Presurgical Evaluation
The presurgical evaluation is a systematic, multidisciplinary process that determines whether a patient with drug-resistant epilepsy is a suitable candidate for surgical intervention and, if so, identifies the epileptogenic zone for targeted treatment. Despite level I evidence supporting the superiority of epilepsy surgery over continued medical therapy for drug-resistant temporal lobe epilepsy, surgery remains underutilized—with significant delays from the onset of drug resistance to surgical referral averaging 10–20 years in many series. The International League Against Epilepsy (ILAE) recommends referral for surgical evaluation as soon as drug-resistant epilepsy is identified, regardless of epilepsy type. A comprehensive epilepsy center provides the multidisciplinary expertise required for the evaluation, including epileptologists, neurosurgeons, neuroradiologists, neuropsychologists, and psychiatrists. Patients treated at comprehensive epilepsy centers have reduced premature mortality compared with those managed in community practice, regardless of whether they ultimately undergo surgery.
Bottom Line
- Drug-resistant epilepsy is defined as failure of two or more appropriate antiseizure medications at adequate doses; 13–37% of people with epilepsy meet this criterion
- Referral timing: The ILAE recommends surgical referral as soon as drug resistance is identified; delays of 10–20 years to evaluation remain common and contribute to excess morbidity and mortality
- Phase I evaluation includes video-EEG monitoring, MRI epilepsy protocol, neuropsychological testing, FDG-PET, ictal SPECT, and MEG/MSI to localize the epileptogenic zone noninvasively
- Phase II evaluation (intracranial EEG) is required in 30–40% of surgical candidates; SEEG has largely replaced subdural grids in North America due to lower morbidity and comparable seizure outcomes
- Concordance of MRI, EEG, PET, and clinical data is the strongest predictor of surgical success; validated tools (Epilepsy Surgery Grading Scale, Seizure Freedom Score) help quantify the likelihood of seizure freedom
- Language lateralization: fMRI has largely replaced the Wada test for language lateralization, though the Wada test remains important for memory lateralization in selected cases
- Multidisciplinary conference: The surgical decision is made at a multidisciplinary epilepsy surgery conference integrating all evaluation data
Identifying Surgical Candidates
Definition of Drug-Resistant Epilepsy
The ILAE defines drug-resistant epilepsy as failure to achieve seizure freedom with adequate trials of two tolerated and appropriately chosen antiseizure medications, whether used as monotherapy or in combination. By this definition, 13–37% of people with epilepsy have drug-resistant seizures. After the first two medications fail, the probability of achieving seizure freedom with subsequent medication trials drops to approximately 5% per additional agent. Ongoing seizures carry significant risks including cognitive decline, psychosocial disability, physical injury, and a 2–3-fold increase in mortality—largely driven by sudden unexpected death in epilepsy (SUDEP).
Indications for Surgical Evaluation
The ILAE recommends referral for surgical evaluation for all patients with drug-resistant epilepsy with disabling seizures, as well as for patients with epileptogenic lesions in noneloquent cortex regardless of treatment response. Early surgery may be particularly beneficial in children (sparing neurodevelopmental effects of seizures and medications) and people of childbearing potential (avoiding teratogenic medications). Despite these recommendations, significant barriers to timely referral persist:
- Clinician barriers: Failure to appreciate drug resistance; lack of knowledge regarding surgical indications, benefits, and risks; underappreciation of risks of ongoing seizures
- Systemic barriers: Insurance coverage, cost, geographic distance from comprehensive epilepsy centers; racial and ethnic disparities (Black and Hispanic patients are less likely to undergo surgery)
- Patient barriers: Fear of brain surgery, insufficient education about surgical options, cultural factors
Predictive Tools for Surgical Candidacy
Several validated tools using clinical and testing features available at the time of referral can help neurologists determine the likelihood of seizure freedom with resective surgery and prioritize referrals:
| Tool | Variables | Scoring | Interpretation |
|---|---|---|---|
| Epilepsy Surgery Grading Scale | IQ, motor seizure features, MRI findings (normal, MTS, temporal/extratemporal lesion, multiple lesions), interictal EEG pattern, EEG-MRI concordance | Score range −1 to 15 | Grade I (8–10): >70% seizure freedom; Grade II (4–7): 40–60%; Grade III (<4): <40% |
| Seizure Freedom Score | Normal MRI (0/1), history of tonic-clonic seizures (0/1), >20 seizures/month (0/1), epilepsy duration >5 years (0/1) | Score 0–4 | 3–4: >70% seizure freedom; 2: 40–69%; ≤1: <40% |
| Epilepsy Surgery Evaluation Tool | Eight multiple-choice questions on seizure type, severity, frequency, duration, medications, MRI, and EEG | Online tool (epilepsycases.com) | Determines appropriateness and priority of referral |
Favorable Prognostic Factors for Surgical Success
- Identifiable structural lesion on MRI (especially mesial temporal sclerosis or low-grade tumor)
- Concordance between MRI lesion location, EEG seizure-onset zone, and semiology
- Normal intellect (IQ ≥70)
- Single seizure type with a consistent aura
- Temporal lobe epilepsy (better outcomes than extratemporal)
- Shorter epilepsy duration before surgery
- Absence of bilateral or generalized interictal epileptiform discharges
Phase I (Noninvasive) Evaluation
Video-EEG Monitoring
Inpatient video-EEG monitoring is the cornerstone of the presurgical evaluation. The goals are to confirm the diagnosis of epilepsy (excluding nonepileptic events), classify seizure type, identify the seizure-onset zone, and characterize seizure semiology. Key elements include:
- Interictal discharges: Lateralize and localize the irritative zone; unilateral temporal interictal epileptiform discharges concordant with MRI are strongly predictive of good surgical outcomes
- Ictal onset pattern: Regional seizure onset on scalp EEG is more favorable than diffuse or bilateral onset
- Seizure semiology: Provides lateralizing and localizing information (e.g., unilateral dystonic posturing lateralizes to the contralateral hemisphere; early unforced head version lateralizes to the contralateral hemisphere)
- Monitoring duration: Typically 5–14 days; antiseizure medications are often reduced to capture seizures; at least 3–5 habitual seizures should be recorded
MRI Epilepsy Protocol
A dedicated MRI epilepsy protocol using 3T (or increasingly 7T) MRI differs substantially from a routine brain MRI and is essential for identifying epileptogenic lesions. Key sequences include:
| Sequence | Key Pathology Detected | Technical Notes |
|---|---|---|
| 3D T1-weighted (1 mm isotropic) | Cortical thickness abnormalities, gray-white junction blurring (FCD), volumetric analysis | Enables morphometric analysis and volumetry; essential for MRI postprocessing |
| 3D FLAIR (1 mm isotropic) | Signal abnormalities in cortex and subcortical white matter; hippocampal sclerosis signal changes; FCD, gliosis | Coronal oblique perpendicular to hippocampal axis improves detection of MTS |
| Coronal T2 | Hippocampal sclerosis (atrophy + signal increase); hippocampal internal structure | Thin slices (2–3 mm) perpendicular to long axis of hippocampus |
| SWI/GRE | Cavernous malformations, calcifications, hemosiderin | Susceptibility-weighted imaging detects occult vascular malformations |
| 3D T2 (submillimetric) | Periventricular nodular heterotopia, polymicrogyria, small FCD | Complements FLAIR; superior for cortical surface morphology |
Advanced MRI techniques include 7T MRI (improves detection of subtle FCD and hippocampal subfield abnormalities), morphometric analysis (automated computational tools to detect FCD), and DTI tractography (maps white matter pathways to predict surgical deficits). Up to 30% of patients with drug-resistant focal epilepsy have a normal 3T MRI (MRI-negative epilepsy), and advanced postprocessing techniques may identify previously occult lesions in a subset of these patients.
FDG-PET
Interictal FDG-PET identifies areas of glucose hypometabolism corresponding to the epileptogenic zone. It is most useful in the following scenarios:
- Temporal lobe epilepsy: 80–90% sensitivity for detecting temporal hypometabolism in MTLE, even in MRI-negative cases
- Extratemporal epilepsy: Lower sensitivity (~45–60%) but can provide lateralizing information
- MRI-negative epilepsy: PET hypometabolism concordant with EEG can support a surgical plan
- Concordance value: PET hypometabolism concordant with the MRI lesion and EEG onset zone strengthens surgical candidacy
Ictal SPECT
Single-photon emission computed tomography (SPECT) captures ictal cerebral blood flow by injecting a radiotracer during a seizure. Subtraction ictal SPECT co-registered with MRI (SISCOM) increases localization accuracy. Key points:
- Requires rapid injection (within 30 seconds of seizure onset for optimal results)
- Sensitivity: 70–90% for temporal lobe seizures, lower for extratemporal
- Most useful when MRI is nonlesional or when other data are discordant
- Late injection (>45 seconds) may show seizure propagation patterns rather than onset zone, reducing specificity
MEG/MSI
Magnetoencephalography (MEG) detects the magnetic fields generated by interictal epileptiform discharges with superior spatial resolution compared to scalp EEG. Magnetic source imaging (MSI) integrates MEG dipole sources with MRI. MEG is particularly useful in:
- MRI-negative extratemporal epilepsy: Can localize the irritative zone when EEG lateralization is unclear
- Deep cortical sources: MEG detects tangential dipoles from sulcal cortex better than EEG
- Guiding intracranial electrode placement: MEG source clusters inform SEEG implantation strategy
- Added value: Concordance of MEG with other modalities increases confidence in the surgical plan
Neuropsychological Testing
Formal neuropsychological evaluation serves multiple purposes in the presurgical workup:
- Localizing and lateralizing: Patterns of cognitive deficits (e.g., verbal memory impairment with left temporal lobe epilepsy, visuospatial deficits with right temporal lobe epilepsy) support localization
- Baseline assessment: Establishes preoperative cognitive function for comparison with postoperative performance
- Risk assessment: Identifies patients at high risk for postoperative cognitive decline (e.g., strong verbal memory in the context of left temporal lobe surgery)
- Counseling: Helps set patient expectations regarding potential cognitive effects of surgery
Language and Memory Lateralization
Wada Test (Intracarotid Amobarbital Procedure)
The Wada test involves sequential injection of amobarbital (or etomidate/methohexital) into each internal carotid artery to transiently anesthetize one hemisphere while testing language and memory function in the contralateral hemisphere. The test determines:
- Language dominance: Critical for planning temporal lobe resections near language cortex
- Memory lateralization: Assesses the ability of the contralateral hippocampus to support memory if the ipsilateral hippocampus is resected
- Limitations: Invasive (catheter angiography), poor test-retest reliability, variable arterial perfusion territories, risk of stroke (<1%), seizures, and anaphylaxis
Functional MRI for Language Lateralization
fMRI has largely replaced the Wada test for language lateralization at most comprehensive epilepsy centers. Language paradigms (verb generation, semantic decision, sentence completion) reliably identify the dominant hemisphere with >90% concordance with Wada results. Key advantages and limitations:
| Feature | fMRI | Wada Test |
|---|---|---|
| Language lateralization | >90% concordance with Wada; preferred first-line test | Gold standard but invasive; reserved for discordant fMRI results |
| Memory lateralization | Hippocampal fMRI protocols exist but have lower reliability and are not yet validated as Wada replacement | Remains the standard for memory lateralization in temporal lobe epilepsy surgery |
| Invasiveness | Noninvasive; repeatable | Invasive (catheter angiography); associated with procedural risks |
| Patient cooperation | Requires task performance in the scanner; may be difficult for young children or cognitively impaired patients | Requires cooperation during hemisphere anesthesia; may be difficult to interpret if patient is confused |
| Availability | Widely available at comprehensive epilepsy centers | Declining use; expertise limited to select centers |
When Is the Wada Test Still Needed?
- fMRI shows atypical or bilateral language representation and resection near language cortex is planned
- Memory lateralization is needed before temporal lobe resection (fMRI memory protocols are not yet a reliable substitute)
- fMRI is nondiagnostic due to motion artifact, poor task performance, or technical limitations
- Concern for contralateral hippocampal insufficiency in patients with bilateral hippocampal abnormalities
Phase II (Invasive) Evaluation: Intracranial EEG
Indications for Intracranial EEG
Approximately 30–40% of patients undergoing presurgical evaluation require intracranial EEG (iEEG) monitoring to further delineate the epileptogenic zone and its relationship to eloquent cortex. Common indications include:
- Discordant or inconclusive Phase I data
- MRI-negative epilepsy with lateralized EEG findings
- Suspicion of seizure onset from or near eloquent cortex
- Bilateral independent temporal lobe seizure onsets on scalp EEG
- Multifocal interictal epileptiform discharges requiring localization of the primary seizure-onset zone
- Extratemporal epilepsy with broad or uncertain localization on noninvasive testing
SEEG vs. Subdural Grid Electrodes
| Feature | SEEG (Depth Electrodes) | Subdural Grids |
|---|---|---|
| Implantation | Stereotactic via small twist-drill holes; robot-assisted in many centers | Open craniotomy for grid/strip placement |
| Spatial coverage | Samples deep structures (hippocampus, insula, cingulate, deep sulci, periventricular); bilateral implantation feasible | Primarily cortical surface; limited access to deep structures; difficult to place bilaterally |
| Complication rate | Hemorrhage 1–4%; infection 1–2%; long-term deficits rare; overall rate ~25–50% of subdural electrodes | Higher rates of hemorrhage, infection, CSF leak, cerebral edema; overall complications 10–15% |
| Cortical mapping | Possible but limited by electrode geometry; cannot perform detailed cortical surface mapping | Excellent for detailed cortical surface mapping of motor, sensory, and language cortex |
| Seizure-free outcomes | Engel I at 1 year: ~58%; at 2 years: ~57% | Engel I at 1 year: ~46%; at 2 years: ~44% |
| Current trend | Dominant modality in North America and Europe since ~2015; facilitated by robotic-assisted implantation | Declining use; reserved for specific indications (e.g., neocortical surface mapping near eloquent cortex) |
SEEG Technique
Stereoelectroencephalography (SEEG) involves the stereotactic placement of multiple depth electrodes (typically 8–16 per patient) through small twist-drill holes in the skull. Each electrode contains 8–18 contacts sampling tissue along its trajectory from cortical surface to deep structures. Key technical aspects:
- Planning: Electrode trajectories are planned based on a hypothesis-driven approach, targeting the suspected epileptogenic network based on Phase I data
- Robot-assisted implantation: Robotic arms (ROSA, Neuromate) improve speed and accuracy of electrode placement; median target error <2 mm
- Bilateral sampling: SEEG readily permits bilateral implantation without a second craniotomy, which is particularly useful in suspected bitemporal epilepsy
- Monitoring duration: Typically 7–14 days; antiseizure medications reduced to capture seizures
- Thermocoagulation: SEEG contacts can deliver radiofrequency thermocoagulation to ablate small epileptogenic foci at the time of monitoring, providing a minimally invasive therapeutic option in selected cases
Risks of Intracranial EEG Monitoring
- SEEG: Intracerebral hemorrhage (1–4%); infection (1–2%); electrode malposition requiring repositioning; long-term neurologic deficits are rare (<1%)
- Subdural grids: Subdural/epidural hemorrhage (~4%); infection (~5%); CSF leak; cerebral edema from mass effect; need for repeat craniotomy for removal
- Both modalities: Risk of provoking seizures during monitoring with medication reduction; rare status epilepticus requiring emergent treatment; patient discomfort during prolonged monitoring
- Decision to proceed: If intracranial monitoring fails to localize a single seizure-onset zone or reveals overlap with eloquent cortex, resection may not be offered—palliative neuromodulation may be considered instead
Data Concordance and Surgical Decision-Making
The Concordance Model
The fundamental principle of presurgical evaluation is the convergence of multiple independent data sources toward a single epileptogenic zone hypothesis. No single test is sufficient to determine surgical candidacy. The degree of concordance among the following domains drives the surgical decision:
- Seizure semiology (lateralizing and localizing features)
- Scalp EEG (interictal discharges and ictal onset zone)
- MRI (structural lesion and its location)
- Neuropsychological profile (cognitive lateralization)
- FDG-PET (hypometabolism)
- Ictal SPECT (ictal hyperperfusion)
- MEG (epileptiform source localization)
When all modalities point to the same region, the probability of postoperative seizure freedom exceeds 70%. Discordance among modalities lowers expected outcomes and often prompts Phase II evaluation. The final surgical decision is made at a multidisciplinary epilepsy surgery conference where all data are reviewed by the team.
The Multidisciplinary Epilepsy Surgery Conference
The epilepsy surgery conference is the decision-making forum at comprehensive epilepsy centers. Typical attendees and their roles include:
| Team Member | Contribution |
|---|---|
| Epileptologist(s) | Seizure semiology, EEG interpretation, overall case synthesis and treatment recommendation |
| Epilepsy neurosurgeon | Surgical feasibility, operative approach, risk assessment, prior surgical experience with similar cases |
| Neuroradiologist | MRI interpretation, identification of subtle lesions, correlation with functional imaging |
| Neuropsychologist | Cognitive profile interpretation, lateralization/localization data, risk of postoperative cognitive decline |
| Nuclear medicine specialist | PET and SPECT interpretation |
| Psychiatrist/social worker | Psychosocial readiness, psychiatric comorbidities, postoperative support planning |
| Epilepsy nurse/coordinator | Patient education, coordination of testing, family communication |
Possible conference outcomes include: recommendation for resective or ablative surgery; recommendation for intracranial EEG monitoring (Phase II); recommendation for palliative surgery or neuromodulation; or determination that the patient is not a suitable surgical candidate at this time.
When Surgery Is Not Offered
- Discordant data that cannot be resolved with Phase II monitoring
- Multifocal or bilateral epilepsy without a dominant focus—consider neurostimulation (VNS, DBS, or RNS)
- Epileptogenic zone overlaps extensively with eloquent cortex—risk of unacceptable neurologic deficit; consider RNS for closed-loop stimulation
- Generalized or genetic epilepsy without a focal structural substrate—optimize medications, consider dietary therapy or VNS/DBS
- Patient preference after informed discussion of risks and benefits
- The decision is not final; patients can return for reevaluation, especially as imaging and surgical techniques evolve
Special Populations in Presurgical Evaluation
Pediatric Considerations
Children with drug-resistant epilepsy benefit from early surgical referral, as ongoing seizures can have devastating effects on neurodevelopment. Unique aspects of the pediatric presurgical evaluation include:
- Etiology spectrum: Focal cortical dysplasia, tuberous sclerosis complex, hemispheric malformations, and low-grade tumors are more common in the pediatric population than adults
- MRI interpretation: Myelination changes in young children may obscure lesions on standard MRI; serial imaging may be needed as myelination matures
- Video-EEG challenges: Young children may not cooperate with monitoring; prolonged sedation-free recordings are difficult; seizure semiology may be less localizing in children due to immature cortical networks
- Neuropsychological testing: Must be age-appropriate; developmental trajectory tracking may be more informative than a single assessment
- Brain plasticity: Younger children have greater capacity for functional reorganization, particularly language, which may reduce the risk of postoperative deficits after hemispherectomy or dominant-hemisphere surgery
- Urgency: In catastrophic epilepsies (infantile spasms, epileptic encephalopathies), very early surgery (even in the first year of life) may prevent irreversible developmental regression
MRI-Negative Epilepsy
Up to 30% of patients with drug-resistant focal epilepsy have no identifiable lesion on 3T MRI (MRI-negative epilepsy). These patients can still be surgical candidates, but the evaluation pathway is more complex and outcomes are generally less favorable:
- Advanced MRI: 7T MRI and computational postprocessing (morphometric analysis, FLAIR quantification) can reveal subtle FCD in up to 20–30% of previously MRI-negative cases
- Multimodal imaging: FDG-PET, ictal SPECT/SISCOM, and MEG provide complementary localizing information; concordance among multiple modalities can substitute for a visible lesion in supporting a surgical hypothesis
- Phase II is frequently required: Most MRI-negative patients require intracranial EEG (SEEG) before resective surgery can be planned
- Outcomes: Engel I rates of 30–45% for MRI-negative epilepsy (vs. 60–70% for lesional temporal lobe epilepsy); patients should be counseled about the lower probability of seizure freedom
- Alternative approaches: Neuromodulation (RNS, VNS, DBS) may be considered when resection is not feasible or has failed in MRI-negative cases
Pitfalls in the Presurgical Evaluation
- Dual pathology: The presence of two epileptogenic substrates (e.g., MTS + FCD, MTS + cavernoma) may not be apparent on initial evaluation; residual seizures after resection of one lesion should prompt reassessment for a second focus
- Falsely localizing EEG: Contralateral or diffuse EEG patterns may occur with focal epilepsy, particularly mesial frontal, insular, or orbitofrontal origin; do not rely solely on scalp EEG localization
- Incidental MRI findings: Not every MRI lesion is epileptogenic; a cavernoma or FCD may be incidental if the electroclinical data point elsewhere; concordance is essential
- Pseudolocalization of semiology: Some seizure signs are lateralizing but not localizing (e.g., automatisms lateralize to the ipsilateral hemisphere but can arise from temporal or frontal foci); misinterpretation can lead to incorrect surgical targeting
- Confirmation bias: The multidisciplinary team must guard against interpreting ambiguous data to fit a preexisting hypothesis; discordance should prompt Phase II evaluation, not be explained away
Benefits of Epilepsy Surgery
The evidence supporting epilepsy surgery is robust. A 2015 Cochrane systematic review found that 64% of more than 16,000 surgically treated patients achieved at least 1 year of freedom from disabling seizures. The pooled analysis of two randomized controlled trials comparing surgery with continued medical therapy found that 70% of surgical patients vs. 7% of medically managed patients were seizure-free at 1 year (relative risk reduction 9.8; 95% CI 4.8–20.2). Additional benefits include:
- Quality of life: About half of surgically treated patients experience clinically significant improvement in quality of life; this correlates with but is not entirely dependent on seizure freedom
- Mortality reduction: Surgery is associated with lower overall mortality and lower SUDEP incidence, with the greatest benefit in patients who become seizure-free
- Cost-effectiveness: Temporal lobe surgery becomes cost-effective after 4 years (3 years if indirect costs are included)
- Employment: A 2022 meta-analysis found a 20% gain in employment following epilepsy surgery
- Medication reduction: Successful surgery may allow reduction or discontinuation of antiseizure medications, eliminating side effects and teratogenic risks
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