Epilepsy Surgery

Anatomy and Physiology

To ensure maximal efficacy from epilepsy surgery, the disconnections of the following epileptic neural networks involved in the seizure semiology need to be performed:[2]

1. Resection of the cortico-thalamic tract 
2. Resection of the medial temporal structures
3. Total corpus callosotomy and
4. Disconnection of the orbito-fronto-hypothalamic tract

Indications

The main indication for the epilepsy surgery is the drug resistance as per the consensus from the Task Force of the International League Against Epilepsy (ILAE).[3]

The most common histological findings in specimens from epilepsy surgery have been hippocampal sclerosis in adults and focal cortical dysplasia in children.[4][5]

The absolute indications for performing hemispherectomy include[6]:

1. Sturge-Weber syndrome
2. Cortical dysplasia
3. Hemimegalencephaly
4. Rasmussen syndrome
5. porencephalic cyst and
6. Hemiconvulsion-hemiplegia-epilepsy syndrome

The resective procedure is indicated in patients with lesions amenable to surgical removal.

The epilepsy surgery is also indicated in patients with seizure semiology showing characteristic clinical localisations.[7]

The patterns of epilepsy surgery to be performed in the patients depend upon the multispectral governing factors such as the pathology, seizure semiology, and the clinical characteristics of the patients.[2]

Contraindications

The relative contraindications for the epilepsy surgery include:[7]

1. Concurrent severe  active psychiatric illness
2. Severe medical co-morbidities precluding surgery and
3. Failure in identifying the epileptic foci in the patient (except in patients with repeated drop attacks wherein corpus callosotomy is advocated)

Equipment

The fundamental work up in the patient constitutes of:[4]

1. High-resolution volumetric magnetic resonance imaging (MRI) with seizure protocol including the short tau inversion recovery (STIR), fluid-attenuated inversion recovery (FLAIR), and the susceptibility-weighted imaging (SWI) sequences
2. Video scalp electroencephalography (EEG) and
3. Thorough neuropsychological assessment.

If there is difficulty in the localization of the epileptogenic zone, phase 1 investigations need to be appropriately undertaken

1. Diffusion tensor imaging for assessing the epileptic networks
2. Interictal high-resolution EEG, magnetoencephalography (MEG) and the functional MRI (fMRI) for identifying ictal origins and
3. Interictal positron emission tomography (PET) or interictal PET showing areas of hypometabolism

The assessment of the post-operative neural deficits can be assessed through the  complementary phase 1 investigations constituting of 

1. fMRI, MEG, and Wada test for lateralization and localization of language functions and memory decline
2. Tractography of Meyer loop and visual field testing for visual field defects, and 
3. fMRI and tractography of the pyramidal tract fo assessing motor deficits.

Preparation

Multidisciplinary three-phase approach constituting:[8]

Phase 1:

1. Understanding seizure semiology 
2. Neurophysiology testing
3. MRI seizure protocol
4. Functional neuroimaging
5. Neuropsychological testing

Phase 2: Intracranial EEG monitoring if there is a concordance between the seizure semiology, relevant imaging studies or the neurophysiological testings

Phase 3: Patient focussed resective or disconnection surgery 

Technique or Treatment

Resective surgery is undertaken for patients with epileptogenic foci. The newer dimensions in the minimally invasive techniques have been achieved through the advancements in the stereotactic radiosurgery, MR-guided laser interstitial thermal therapy (MgLiTT), and subdural electroencephalogram (SEEG)-guided radiofrequency thermal coagulation.

In non-lesional epilepsy disorder, there has been a gradual transition from the previously performed major anatomic resective procedures (hemispherectomy) to the currently advocated disconnection procedures (hemispherotomy).[4]

In certain patients who are not a suitable candidate for epilepsy surgery, seizure reduction can be attainable through neuromodulatory techniques such as vagal nerve stimulation and deep brain stimulation.

Peri-Insular Hemispherotomy

The salient steps in the surgical procedures for a peri-insular hemispherotomy include:[6]

1. Suprasylvian block
2. Infrasylvian block constituting the resection of the frontal and the temporal neocortex exhibiting the feature of the ictal origins, as per the relevant preoperative and intraoperative investigations
3. Transection of the corona radiata
4. Anterolateral temporal lobectomy/disconnection of the temporal lobe via incising fornix at the splenium
5. Transventricular corpus callosotomy
6. Frontoparietal disconnection
7. Subpial insular resection

The details of the procedure include:[8]

Patient Positioning

Supine with head turned to the contralateral side, ensuring the temporal lobe remains at the apex and parallel to the floor. 

Osteoplastic Craniotomy

This ensures better cosmetic results through enhanced wound healing and preservation of the atrophy of the temporalis muscle.

Lateral Disconnection

A corticectomy in the middle frontal gyrus extending into the anterior horn of the lateral ventricle disconnects the fibers of the corona radiata and the internal capsule. Plugging of the foramen of Monro is advocated for minimizing the risk of hydrocephalus. The corticectomy at the supramarginal gyrus is then carried out along the atrium into the temporal horn. Subpial resection of the insula is carried after the frontotemporal operculectomy, which facilitates surgical access. The frontal and temporal opercula are resected to expose the insula.

Mesial Temporal Resection

The anterior temporal lobectomy is performed until the subpial visualization of the oculomotor nerve, and MCA bifurcation at the limen insulae is confirmed. The subpial resection ensures that the midbrain/brainstem is not inadvertently entered. Amygdalectomy is then undertaken till the junction of the inferior choroidal point and the MCA bifurcation, followed by hippocampectomy. It is of paramount importance to remain lateral to the choroid fissure throughout the procedure so as to prevent damage to the basal ganglion and the thalamus. 

Transventricular Corpus Callosotomy

Corpus callosotomy taking pericallosal arteries and the falx as the anatomic landmarks, aided with neuronavigation, is carried out posteriorly till the atrium and the tentorial edge are visualized. The anterior dissection is limited 5 mm anterior to the foramen of Monro to prevent hypothalamic injury.

Frontobasal and Parietooccipital Disconnections

This completes the disconnection of the neural network involved in the seizure semiology.

Closure

External ventricular drain (EVD) is placed. The water seal closure of the dura aided with a dural substitute is carried out. The osteoplastic bone flap is replaced and fixed with mini plates. The wound is closed in layers.

The surgical steps, while undertaking other epilepsy surgical procedures, can be summarised as:

Anterior Temporal Lobectomy and Amygdalohippocampectomy

This includes resection of the lateral and mesial temporal structures either separately or en bloc [9][10][9]

1. Pterional craniotomy and durotomy are performed in the standard fashion.
2. A point is marked in the middle temporal gyrus (5.5 cm from the temporal tip on the non-dominant hemisphere and 4.5 cm on the dominant side)  
3. Sylvian dissection exposing the insula and extending to the lateral uncus is performed.
4. The temporal pole is resected and extended to the collateral sulcus.
5. The temporal horn is entered, and the temporal stem along with the temporal neocortex is excised at the level of the inferior circular sulcus. 
6. Mesial temporal resection is performed with the aid of ultrasonic aspirator preserving the arachnoid plane.
7. Uncus paralleling the M1 segment of MCA till the limen insulae is performed. 
8. The amygdalectomy is performed at the intersection of the choroidal point and the limen insulae. 
9. The hippocampal sulcus is exposed, its feeder controlled, and the hippocampus and the parahippocampus are removed en bloc—the posterior limit of the resection in the midbrain tectum. 

Selective Amygdalohippocampectomy

This is aimed to preserve the temporal neocortex, can be performed via the subtemporal, transcortical, or via trans-Sylvian approaches.[10]

Multiple Subpial Transections

Indication- surgical management in cases of ictal origins within the eloquent cortex

Salient steps in the procedure include:[11]

1. A right-angled hook is inserted subcortically across the gyrus form one sulcus to the other.
2. The hook is then dragged in the subpial plane back to the entry point.
3. Such cuts, perpendicular to the gyrus, are repeated every 5 mm from one sulcus to the other.
4. This disrupts the intercortical neural connectome preserving the subcortical fibers.

Vagal Nerve Stimulation

Achieves up to 50% reduction in the seizure frequency in patients who are high-risk candidates for major surgical procedures. It includes placement of the helical electrodes on the left vagus nerve (right vagus was found to activate the efferents innervating the SA node in the heart during experimental studies) 8 cm above the clavicle, with intermittent stimulation (30 seconds of 30 Hz) from subcutaneous prosthesis placed in the chest.[12] The major seizure suppressive role is governed predominantly by the subdiaphragmatic vagal branch.

Complications

The major complications of the epilepsy surgery include:[6]

1. Superficial hemosiderosis (anatomic hemispherectomy)
2. Progressive hydrocephalus (anatomic hemispherectomy)
3. Postoperative infarction from injury to the Sylvian vein
4. Sinus thrombosis
5. Memory decline and anomic aphasia (temporal lobe resection)
6. Vasospasm and with contralateral superior quadrantanopia following the injury to Meyer's loop (amygdalohippocampectomy)
7. Disconnection syndrome (corpus callosotomy)
8. Hemiparesis and dysphagia (multiple subpial transections)
9. Cough and hoarseness of voice (vagal nerve stimulation)

Clinical Significance

The fundamental rationale of epilepsy surgery is the attainment of the seizure-free profile of the patients in the post-operative period. However, this is accomplished in only up to 80% of cases. The automated fiber quantification from the tractography study has identified the following variables contributing to the persistent occurrence of postoperative seizures.[13]

1. Diffusion abnormalities in the dorsal fornix 
2. Diffusion abnormalities in the contralateral parahippocampus and
3. Insufficient resection of the uncinate fasciculus.

Diffusion tensor imaging also ensures hemispheric disconnection as well as identify the residual connectome hindering seizure-free status.[6] MEG cluster confined to the resection area is the most important variable governing the prospects of seizure freedom in the patient.[6] The extratemporal genesis of epilepsy and ill-defined ictal origin connote unfavorable outcomes. Seizure recurrence within two years of surgery also is a poor clinical marker.[14] Rapid seizure spread due to neural spurting outside the zone of resection plays a significant role in postoperative seizure recurrence.[15]

Enhancing Healthcare Team Outcomes

Surgical pearls in preventing complications:[8]

1. Resecting insular block provides a window for subsequent disconnection procedures
2. The most common reasons for seizure persistence include incomplete corpus callosotomy, frontoparietal disconnection, and incomplete insular resection. 
3. The early placement of cottonoid at the foramen of Monro minimizes, along with the prophylactic EVD placement, minimizes the risk of hydrocephalus.  
4. The osteoplastic bone flap ensures better cosmetic results with reduced risk of the surgical site infection. 

Surgical and functional outcomes:[8]

Functional hemispherectomy/hemispherotomy provides equivocal seizure control (80% seizure-free Engel class 1 at two years) in comparison to that of the anatomical hemispherectomy. The pediatric population also shows significant neurocognitive and behavioral improvement following epilepsy surgery.

Motor deficits also gradually improve, with 84% of patients eventually being able to walk independently or with minimal assistance.

Nursing, Allied Health, and Interprofessional Team Interventions

The main reasons for the delayed or non-referral of suitable candidates for the epilepsy surgery include:[16][17]

1. Unknown bias against the surgical mode of management 
2. Unawareness of proper dichotomization of patients
3. Hesitancy for brain surgery by overestimating the operative risks

Nursing, Allied Health, and Interprofessional Team Monitoring

Early referral for epilepsy surgery is the key factor determining the outcome of the surgery. 

A multidisciplinary approach in a high-volume epilepsy center needs to be promoted for improved patient outcomes.[8]

A significant proportion of patients, though not seizure-free, will at least have improved quality of life owing to the substantial reduction in their seizure frequency.[18]

Since epilepsy surgery has shown to be cost-effective, even when the probability of being a suitable candidate is as low as 5%, harmonization and dissemination of the role of epilepsy surgery is the need of the hour.