# Surgical Treatment of Epilepsy

## Introduction

Epilepsy surgery is a critical treatment option for patients with drug-resistant epilepsy, defined as failure of two or more appropriately chosen and tolerated antiseizure medications. Approximately 30 percent of epilepsy patients develop drug-resistant epilepsy, and surgical intervention offers the potential for seizure freedom when the epileptogenic zone can be identified and safely resected.

## Presurgical Evaluation

### Non-Invasive Phase

The presurgical workup begins with video-EEG monitoring, involving prolonged recording to capture habitual seizures and localize ictal onset. MRI of the brain with an epilepsy protocol uses high-resolution T1, T2, and FLAIR sequences to identify structural lesions such as hippocampal sclerosis, focal cortical dysplasia, or tumors. PET scanning demonstrates interictal hypometabolism that helps lateralize temporal lobe epilepsy. Neuropsychological testing establishes baseline cognitive function and helps lateralize language and memory. Ictal SPECT, performed by injecting radiotracer during a seizure, demonstrates hyperperfusion at the seizure focus. Magnetoencephalography localizes interictal epileptiform discharges with high spatial resolution.

### Invasive Monitoring

When non-invasive data are discordant or insufficient, invasive monitoring is performed. Subdural grid and strip electrodes provide cortical surface recording for precise mapping of the seizure onset zone and functional cortex. Stereoelectroencephalography uses depth electrodes placed via stereotactic guidance for three-dimensional localization of seizure networks. Indications for invasive monitoring include discordant non-invasive data, bilateral or extratemporal seizure onset, and proximity of the suspected focus to eloquent cortex.

## Surgical Procedures

### Resective Surgery

Anterior temporal lobectomy is the gold standard procedure for mesial temporal sclerosis and includes removal of the amygdala and hippocampus (amygdalohippocampectomy). Selective amygdalohippocampectomy achieves the same mesial resection while preserving lateral temporal neocortex. Lesionectomy involves resection of a focal cortical dysplasia, cavernoma, or tumor-associated epilepsy focus. Tailored cortical resection is guided by intraoperative electrocorticography to define resection margins based on real-time electrophysiology.

### Disconnection Surgery

Corpus callosotomy is a palliative procedure primarily indicated for drop attacks from atonic or tonic seizures, reducing seizure generalization by interrupting interhemispheric spread. Functional hemispherectomy is performed for hemispheric syndromes including Rasmussen encephalitis, Sturge-Weber syndrome, and hemimegalencephaly, disconnecting the entire affected hemisphere. Multiple subpial transections disrupt horizontal fiber spread while preserving vertical cortical columns, used when the epileptogenic zone overlaps eloquent cortex that cannot be resected.

### Neuromodulation and Ablation

Vagus nerve stimulation is a palliative device implanted for patients who are not candidates for resective surgery, providing modest seizure reduction. Responsive neurostimulation is a closed-loop system that detects seizure activity and delivers stimulation directly at the seizure focus. Deep brain stimulation of the anterior nucleus of the thalamus reduces seizure frequency in drug-resistant focal epilepsy. Laser interstitial thermal therapy provides MRI-guided ablation of mesial temporal structures or other focal lesions through a minimally invasive stereotactic approach.

## Outcomes and Classification

### Engel Classification

Surgical outcomes are graded using the Engel classification. Class I indicates freedom from disabling seizures. Class II represents rare disabling seizures. Class III indicates worthwhile improvement. Class IV means no worthwhile improvement.

| Engel Class | Outcome | Definition |
|-------------|---------|------------|
| I | Seizure-free | Free from disabling seizures |
| II | Rare seizures | Rare disabling seizures |
| III | Worthwhile improvement | Meaningful reduction in seizure burden |
| IV | No improvement | No worthwhile improvement |

### Expected Outcomes

Anterior temporal lobectomy for mesial temporal sclerosis achieves Engel Class I outcomes in 60 to 80 percent of patients at two years. Lesionectomy with a concordant MRI lesion produces seizure freedom in 60 to 70 percent. Extratemporal resections achieve seizure freedom in 40 to 60 percent. Hemispherectomy in appropriately selected candidates yields seizure freedom in 70 to 85 percent.

| Procedure | Engel Class I Rate |
|-----------|--------------------|
| ATL for mesial temporal sclerosis | 60-80% |
| Lesionectomy (concordant MRI lesion) | 60-70% |
| Extratemporal resection | 40-60% |
| Hemispherectomy (selected candidates) | 70-85% |

## Complications

Neurological deficits include visual field cuts, specifically superior quadrantanopia with anterior temporal lobectomy from disruption of Meyer's loop, hemiparesis, and language deficits. Memory decline is a risk with dominant temporal resection; the Wada test or functional MRI is used preoperatively to lateralize language and memory and assess the risk of postoperative decline. Infection and hemorrhage represent standard surgical risks at 1 to 3 percent. Hydrocephalus occurs particularly after hemispherectomy.

## Clinical Pearls

Early referral to a comprehensive epilepsy center is critical; the average delay from epilepsy diagnosis to surgery remains over 20 years despite Level I evidence supporting surgical intervention. MRI-negative epilepsy can still be surgical, but outcomes are inferior to lesional cases and require thorough invasive monitoring for adequate localization. The randomized ERSET trial demonstrated the superiority of surgery over continued medical therapy for temporal lobe epilepsy, providing Class I evidence that surgery should be offered rather than withheld. Stereoelectroencephalography has largely replaced subdural grids in many centers due to lower morbidity and the ability to sample deep structures including the insula, cingulate, and mesial temporal regions.

## References
1. 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.
2. Engel J Jr, McDermott MP, Wiebe S, et al. Early surgical therapy for drug-resistant temporal lobe epilepsy: a randomized trial (ERSET). JAMA. 2012;307(9):922-930.
3. Jobst BC, Cascino GD. Resective epilepsy surgery for drug-resistant focal epilepsy: a review. JAMA. 2015;313(3):285-293.
4. Gonzalez-Martinez J, Bulacio J, Alexopoulos A, et al. Stereoelectroencephalography in the "difficult to localize" refractory focal epilepsy. Epilepsia. 2014;55(8):1199-1209.