Epilepsy surgery is indicated in 30% to 40% of patients with refractory seizure disorders despite being on one year of therapy with an adequate dosage of two antiepileptic therapy.[1] Though the anterior temporal lobectomy has been the prototype epilepsy surgery, there has been a paradigm shift in the surgical principle of ensuring maximum disconnection of the epileptic neural pathways, in sharp contrast to the previous concepts of governing maximal surgical resection.
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]
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]:
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]
The relative contraindications for the epilepsy surgery include:[7]
The fundamental work up in the patient constitutes of:[4]
If there is difficulty in the localization of the epileptogenic zone, phase 1 investigations need to be appropriately undertaken
The assessment of the post-operative neural deficits can be assessed through the complementary phase 1 investigations constituting of
Multidisciplinary three-phase approach constituting:[8]
Phase 1:
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
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]
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]
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]
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.
The major complications of the epilepsy surgery include:[6]
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]
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]
Surgical pearls in preventing complications:[8]
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.
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.
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[16] | de Flon P,Kumlien E,Reuterwall C,Mattsson P, Empirical evidence of underutilization of referrals for epilepsy surgery evaluation. European journal of neurology. 2010 Apr; [PubMed PMID: 20039934] |
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