For patients with drug-resistant focal epilepsy (failure of two appropriately chosen anti-seizure medications), resective surgery offers the prospect of seizure freedom in 50–70% with the right anatomic substrate. The presurgical evaluation is a multi-stage process that combines clinical semiology, scalp EEG, MRI, functional imaging, neuropsychological testing, and — in many cases — intracranial EEG to identify the epileptogenic zone and assess the risk of postoperative deficits. This page covers the staged evaluation, the role of intracranial monitoring (subdural grids and stereo-EEG), the Wada test, functional mapping, and the criteria for surgical candidacy.

When to Consider Surgical Evaluation

  • Drug-resistant epilepsy: failure of two appropriate ASMs, each at therapeutic dose, used singly or in combination.
  • ~30% of focal epilepsy patients meet this criterion.
  • Should occur early — within 2 years of resistance, ideally — not after a decade of medication trials.
  • Surgical candidacy assessed at a specialized epilepsy center.

Phase I (Non-Invasive) Evaluation

Detailed Clinical History

  • Seizure semiology (auras, ictal behavior, post-ictal features).
  • Frequency and pattern.
  • Risk factors: febrile seizures, head injury, encephalitis, family history.
  • Previous treatments and response.

Video EEG Monitoring (vEEG)

  • Days to weeks at epilepsy monitoring unit (EMU).
  • Captures multiple seizures with video for semiology analysis.
  • Goal: capture 3+ seizures with consistent ictal onset zone.
  • ASMs tapered during admission to facilitate seizure capture.
  • Identifies ictal onset zone (IOZ), early propagation, semiology.

MRI Brain

  • 3T epilepsy protocol with thin coronal cuts through temporal lobes.
  • Sensitive to: mesial temporal sclerosis (hippocampal atrophy + T2 hyperintensity), focal cortical dysplasia, tumors, cavernomas, dual pathology.
  • Concordant MRI + EEG findings dramatically improve surgical outcomes.
  • “Non-lesional” cases (normal MRI) have lower surgical success rates and often require intracranial evaluation.

Neuropsychological Testing

  • Cognitive baseline.
  • Detects lateralization (e.g., verbal memory deficit suggests left temporal).
  • Predicts post-surgical cognitive outcome.
  • Essential before considering dominant-hemisphere resection.

Functional Imaging

Interictal FDG-PET

  • Hypometabolism in epileptogenic zone (interictal).
  • Sensitive for temporal lobe epilepsy.
  • Concordant with EEG and MRI strengthens lateralization.

Ictal SPECT

  • Radiotracer injected during a captured seizure.
  • Hyperperfusion localizes ictal onset zone.
  • SISCOM (subtraction ictal-interictal co-registered to MRI): improves localization.
  • Particularly useful for non-lesional or non-temporal cases.

MEG (Magnetoencephalography)

  • Records magnetic fields generated by cortical activity.
  • Excellent source localization (1–2 cm accuracy).
  • Useful for non-lesional cases or when scalp EEG is ambiguous.
  • Not universally available.

fMRI (Functional MRI)

  • BOLD response during specific tasks.
  • Functional language and motor mapping.
  • Increasingly replaces invasive Wada in many centers.
  • Memory lateralization more challenging than language.

Phase II (Invasive) Evaluation

Required when Phase I is non-localizing, discordant, or near eloquent cortex.

Subdural Electrode Grids

  • Surface arrays placed via craniotomy.
  • Cover broad cortical areas.
  • Limited depth coverage.
  • Allow extraoperative functional mapping (language, motor).
  • Used when convexity cortex involvement suspected.

Stereo-EEG (sEEG)

  • Multiple depth electrodes placed via small drill holes using stereotactic guidance.
  • Sample deep structures (mesial temporal, insula, cingulate, deep frontal, parietal opercular).
  • Less morbidity than craniotomy.
  • Becoming dominant approach in many centers.
  • Allows 3D sampling of multiple potential ictal onset zones.

Recording Goals During Intracranial Monitoring

  • Capture multiple habitual seizures with localized ictal onset.
  • Map seizure propagation pathways.
  • Identify interictal high-frequency oscillations (HFOs) — research/emerging clinical use.
  • Perform functional mapping of eloquent cortex.

Cortical Stimulation

  • Bipolar stimulation between adjacent electrodes.
  • Map functional cortex:
    • Motor: stimulation produces movement.
    • Sensory: stimulation produces tingling.
    • Language: stimulation during task disrupts naming, reading, or speech.
  • Identifies cortex to spare during resection.

Wada Test (Intracarotid Amobarbital Procedure)

  • Amobarbital injected into one carotid artery temporarily anesthetizes that hemisphere.
  • Tests language and memory in each hemisphere separately.
  • Determines language dominance and memory adequacy of the contralateral hemisphere.
  • Largely replaced by fMRI for language lateralization.
  • Still used for memory assessment in some centers.
  • Risk: stroke, vasospasm; requires invasive angiography.

Surgical Procedures

Anterior Temporal Lobectomy (ATL)

  • Standard procedure for mesial temporal lobe epilepsy with hippocampal sclerosis.
  • Resect anterior 2.5–4 cm of temporal lobe, including hippocampus.
  • Seizure freedom at 5 years: ~50–70%.
  • Better outcomes for lesional cases.

Selective Amygdalohippocampectomy

  • Spares lateral temporal cortex.
  • Lower cognitive impact in dominant hemisphere.
  • Slightly lower seizure freedom rates than ATL in some series.

Lesionectomy

  • Removal of structural lesion (tumor, cavernoma, dysplasia).
  • Often combined with adjacent epileptogenic cortex.

Multilobar Resection

  • For extensive epileptogenic zones.
  • Higher risk of deficits.

Disconnection Procedures

  • Corpus callosotomy: for drop attacks in LGS; reduces but doesn’t eliminate seizures.
  • Hemispherectomy: for unilateral hemispheric pathology with already-established deficit.

Laser Interstitial Thermal Therapy (LITT)

  • Minimally invasive thermal ablation of epileptogenic zone.
  • Stereotactic placement of laser probe.
  • For mesial temporal sclerosis, periventricular nodular heterotopia, hypothalamic hamartomas.
  • Less morbidity; potentially fewer cognitive deficits than open resection.

Responsive Neurostimulation (RNS)

  • Closed-loop intracranial device.
  • Detects ictal onset and delivers electrical stimulation.
  • For patients not candidates for resection (bilateral, eloquent cortex).
  • Modest seizure reduction; long-term improvement.

Deep Brain Stimulation (DBS)

  • Anterior thalamic nucleus DBS approved for refractory focal epilepsy.
  • Modest seizure reduction.
  • Alternative when resection not feasible.

Vagus Nerve Stimulation (VNS)

  • Cervical vagus nerve stimulation.
  • Modest seizure reduction (~50% in 30–50% of patients).
  • For patients not candidates for resection or as adjunct.

Outcomes and Predictors

Engel Classification (Outcome)

  • Class I: free of disabling seizures.
  • Class II: rare disabling seizures.
  • Class III: worthwhile improvement.
  • Class IV: no worthwhile improvement.

Predictors of Good Outcome

  • Concordant MRI lesion + EEG localization.
  • Mesial temporal sclerosis on MRI.
  • Concordant ictal onset on intracranial EEG with anatomic lesion.
  • Shorter duration of epilepsy before surgery.
  • Concordant neuropsychological lateralization.

Predictors of Poor Outcome

  • Non-lesional MRI.
  • Discordant or multifocal findings.
  • Bilateral mesial temporal sclerosis.
  • Extra-temporal onset (frontal, parietal, occipital lower success than temporal).
  • Long duration of refractory epilepsy.

Considerations for Non-Lesional Cases

  • Normal MRI is the most common reason for surgical candidate exclusion.
  • Advanced techniques may identify subtle lesions:
    • Specialized MRI (3D FLAIR, advanced post-processing).
    • Combined PET-MRI co-registration.
    • SISCOM.
    • MEG source localization.
    • High-density EEG with source modeling.
  • Even with these, surgical success rates for non-lesional cases are 30–50% (vs 50–70% for lesional).

🔍 Did You Know?

Despite robust evidence that resective surgery for mesial temporal lobe epilepsy produces seizure freedom in 50–70% of well-selected patients, the average time from epilepsy diagnosis to surgical evaluation in the United States is approximately 20 years. This “treatment gap” represents one of the most consequential delays in modern neurology. The reasons are multifactorial: clinical inertia (“just one more medication”), patient anxiety about brain surgery, lack of awareness about evolving outcomes, geographic and insurance barriers to specialized epilepsy centers, and the slow accumulation of refractory cases in general neurology practices. The clinical consequence is profound: each year of refractory epilepsy adds cognitive decline, increased mortality risk (SUDEP — sudden unexpected death in epilepsy), social impairment, and reduced quality of life, all of which surgery could potentially prevent. The 2017 American Academy of Neurology guideline explicitly recommends that any patient meeting criteria for drug-resistant epilepsy (failure of two adequate ASM trials) should be referred to a specialized epilepsy center for surgical evaluation. For practicing neurologists, the practical implication is clear: do not wait through years of failed medication trials; refer for evaluation early, even if surgery is ultimately not pursued. The evaluation itself often clarifies diagnosis (some patients turn out to have non-epileptic events), and the conversation about surgery can be more thoughtful when there’s time to explore options.

Pitfalls and Pearls

  • Drug-resistant epilepsy: failure of two adequate ASM trials; ~30% of focal epilepsy.
  • Refer early: surgical evaluation within 2 years of resistance, not after decades.
  • Phase I non-invasive: vEEG, MRI, neuropsych, PET, SPECT, MEG, fMRI.
  • Phase II invasive: subdural grids or stereo-EEG when Phase I non-localizing.
  • Stereo-EEG: increasingly preferred for deep structures and 3D sampling.
  • Wada: largely replaced by fMRI for language; still used for memory.
  • Cortical stimulation: maps eloquent cortex; spares during resection.
  • ATL: 50–70% seizure freedom at 5 years for mesial temporal sclerosis.
  • SAH (selective amygdalohippocampectomy): spares lateral temporal; lower cognitive impact in dominant.
  • LITT: minimally invasive thermal ablation; emerging alternative.
  • Hemispherectomy: unilateral hemispheric pathology with established deficit.
  • RNS: responsive neurostimulation for non-resectable cases.
  • Anterior thalamic DBS: FDA-approved for refractory focal epilepsy.
  • VNS: modest reduction; adjunctive.
  • Predictors of good outcome: concordant MRI + EEG + neuropsych.
  • Non-lesional MRI: lower success rates; advanced techniques may help.
  • Engel Class I: seizure free (goal).
  • 20-year average delay to surgery: too long; refer early.

References

  1. Engel J Jr, McDermott MP, Wiebe S, et al. Early surgical therapy for drug-resistant temporal lobe epilepsy: a randomized trial. JAMA. 2012;307(9):922-930.
  2. Wiebe S, Blume WT, Girvin JP, Eliasziw M. A randomized, controlled trial of surgery for temporal-lobe epilepsy. N Engl J Med. 2001;345(5):311-318.
  3. Jobst BC, Cascino GD. Resective epilepsy surgery for drug-resistant focal epilepsy: a review. JAMA. 2015;313(3):285-293.
  4. Kwan P, Arzimanoglou A, Berg AT, et al. Definition of drug resistant epilepsy: consensus proposal by the ad hoc Task Force of the ILAE Commission on Therapeutic Strategies. Epilepsia. 2010;51(6):1069-1077.
  5. So NK, Lüders HO. Practical algorithm for surgical evaluation of patients with refractory focal epilepsy. Epilepsy Behav. 2002;3(5S):S2-S15.
  6. Iida K, Otsubo H. Stereoelectroencephalography: indication and efficacy. Neurol Med Chir (Tokyo). 2017;57(8):375-385.