Surgical treatment of epilepsy - Initial experience from a comprehensive epilepsy program in coastal South India

When should epilepsy surgery be considered?

Delaying surgery until a patient’s lifestyle has become established could result in the surgery having little impact on the individual’s quality of life. Therefore, surgery should be considered early in suitable patients to effectively eliminate seizures.^[2,3,8-10] In addition, in certain surgically remediable epilepsy syndromes, surgery needs to be considered early even without waiting unduly to establish drug resistance. We have adopted a policy of early surgery in appropriate patients, and accordingly, the mean age in our series was 22 years and the median epilepsy duration before surgery was 7 years.

Defining refractoriness to medications

We follow a simple “rule of two” to define refractoriness – “If after 2 years of appropriate ASMs (closely monitored trials of ASM including two separate trials with a single medication and one trial with a combination of two medications), the patient continues to have more than two seizures every month, we recruit the patient for consideration for surgery.^[6,11] Before categorizing a patient as having DRE, pseudo-refractoriness due to suboptimal dosage of ASMs, diagnostic errors, inappropriate choice of ASMs, poor compliance to drugs, and unhealthy lifestyles (such as erratic sleep, overindulgence in alcohol and recreational drugs) should be ruled out.^[6,11,12]

Surgically remediable epilepsy syndromes

Surgically remediable epilepsy syndromes refer to epilepsy associated with cortical dysplasia, mesial temporal sclerosis, benign tumors (eg: ganglioglioma, Dysembryoplastic neuroepithelial tumour (DNET)), vascular malformations, and specific paediatric epilepsy conditions such as hemimegaloencephaly and Rasmussen’s encephalitis [Figure 1].^[11,13] As has been described in the literature, the most prevalent surgically treatable lesional adult epilepsy syndrome in our series was mesial temporal lobe epilepsy secondary to mesial temporal lobe sclerosis. However, in childhood epilepsy cases, focal cortical dysplasia is more dominant.^[3,8,13]

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Figure 1:

Magnetic resonance imaging images of selected patients with surgically remediable epilepsy syndromes. (a) Right temporal mesial temporal sclerosis (Patient 8); (b) Right temporal gangliogliomas (Patient 14); (c) Right temporal uncal low-grade glioma (Patient 14); (d) Right temporal cavernoma (Patient 29); (e) Right temporal dysembryoplastic neuroepithelial tumor (Patient 20); (f) Rasmussen’s encephalitis (Patient 5).

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Contraindications for surgery

Epilepsy surgery is contraindicated in primary generalized epilepsy, minor seizures that do not significantly impact the patient’s quality of life, progressive medical or neurological disorders, active psychosis, behavioral problems that interfere with rehabilitation, and poor contralateral memory function.^[11,13]

Presurgical evaluation

Presurgical evaluation requires a multidisciplinary approach comprising neurologists, neurosurgeons, radiologists, psychologists, psychiatrists, and medico-social workers.^[6,13-15] The objective of the presurgical evaluation is (a) to confirm the diagnosis of epileptic seizures while ruling out nonepileptic behavioral events, (b) identify the lesion causing the seizures, (c) choose suitable surgical candidates based on an optimal combination of electro-clinical-radiological correlation, and (d) ensure that surgery will not lead to significant neuropsychological impairments.^[6,13] The key components of the presurgical evaluation include a thorough clinical history and physical examination, video-EEG monitoring, high-resolution MRI, neuropsychological testing, and evaluation of psychosocial well-being. The second stage may involve functional studies such as SPECT and PET, advanced modalities like magnetic source imaging (MSI), EEG-functional MRI (EEG-fMRI), and electrical source imaging (ESI). Invasive intracranial monitoring is reserved as the last resort. Wada test and functional MRI for language and memory functions, cortical stimulation, and mapping during intracranial monitoring help in predicting and minimizing deficits.^[6,13-15] In recent years, stereotactically placed intracranial electrodes and EEG recording (stereo-EEG) have made invasive monitoring less traumatic and more accurate.^[16] However, presurgical evaluation needs to be customized and optimized and costly advanced and invasive monitoring should be judiciously reserved for few complex cases. A good majority of epilepsy surgery work can be performed with basic investigations as was observed in our series where we performed invasive monitoring in only one patient.

Surgical treatment

The goal of epilepsy surgery is the complete removal/ disconnection of the “epileptogenic zone,” which is defined as the area necessary and sufficient for initiating seizures, and resection/disconnection which is associated with sustained seizure freedom.^[2,6,13] Surgical techniques can be of three types - resective, disconnection, or augmentative.

Complications of epilepsy surgery

The rate of complications of epilepsy surgery has significantly reduced over the years to <3% risk of morbidity.^[8,24-33] Complications that can arise from anterior temporal lobectomy include homonymous superior quadrantanopia, which can result from the involvement of the optic tract or radiation, as well as language deficits and manipulation hemiplegia caused by vascular injury or spasm affecting the Sylvian vessels and anterior choroidal artery branches. Frontal lobe resections may cause personality changes, while resection of the dominant inferior parietal lobule may lead to Gerstmann syndrome. Functional hemispherotomy has minimised risk of hydrocephalus and superficial cerebral hemosiderosis, the two most dreaded complications seen following hemispherectomy. Transient aggravation of hemiparesis following hemispherotomy is common, especially for hand functions, but permanent deficits such as hemiplegia are uncommon. Risk of infracts, brain swelling, and perioperative mortality is <1%, but visual field defects remain an expected complication following hemispheric and quadrantic disconnection. Callosotomy can result in the syndrome of mutism, apraxia of the non-dominant leg, and incontinence, although these are often transient.^[31,33]

Outcome

Multiple trials have provided conclusive evidence that surgery is more effective than prolonged medical therapy in patients with refractory temporal lobe epilepsy where long-term seizure freedom can be achieved in about 70% (62–83%) of cases with low morbidity and mortality (<1%).^{[1,8,18,19,33,34]} With extratemporal resective surgeries, the outcome is generally 10% less than temporal lobe surgeries. In hemispherotomies, the probability of achieving 1-year remission is 57% by 1 year, 70% by 2 years, and 77% by 7 years.^[25-29] Large series on quadrantic disconnection surgeries are limited, but available reports suggest that temporo-parieto-occipital disconnection surgeries provide a good outcome in nearly 75% of patients.^[24]

Prolonged longitudinal follow-up is essential for accurate assessment of seizure outcome, since initially seizure-free patients may relapse, and some patients with early postoperative seizure may become seizure-free subsequently (running-down phenomenon).^[33,34] The proportion of seizure-free adults in whom ASMs have been withdrawn varied widely across studies, from 19% to 63% after around 5 years of seizure freedom. Nearly two-thirds of our patients (66.6%) had an Engel’s Class I outcome, five patients had Engel’s Class II outcome, while three patients had Class III outcome and one patient did not have any worthwhile improvement. Suboptimal outcome in three patients with ganglioglioma (Class III on two and Class IV in one) is probably related to incomplete excision as the lesions were near eloquent areas. Similarly, the two patients with cortical dysplasia had Class II outcome probably related to residual epileptogenic zone in the tail of the dysplasia. However, it needs to be emphasized that treatment outcome in epilepsy is not completely about seizure freedom and ASM withdrawal. Even a moderate reduction in seizure frequency can result in psychosocial, educational, and occupational and overall improvement of the quality of life.^[33,34]

Determinants of postoperative seizure outcome

Few predictors of good outcomes after epilepsy surgery include younger age at surgery, shorter duration of epilepsy, absence of a history of status epilepticus, presence of a well-defined radiological lesion, concordance between clinical, EEG, and imaging data, absence of intellectual impairment, favorable psychosocial and occupational conditions, and the absence of comorbidities.^{[8,18-20,33,34]} In patients undergoing hemispherotomy, favorable predictors include acquired etiologies such as Rasmussen’s encephalitis, porencephaly secondary to perinatal stroke, and Sturge–Weber syndrome. Cortical malformations such as hemimegalencephaly, cortical dysplasia, contralateral hemispheric pathology, and a long duration of epilepsy before surgery imply poor outcome following hemispherotomy.^[25-29]