|
| Surgical Approach | Large open craniotomy with dural opening | Percutaneous 2 mm twist-drill burr holes |
| Implantation Technique | Manual placement by neurosurgeon | Robotic stereotaxy (ROSA, Neuromate) |
| Recording Geometry | Continuous 2D planar gyral surface | Discrete 3D volumetric network |
| Sulcal Fundus Sampling | Blind (cannot record sulcal depths) | Direct sampling of bottom-of-sulcus dysplasias |
| Deep Structure Access | Cannot sample insula or hippocampus | Direct orthogonal/oblique insular & hippocampal access |
| Bilateral Implantation | Rarely feasible (requires bilateral craniotomies) | Routinely performed with minimal added risk |
| Functional Mapping | Superb 2D continuous margin delineation | Point-by-point along 1D linear tracks |
| Morbidity & Infection | Higher (mass effect, CSF leak, hemorrhage, ~5-10%) | Extremely low (<1% serious complications) |
| Post-Operative Recovery | Substantial pain, facial/scalp edema | Minimal pain; rapid patient mobilization |
| Explantation Method | Requires second major open craniotomy | Simple bedside pull or minor surgical removal |
4. Spatial Sampling Dynamics: 2D Surface vs. 3D Volumetric Networks
The fundamental neurophysiological distinction between subdural grids and stereo-EEG is the spatial geometry of the sampled neuronal populations.
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| 2D GYRAL SURFACE vs. 3D NETWORK SAMPLING |
| |
| [ SUBDURAL GRID: 2D SURFACE ] [ STEREO-EEG: 3D NETWORK ] |
| |
| Contact 1 Contact 2 Contact 8 (Neocortex) |
| +---------+ +---------+ | |
| ~~~|=========|~~|=========|~~~ Gyral Crown v |
| +---------+ +---------+ +---------+ Gyral Crown |
| \ / | Contact | |
| \ Sulcus / +---------+ |
| \ / | |
| \ / v |
| \====/ +---------+ Sulcal Bank / |
| Sulcal | Contact | Fundus |
| Fundus +---------+ |
| (BLIND) | |
| v |
| +---------+ Deep White |
| | Contact | Matter |
| +---------+ |
| | |
| v |
| +---------+ Mesial Structure|
| |Contact 1| (Hippocampus/ |
| +---------+ Amygdala) |
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- The Gyral Crown Blind Spot: Over 60% to 70% of the human cerebral cortex is buried within sulcal banks and fundi. Subdural grids rest exclusively on gyral crowns and are physically separated from sulcal fundi by cerebrospinal fluid. Consequently, a Focal Cortical Dysplasia (FCD Type IIb) located at the bottom of a sulcus ("bottom-of-sulcus dysplasia") generates high-amplitude discharges that are attenuated or missed by surface grids until the seizure propagates broadly to the gyral surface.
- SEEG Volumetric Sampling: SEEG depth electrodes traverse perpendicularly or obliquely across the gyral crown, both banks of the sulcus, the subcortical white matter, and deep gray nuclei, capturing the true origin of sulcal, insular, and mesial temporal epileptogenesis.
5. Electrode Numbering, Orientation, and Nomenclature
There is no universal rule that “contact 1 is deepest” for every SEEG lead or that a specific corner is contact 1 on every grid. Manufacturers, centers, surgical teams, and planning systems use different conventions. Before acquisition, obtain the operative implant map and imaging-based contact localization, verify patient and laterality, and reconcile every physical connector label with the acquisition label. A useful label is unambiguous—for example, left amygdala depth contacts 1–12—but the numeric direction must be defined by that patient’s map.
Never infer anatomy from a familiar naming convention. If the map says contact 1 is distal/mesial, document it that way; if the system numbers in the opposite direction, follow the approved map. After any reconnection, compare the jackbox, acquisition screen, imaging map, and prior record. Escalate a discrepancy before stimulation, clipping, or clinical localization.
6. Impedance, Display, and Signal Integrity
Intracranial contacts have lower amplitudes, different impedance characteristics, and far more local fields than scalp electrodes. Use the amplifier manufacturer’s approved impedance test and the facility’s intracranial protocol; do not apply a scalp-electrode threshold automatically. Some systems limit or prohibit continuous impedance checks because test current or switching can contaminate the record or interact with implanted contacts. Establish a baseline after connection, look for abrupt changes, and assess suspected failure with morphology, neighboring contacts, connector inspection, and the approved test sequence.
Intracranial recordings may contain low-voltage fast activity as well as high-amplitude artifacts or afterdischarges. Use sufficient sampling bandwidth and dynamic range, preserve raw data, and make display-filter changes only for review. Avoid a notch filter as a substitute for correcting a connection or grounding problem. A single noisy contact may be omitted from a display montage when authorized, but the raw channel and troubleshooting record remain preserved.
7. Reference, Ground, and Connector Safety
Use only the reference and ground plan approved for the implant and recording system. The neurosurgical team identifies dedicated reference and ground contacts or another validated arrangement. Never improvise a connection, combine grounds between independent devices, or use an electrode that has an uncertain anatomic location. Ground loops and shared pathways can add artifact and create electrical risk.
Before connecting auxiliary devices or stimulators, coordinate with biomedical engineering and the responsible clinician. Confirm compatible, isolated, medical-grade equipment; intact cables; approved connectors; current inspection status; and the facility’s equipotential/grounding design. Keep fluids away from connectors and provide strain relief so a patient movement cannot pull the implant or jackbox.
8. Stimulation and Electrical Safety
Direct cortical or depth-electrode stimulation is performed only under a physician-approved mapping order and protocol. The stimulation system, pulse parameters, maximum current or charge density, contact pair, train duration, and emergency response are specific to the electrode, generator, anatomic target, and patient. The technologist confirms the selected contacts against the implant map, performs a verbal read-back, documents parameters and responses, and watches for afterdischarges or seizures.
Medical electrical classifications and leakage-current acceptance limits depend on the device’s intended applied part and applicable standards. The safe operational rule is not to memorize one universal microampere number for all intracranial systems. Use only equipment approved for its intended patient connection, with biomedical-engineering acceptance testing and the manufacturer’s instructions. Remove unapproved consumer electronics and avoid improvised adapters or extension cords.
If afterdischarges, a clinical seizure, unexpected pain, new deficit, connector heating, smoke, fluid ingress, or electrical fault occurs, stop stimulation immediately and follow the physician/facility emergency protocol. Preserve and annotate the recording. Do not independently deliver a rescue stimulus or apply a solvent or fluid to the implant site.
9. Cable Management and Complication Surveillance
Support the jackbox and cable bundle so tension is transferred to an approved anchor rather than the implanted lead. Maintain the prescribed head dressing, keep connectors dry, and count/inspect connectors at handoff. Document any disconnection and the exact reconnection map.
Watch continuously for clinically important changes: new severe headache, vomiting, reduced responsiveness, new focal deficit, unequal pupils, fever, drainage, bleeding, swelling, or an abrupt widespread EEG change. These can reflect hemorrhage, edema, infection, lead movement, or other complications. Notify nursing and the responsible physician immediately under the critical-value policy; the technologist describes observations and does not independently diagnose the complication.
The essential intracranial workflow is verification: patient, laterality, implant map, contact labels, approved equipment, reference/ground plan, stimulation pair, and post-procedure documentation. These checks prevent a labeling convention or connector error from becoming a wrong-site clinical conclusion.
A 28-year-old patient with drug-resistant focal epilepsy undergoes Phase I presurgical evaluation. Continuous scalp video-EEG demonstrates ambiguous left frontotemporal seizure onsets, 3T MRI is completely normal (MRI-negative / non-lesional), and 18F-FDG PET demonstrates hypometabolism involving both the left anterior insular cortex and the left mesial temporal structures. Which invasive monitoring modality is most appropriate to evaluate this 3D network epileptogenic hypothesis?
An operative implant map explicitly states that contact 1 on a left hippocampal depth lead is the distal/mesial contact and contact 12 is superficial. How should the technologist label the acquisition?
During pre-monitoring technical verification of an invasive intracranial recording system, the technologist performs an impedance check on all contacts of a newly implanted 10-contact SEEG depth lead. Contacts 1 through 8 display impedances between 0.8 and 1.4 kΩ, while Contact 9 displays an impedance of 18.5 kΩ and Contact 10 displays an impedance of 45.0 kΩ. How should the technologist interpret and troubleshoot these findings?
Which electrical-safety approach is appropriate when connecting an intracranial EEG implant to recording and stimulation equipment?