During the pre-surgical evaluation of drug-resistant epilepsy, the assessment of the extent of the epileptogenic zone and its organization is a crucial objective. Indeed, the epileptogenic zone may be organized as a simple focal lesional site or as a more complex network (often referred to as the 'epileptogenic network') extending beyond the lesion. This distinction is particularly relevant in developmental lesions such as focal cortical dysplasias or dysembryoplastic neuroepithelial tumours and may determine both the surgical strategy and the prognosis. In this study, we have quantified the epileptogenic characteristic of brain structures explored by depth electrodes in 36 patients investigated by stereoelectroencephalography and suffering from focal drug-resistant epilepsy associated with focal cortical dysplasias or dysembryoplastic neuroepithelial tumours. This quantification was performed using the 'Epileptogenicity Index' method that accounts for both the propensity of a brain area to generate rapid discharges and the time for this area to get involved in the seizure. Epileptogenicity Index values range from 0 (no epileptogenicity) to 1 (maximal epileptogenicity). We determined Epileptogenicity Index from signals recorded in distinct brain structures including the lesional site. We studied the type of epileptogenic zone organization (focal versus network) and looked for a correlation with clinical data and post-surgical outcome. Mean Epileptogenicity Index in lesional regions was 0.87 (+/-0.25), and 0.29 (+/-0.30) in 'non-lesional' structures. The number of highly epileptogenic structures (defined by Epileptogenicity Index value >or=0.4) was 3.14 (+/-1.87) in the whole population. We found that 31% of patients had only one epileptogenic structure (N(EI>or=0.4) = 1), therefore disclosing a strictly focal epileptogenic zone organization while 25 patients had more than one epileptogenic region, disclosing a network (61%) or bilateral (8%) epileptogenic zone organization. We observed a trend for a difference in seizure outcome according to the type of epileptogenic zone organization. Indeed, 57% of patients with network organization and 87% with focal organization were seizure-free while none of those with bilateral organization became seizure-free. The determination of Epileptogenicity Index computed from electrophysiological signals recorded according to the stereoelectroencephalography technique is a novel tool. Results suggest that it can help in the delineation of the epileptogenic zone associated with brain lesions and that it could be used in the definition of the subsequent surgical resection.

Local and remote epileptogenicity in focal cortical dysplasias and neurodevelopmental tumours

OBJECT Conflicting conclusions have been reported regarding several factors that may predict seizure outcomes after hemispheric surgery for refractory epilepsy. The goal of this study was to identify the possible predictors of seizure outcome by pooling the rates of postoperative seizure freedom found in the published literature. METHODS A comprehensive literature search of PubMed, Embase, and the Cochrane Library identified English-language articles published since 1970 that describe seizure outcomes in patients who underwent hemispheric surgery for refractory epilepsy. Two reviewers independently assessed article eligibility and extracted the data. The authors pooled rates of seizure freedom from papers included in the study. Eight potential prognostic variables were identified and dichotomized for analyses. The authors also compared continuous variables within seizure-free and seizure-recurrent groups. Random- or fixed-effects models were used in the analyses depending on the presence or absence of het...

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Hemispheric surgery for refractory epilepsy: a systematic review and meta-analysis with emphasis on seizure predictors and outcomes

Crossed Uncrossed Differentials (CUDs) have long been used as surrogate for the interhemispheric transfer time (IHTT). Evidence is presented that macular vision is the province of the major hemisphere, wherein all commands are initiated regardless of the laterality of the effectors of such commands. Using clinical and time-resolved data it is shown also that the above arrangement (i.e. neural handedness) corresponds to the subject's behavioral avowed (avowed, self-declared) handedness only in a statistical sense; with a substantial minority of humanity displaying a disparity of neural and behavioral handedness. Evidence is provided that the negative CUD in previously reported studies was a reflection of such incongruity in those subjects studied. Thus, to lateralize the command center it is sufficient to determine the reaction time of two symmetrically located effectors on the body. The side with longer reaction time is ipsilateral to the major hemisphere, with the difference of the two sides commensurate to transcallosal IHTT.

Crossed-uncrossed difference (CUD) in a new light: anatomy of the negative CUD in Poffenberger's paradigm.

Publisher Summary This chapter illustrates the principles that guide clinicians in selecting patients for epilepsy surgery and in estimating their prognosis; they also exemplify uncertainties surrounding these very issues. Two cases are discussed here. The first patient was a 45-year-old right-handed male who had his first seizure at the age of 2 months in the context of a fever. He started having recurrent seizures as an adolescent. Over the years, the patient was treated with multiple antiepileptic drug (AEDs) including phenobarbital, phenytoin, lamotrigine, primidone, valproic acid, and most recently levetiracetam and carbamazepine. He had a vagal nerve stimulation (VNS) placed in 1997 that resulted in decreased intensity of seizures, but no change in frequency. The second patient is a 24-year-old left-handed Caucasian woman whose seizures likely began at age 3 or 4 with nocturnal episodes of emesis but she was not diagnosed until age 6 when she suffered her first convulsive seizure. Carbamazepine was initiated but she continued to have a few complex partial seizures per month. These seizures increased in frequency during her teens and at the age 16 she was switched to gabapentin and felbamate due to weight gain and intractability. Seizure frequency improved on felbamate but occasional breakthrough seizures persisted. She was offered either an intracranial depth electrode study or a focal right occipital resection under the guidance of electrocorticography. Although the patient was informed that a right occipital resection may fail to produce complete seizure control, possibly leading to a subsequent intracranial study and right (dominant) temporal resection, she decided to pursue a focal occipital resection. She has been followed for over 2 years since surgery and has surprisingly remained seizure-free on her prior medical regimen.

Why Do Some Patients Have Seizures After Brain Surgery While Others Do Not

Objective: Reports on bilateral epilepsy surgical interventions are anecdotal because of the possible neurological deficits caused by them. Methods: We report on a four-year-old amaurotic child with catastrophic epilepsy due to bilateral occipital cortical dysplasia. After video-EEG monitoring and intraoperative electrocorticography he underwent a two-step bilateral occipital lobectomy. Results: The first resection resulted in only temporary seizure cessation; however, he became seizure-free after the second operation (follow-up: 20 months). Conclusion: Patients with catastrophic epilepsy due to bilateral epileptogenic lesions but without a high risk of additional post-surgical deficit may be good candidates for epilepsy surgery.

M. Hegyi

Papers

Bilateral epilepsy surgery in a 4-year-old child