22 Epilepsy: Extra-Temporal Surgery with Invasive Monitoring
Abstract
The goal of epilepsy surgery is the complete resection or disconnection of the cortical and subcortical areas that are responsible for the generation and early spread of seizures. This is known as the epileptogenic zone (EZ). The EZ may overlap with eloquent cortex, which must be preserved. Standard, non-invasive monitoring provides a rather broad overview of the anatomical location of epileptogenic areas and respective cortical functioning. The goal of invasive monitoring is to better understand the anatomical boundaries of the EZ as well as cortical and subcortical function. This chapter discusses the indications, advantages, disadvantages, and the role of invasive monitoring in medically refractory extra-temporal focal epilepsy, focusing primarily on subdural grid and strip and stereo-electroencephalography (SEEG) methodologies.
22.1 Patient Selection
To best define the anatomical location of the epileptogenic zone (EZ) and its proximity to cortical and subcortical eloquent areas, a range of non-invasive tools can be used. Such tools include the analysis of patient semiology, magnetocencephalography (MEG), Magnetic resonance imaging (MRI), ictal single-photon emission computed tomography (SPECT), functional MRI (fMRI), magnetic resonance spectroscopy (MRS) and positron emission tomography (PET). These methods are complementary to one another and may define cortical zones of interest as symptomatogenic, irritative, ictal, and functionally deficient in addition to the EZ, which is defined as the minimal amount of brain tissue that needs to be removed or disconnected to achieve seizure freedom. 1 However, under circumstances in which: (1) the non-invasive data is insufficient to precisely define the location of the hypothetical EZ, (2) there is suspicion for early involvement of eloquent cortical and subcortical areas or, (3) there is the possibility for multi-focal seizures, invasive monitoring may be indicated. 2 , 3 , 4
22.2 Preoperative Preparation
22.2.1 Localizing the Epileptogenic Zone
Prolonged video-EEG monitoring with analysis of clinical semiology is the standard for diagnosis and localization of the EZ. 1 This noninvasive sampling technique gives an excellent overview of the epileptogenic areas but often only approximates the boundaries of both the irritative zone and the EZ. Scalp EEG detects only epileptiform activity that results from EEG synchronization of large areas of cortex, while recordings are disturbed by the effect of high-resistance structures. 1 , 5 MEG may provide better identification of the epileptic activity localized in a tangential orientation such as the inter-hemispheric fissure or opercular areas, but with limited information regarding the interictal epileptic activity. 6 In malformations of cortical development (MCD), 85%-100% of patients exhibit epileptiform discharges on inter ictal scalp EEG recordings. These discharges can range from lobar to lateralized, and from non-localizing to diffuse. In some cases of subependymal heterotopia, they can include generalized spike-wave patterns. 7 The spatial distribution of interictal spikes is usually more extensive than the structural abnormality when assessed by intraoperative inspection or visual MRI analysis. 7 , 8 For these reasons, when subtle forms of cortical dysplasia are suspected as the pathological substrate of medically intractable epilepsy, mainly in patients with extra-temporal epilepsy and non-lesional imaging, extraoperative invasive monitoring is recommended to provide a more accurate and effective localization of the EZ. 2 , 9 , 10 , 11
22.2.2 Localization of the Functional/Eloquent Zone
Localization of functional areas in the brain, and the anatomical boundaries of these areas with the EZ, is an essential part in the process of developing an adequate and individualized surgical strategy. 1 , 12 , 13 An understanding of the functional status of the involved region(s) and its anatomical or pathological correlation is essential. 1 , 12 , 14 For example, some focal MCD-related lesions that are characterized by significant FLAIR signal increase on MRI and are located in anatomically functional areas such as Broca’s area, are not functional on direct electrical stimulation. 15 Additionally, the same lesions may show no evidence of intrinsic epileptogenicity when assessed by mapping of the ictal onset zones (▶ Fig. 22.1). 16 On the other hand, MCD lesions with mild or no FLAIR signal increase may be functional and at times epileptogenic with persistent eloquent function in MCD devoid of balloon cells. 9 , 17 Similar electrocorticogram (EcoG) patterns have been reported in patients with low-grade glial tumors (DNET and ganglioglioma), whereas dysplastic and epileptic cortical areas were found immediately surrounding these lesions. 18 Functional cortex may be displaced. Of course, the precise location of eloquent cortex that may be in the vicinity or within the limits of the hypothetical EZ is essential information that will guide the completion of a safe operation.
22.3 Operative Procedure
22.3.1 Indications, Advantages, and Disadvantages of the Subdural Method in Evaluating Medically Refractory Extratemporal Lobe Epilepsy
Intracranial electrodes are used to identify the EZ and functional or eloquent cortex. Subdural grids have an advantage, however. They can be left in place long enough to record both spontaneous seizures and inter ictal activity during various stages of arousal. They can be used for continuous preoperative mapping and surveying of adjacent cortex. 9 , 15 , 19 , 20 However, subdural grids also have disadvantages. These are: increased risks of wound infection, flap osteomyelitis, acute meningitis, cerebral edema, increased intracranial pressure, and hemorrhage, greater financial costs, and limited access to deep cortical regions. Two such regions are the mesial orbitofrontal cortex and the anterior cingulate gyrus. 21 , 22 , 23 , 24 While implanted depth electrodes that use a non or semi-stereotactic technique can compensate for these deficiencies, high precision and clear localization of the EZ may be lacking. 25
22.3.2 The Subdural Operative Technique in Evaluating Medically Refractory Extra-temporal Lobe Epilepsy
Pre-operative, non-invasive studies are used to determine areas of coverage. Perioperative antibiotics, dexamethasone and 0.25 g/kg mannitol are given. The craniotomy should allow room for electrode insertion in addition to lesion resection. Stereotactic guidance might be necessary if depth electrodes are needed, as well as a sizable craniotomy if extensive cortical surface area is to be covered. Orbitofrontal access can be obtained if the craniotomy includes the neurosurgical keyhole. Inter-hemispheric access requires an incision to the midline. Planned depth electrodes are inserted first, using stereotactic guidance. Cortical draining veins can impede basal and medial surface electrode placement. These surfaces should be carefully inspected before a grid is positioned beyond the craniotomy edge. Any resistance to placement likely indicates the presence of a draining vein and the trajectory of the array should be adjusted. The lateral cortical surface is covered as the final step. Once in place, each electrode wire is secured to the nearest dural edge with a stitch. The closure is done using standard technique.
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