Presurgical Evaluation

What You'll Learn

Drug-resistant epilepsy = failure of 2 appropriate ASMs; probability of seizure freedom drops to ~5% per subsequent agent — refer early
Phase I (noninvasive): video-EEG, 3T MRI epilepsy protocol, FDG-PET, ictal SPECT/SISCOM, MEG/MSI, neuropsychological testing
Phase II (invasive): SEEG has become the predominant intracranial modality in North America for most indications; subdural grids retain a role when high-density cortical functional mapping is the primary goal; Phase II required in 30–40% of surgical candidates
Language lateralization: fMRI has largely replaced Wada (>90% concordance); Wada still needed for memory lateralization
Concordance model: all modalities concordant → best outcomes; in mTLE/HS with full concordance Engel I reaches 70–80%; extratemporal concordant cases lower (typically 40–60% Engel I); discordance → poorer outcomes or need for Phase II
MRI-negative epilepsy: 20–40% of drug-resistant focal epilepsy (depending on protocol/field strength); 30–45% Engel I (vs. 60–70% for lesional TLE); PET, MEG, SEEG become critical
Best prognostic factors: identifiable MRI lesion, mTLE with HS, concordance across modalities, shorter epilepsy duration

HighYield Pearls

Drug-resistant epilepsy = 2 failed ASMs: Refer EARLY — each subsequent ASM adds only ~5% chance of seizure freedom; do NOT wait for 5+ failures.
Concordance is everything: If video-EEG + MRI + PET + MEG all converge on one zone → resect; if discordant or non-lesional → Phase II SEEG.
Ictal SPECT timing: Tc-99m HMPAO must be injected within seconds of seizure onset (<20 sec ideal; <10–20 sec for extratemporal) — late injection captures propagation, not onset.
SISCOM = subtraction ictal SPECT co-registered with MRI: mismatch between ictal hyperperfusion and interictal hypoperfusion identifies seizure onset zone — gold standard for SPECT analysis.
fMRI replaces Wada for language in most centers; Wada (intracarotid amobarbital) still used selectively for memory lateralization and postoperative deficit risk assessment.
Neuropsych red flags for postop verbal memory decline: intact preoperative verbal memory + planned dominant temporal lobectomy = highest risk; bilateral hippocampal involvement and near-normal baseline also high risk.
SEEG vs subdural grids: SEEG = depth electrodes, deep/multilobar sampling, lower complication rate; subdural grids = high-density cortical mapping including ECS for eloquent cortex.
Engel I outcomes by substrate: MTLE-HS with ATL = 60–70%; FCD with concordant data = 40–60%; non-lesional = 25–40%.
RNS (responsive neurostimulation): bilateral mesial temporal foci, eloquent cortex pathology, or multifocal disease where resection is inadvisable.
NEVER resect eloquent cortex (motor, speech, memory) without ECS mapping — awake craniotomy with cortical stimulation preserves function.

🔍 Quick ReferencePhase I modalities · Localization techniques · Phase II / pitfalls

Phase I (non-invasive) modalities

Video-EEG monitoring → capture ≥3 habitual seizures; gold standard for seizure-onset zone localization
3T MRI epilepsy protocol (HARNESS-MRI) → thin-slice coronal hippocampi, 3D FLAIR, 3D T1 IR for FCD detection
FDG-PET → interictal hypometabolism in epileptogenic zone (80–90% sensitivity in mTLE)
Ictal SPECT (Tc-99m HMPAO) → injected within seconds of seizure onset → SISCOM co-registered with MRI
MEG / magnetic source imaging → interictal magnetic dipole source localization; detects tangential sulcal dipoles missed by EEG
fMRI language/motor mapping → has replaced Wada in most centers (>90% concordance)

Localization concepts

Concordance model → all modalities converge on same zone → resect; discordance → Phase II or poorer outcome
SISCOM mismatch → ictal hyperperfusion + interictal hypoperfusion identifies seizure onset zone
Wada test (intracarotid amobarbital) → transient hemispheric anesthesia tests contralateral memory + language
Verbal memory deficit on neuropsych → language-dominant (usually left) temporal lobe pathology
Visuospatial memory deficit → non-dominant (usually right) temporal lobe pathology
Engel classification → I = seizure-free; II = rare disabling (<3/yr); III = worthwhile improvement; IV = no benefit

Phase II / pitfalls

SEEG (stereo-EEG) → depth electrodes for deep/multilobar sampling; lower complication rate vs subdural grids
Subdural grids → high-density cortical mapping including ECS for eloquent cortex preservation
Electrical cortical stimulation (ECS) during awake craniotomy → language/motor cortex mapping before resection
RNS (responsive neurostimulation) → bilateral mesial temporal, eloquent cortex, or multifocal foci where resection inadvisable
Late SPECT injection → captures propagation, NOT seizure onset — misleading localization
Resection of eloquent cortex without ECS mapping → postoperative motor/language deficit — AVOID

When to Refer for Epilepsy Surgery

Drug-resistant epilepsy (ILAE definition): failure to achieve seizure freedom after adequate trials of 2 tolerated, appropriately chosen ASMs
Probability of seizure freedom drops to ~5% with each subsequent ASM after 2 failures
Earlier referral = better outcomes: average delay from drug resistance to surgery is 10–20 years in many series
ERSET (Engel JAMA 2012): small RCT (n=38, terminated early); 73% of surgical arm vs. 0% medical arm seizure-free during year 2
ILAE recommends referral as soon as drug resistance is identified, regardless of epilepsy type
Comprehensive epilepsy center care reduces premature mortality — even in patients who do not undergo surgery

💎 Board Pearl

Drug-resistant epilepsy = failure of 2 ASMs. After 2 failures, each additional ASM adds only ~5% chance of seizure freedom. Do NOT wait for 5+ ASM failures before referring. Boards test this threshold repeatedly.

Phase I (Noninvasive) Evaluation

Modality	What It Shows	Sensitivity / Specificity	Key Points
Video-EEG monitoring	Capture habitual seizures; identify seizure-onset zone; classify semiology	Gold standard for seizure localization	Typically 5–14 days; capture ≥3–5 habitual seizures; ASMs often reduced; interictal IEDs lateralize irritative zone
3T MRI epilepsy protocol	Structural lesion identification (HS, FCD, tumors, vascular malformations)	1.5T misses ~20% of lesions detected at 3T	ILAE HARNESS-MRI protocol (Bernasconi et al., Epilepsia 2019); key sequences: 3D T1 (1 mm), 3D FLAIR, coronal T2 perpendicular to hippocampus, SWI; NOT a “routine brain MRI”
FDG-PET	Interictal hypometabolism in epileptogenic zone	80–90% sensitivity for mTLE; 45–60% extratemporal	More sensitive than MRI for some subtle lesions; concordance with EEG strengthens surgical candidacy
Ictal SPECT (SISCOM)	Ictal hyperperfusion at seizure-onset zone	70–90% for TLE; lower for extratemporal	Inject as early as possible — optimal <20 sec; <45 sec for TLE; for extratemporal/frontal seizures the window is much narrower (<10–20 sec) because of rapid propagation; SISCOM = subtraction ictal SPECT coregistered to MRI
MEG / MSI	Magnetic source imaging of interictal epileptiform discharges	Complementary to EEG; better for neocortical foci	Most useful in MRI-negative cases; detects tangential dipoles (sulcal cortex) better than EEG; guides SEEG placement
Neuropsychological testing	Baseline cognitive function; lateralization of language/memory	Supports lateralization; predicts postop deficits	Verbal memory deficit = language-dominant (usually left) temporal lobe; visuospatial memory deficit = non-dominant (usually right) temporal lobe; establishes preoperative baseline

MRI Epilepsy Protocol — Key Sequences

ILAE HARNESS-MRI protocol (Bernasconi et al., Epilepsia 2019): minimum core sequences = 3D T1 millimetric, 3D FLAIR, high-resolution 2D coronal T2 perpendicular to hippocampal long axis.

3D T1 (1 mm isotropic): cortical thickness, gray-white junction blurring (FCD), volumetric analysis
3D FLAIR (1 mm isotropic): hippocampal signal abnormalities, FCD, gliosis
Coronal T2 (2–3 mm): perpendicular to long axis of hippocampus — essential for HS detection
SWI/GRE: cavernous malformations, calcifications, hemosiderin deposits

PET vs. SPECT — Key Distinctions

FDG-PET: interictal study; shows hypometabolism; hypometabolism extends beyond epileptogenic zone (localizing but not precise)
Ictal SPECT: inject as early as possible — optimal <20 sec; <45 sec acceptable for TLE; for extratemporal/frontal seizures the window narrows to <10–20 sec due to rapid propagation; late injection = propagation, NOT onset; SISCOM increases accuracy

💎 Board Pearl

FDG-PET = interictal hypometabolism. Ictal SPECT = ictal hyperperfusion. These are OPPOSITE findings, both localizing to the epileptogenic zone. SPECT should be injected as early as possible — optimal <20 sec; <45 sec acceptable for TLE; <10–20 sec for extratemporal/frontal seizures due to rapid propagation. Late injection shows propagation, not origin.

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