Evaluation of Patients for Epilepsy Surgery

This chapter includes an accompanying lecture presentation that has been prepared by the authors: .

Key Concepts

Resection (or ablation) of the epileptogenic zone (EZ) is the main goal of epilepsy surgery. For an effective and safe operation to be performed, the EZ must be localized, as must its anatomic overlap with eloquent cortex.
There are many techniques available to meet these dual goals, but the most important are video- electroencephalography (EEG) and high-resolution MRI. These, together with a concordant history, are sufficient in the majority of cases for planning an appropriate surgical intervention.
The varied data need to be presented in a multidisciplinary setting to ensure that they are considered from multiple expert perspectives and to foster an approach to decision making and surgical planning by consensus.
Invasive monitoring is needed in select cases, and electrodes need to be placed according to a strong anatomo-electro-clinical (AEC) hypothesis. There are two major invasive methods—subdural grid (SDG) recordings and stereo-electroencephalography (SEEG)—with the latter being the optimal choice in most clinical situations.

Goals of Presurgical Evaluation of Patients With Epilepsy

The main goal of epilepsy surgery is the complete resection (or ablation) of the cortical area(s) responsible for the generation of seizures, the epileptogenic zone (EZ). Whether the EZ is conceptualized as a “focus” or a “network,” an anatomo-electro-clinical (AEC) hypothesis on its localization needs to be formulated in order for surgery to be planned; this is the purpose of the epilepsy presurgical evaluation. Because the EZ may anatomically overlap with functional (eloquent) cortex, an additional and equally important goal of the presurgical evaluation is determining the extent to which the EZ overlaps with eloquent cortex. Various noninvasive techniques are used to accomplish to accomplish these goals. These include video-electroencephalography (EEG), high-resolution MRI, positron emission tomography (PET), ictal single-photon emission computed tomography (SPECT), neuropsychological testing, and magnetoencephalography (MEG). In select patients, invasive EEG techniques may be required in order to define the EZ and delimit safe surgical boundaries. Many of these techniques are discussed at length in the chapters that follow. Here we provide an overview of the presurgical evaluation for epilepsy.

Clinical Approach and Techniques Used in the Presurgical Evaluation

Clinical Approach

An epilepsy surgery evaluation should occur after the diagnosis of pharmacoresistant epilepsy, defined as a failure to respond to two or more appropriately chosen and used antiepileptic medications. This evaluation begins with a detailed history taken from the patient. A clear description of the seizure semiology (symptoms and signs) and in particular the aura, if present, is the most important step in an epilepsy surgery evaluation. If possible, an account of the seizure semiology should be obtained from family members or other witnesses, and later compared with the seizures recorded during video-EEG monitoring. There are important risk factors for the development of epilepsy that need to be identified if present, including febrile convulsions, prior head injuries, and CNS infections. In addition, a family history with particular reference to seizures and other neurological illnesses is important to obtain, as there may be a genetic basis relevant to focal epilepsies in some patients. ^, A neurological examination will uncover any relevant neurological deficits.

Necessary Techniques for the Localization of the Epileptogenic Zone

In our opinion the two most important and highest-yield techniques for the localization of the EZ are high-resolution MRI and video-EEG monitoring. If the history is concordant, the findings of these studies may be sufficient to formulate a presurgical hypothesis and propose a surgical strategy.

High-Resolution Magnetic Resonance Imaging

The identification of lesion on MRI greatly informs the anatomic component of the presurgical hypothesis. Recent guidelines proposed a recommended epilepsy MRI protocol (HARNESS-MRI). This protocol includes a three-dimensional (3D; volumetric) T1-weighted sequence, a 3D fluid-attenuated inversion recovery (FLAIR) sequence, and a two-dimensional (2D) coronal T2-weighted sequence acquired perpendicular to the long axis of the hippocampus. This last oblique sequence is optimal for assessing hippocampal abnormalities, specifically to evaluate for hippocampal sclerosis (HS). Gadolinium is needed only if a tumor, vascular lesion, or active infectious process is suspected. A neuroradiologist who is active in an epilepsy surgery program should provide the interpretation, as this yields a greater detection of focal epileptogenic lesions than evaluation by “nonexpert” neuroradiologists.

It is critical to realize that not all lesions are equally epileptogenic and that the EZ may extend beyond the visible boundaries of the lesion. For example, an encephaloclastic lesion such as a perinatal infarction is much less epileptogenic than a focal cortical dysplasia (FCD) ; moreover, the EZ related to an FCD may well extend beyond its margins, at times even into the contralateral hemisphere. The opposite case applies with lesions such as polymicrogyria (PMG) or periventricular nodular heterotopia (PVNH), in which only a small component of the lesion(s) may be epileptogenic. HS is generally considered to be the most predictive lesion type for defining the EZ, although this rule does not hold in cases of so-called “temporal plus” epilepsy, in which the epileptogenic network extends beyond the boundaries of the temporal lobe.

Although finding a lesion on MRI is helpful in formulating a presurgical hypothesis, and may correlate with a better surgical outcome, finding a lesion on MRI is not necessary for defining the EZ, and an apparently normal MRI result does not exclude a patient from epilepsy surgery. Indeed, similarly good surgical outcomes may be achieved in MRI-negative patients if a careful and thorough presurgical methodology is used.

Scalp Video-Electroencephalographic Monitoring

Prolonged video-EEG in a dedicated epilepsy monitoring unit (EMU) is required to confirm the diagnosis of epilepsy and to generate the electroclinical component of the presurgical hypothesis, through analysis of EEG (both interictal and ictal) and seizure semiology. The details of video-EEG monitoring go beyond the scope of this chapter, but a few illustrative examples follow. For example, in a patient with an olfactory aura, automotor (focal impaired awareness) seizures, and right HS on MRI, inferior temporal electrodes should be applied in addition to the standard 10/20 scalp electrodes, as these additional electrodes will better capture the inferior EEG fields generated by a mesial temporal epilepsy. In such a patient, recording only a small number of seizures (one or two) will suffice if they are of right temporal origin—that is, concordant with imaging data. In another patient with a somatosensory aura of the left hemibody and right temporal atrophy on MRI, additional EEG electrodes should be placed over the right central and parietal regions because of the distinct possibility of a posterior perisylvian component to the EZ. This patient would be likely to benefit from additional diagnostic testing to better resolve this question (see later). In a patient with bilateral hippocampal atrophy on MRI, multiple seizures should be recorded, given the likelihood of independent bitemporal seizures, noting the recently identified caveat that true lateralization of epilepsy is such a patient may not be possible during the EMU stay.

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