5 Invasive Electrophysiological Monitoring

10.1055/b-0034-84115

5 Invasive Electrophysiological Monitoring

Jayakar, Prasanna, Miller, Ian

The primary goal of the presurgical evaluation for intractable epilepsy is to accurately define the epileptogenic region (ER). The ER is mainly conceptual and, in practical terms, translates to the minimum amount of tissue that must be resected to ameliorate all seizures. This critical mass of tissue is viewed as a function of the region of seizure onset, seizure propagation patterns, the areas that could become epileptogenic later, and the underlying structural lesion and functional abnormality. As in adults, surgical strategies in childhood are guided by diverse pieces of information obtained from clinical semiology, imaging, and neurophysiological data; the task, however, is more daunting given the greater heterogeneity of etiopathological substrates and maturational factors that significantly influence the clinical presentation and investigative findings.1

The role of invasive electroencephalography (EEG) monitoring (IEM) in the evaluation of childhood epilepsy has evolved. With advances in magnetic resonance imaging (MRI), single photon emission computed tomography (SPECT) and positron emission tomography (PET) imaging, presurgical evaluation in many children can be adequately performed through noninvasive means. In the International League Against Epilepsy (ILAE) study of 543 children, the 20 participating centers worldwide reported using IEM in just more than 25% of their surgeries.2 The increasing use of magnetoencephalography, or three-dimensional EEG dipole source localization algorithms, and the advent of functional MRI to define critical cortex may further diminish the need for IEM. This trend will likely be offset as pediatric centers gain more experience in identifying subtle focal abnormalities using noninvasive tools and aggressively pursue surgical candidacy in increasingly complex cases. This chapter critically examines the continuing role of IEM in presurgical evaluation.

Pragmatic Considerations

IEM is inherently costly, risky, and not devoid of limitations. It is therefore prudent to first consider some practical issues as they relate to a given patient. IEM is preferably undertaken only if prior noninvasive evaluation provides sufficient information as to the side or approximate location of the ER; it should not be used as an “exploratory procedure.” Also, it is prudent to ask the question if a more precise definition of ER using IEM will alter the ultimate surgical strategy and outcome. For example, IEM may be of little use if the noninvasive studies support widespread epileptogenic dysfunction that cannot all be resected and the goals of surgery are mainly palliative or when a “standard” temporal or precoronal frontal lobectomy is contemplated in a child with all noninvasive data indicating an ER contained within the planned resection. Similarly, when the intraoperative electrocorticography (ECoG) reveals almost continuous focal seizure discharges, the additional yield, if any, from IEM rarely justifies a two-stage procedure.

Because of limitations of sampling and interpretation, IEM does not always ensure a clear focality and precise demarcation of the ER. Although its yield cannot be predicted with certainty, our experience suggests that children who are neurodevelopmentally intact, who reveal a restricted focus with an otherwise normal scalp EEG, or who have relatively localized imaging findings that can be specifically targeted by electrodes are likely to accrue the greatest benefit. By contrast, IEM is unlikely to document discrete seizure onsets when patients with normal imaging studies present with spasms or diffuse patterns on the scalp EEG or in patients with multiple subcortical nodular heterotopias, large infiltrative lesions, or extensive multilobar cortical dysplasia.

Lastly, the use of IEM for defining critical cortex to “fine tune” the resection must be justified by the potential limitations of intraoperative functional mapping. Language mapping can rarely be performed intraoperatively in children. Somatosensory responses to median nerve stimulation help to define the central sulcus, but the estimate of sensory and motor regions may be inaccurate. The motor cortex can be crudely mapped by direct electrical stimulation, but general anesthesia often suppresses the ability to elicit consistent responses especially in the young child.

Indications

Although many centers offer IEM, there is no standard pre-surgical protocol, and its use is guided primarily by the availability of other noninvasive tools and the referral pattern at each center. At our center, all children undergo video EEG and MRI scanning; those revealing certain subtypes of MRI lesions with convergent video-EEG data require no further testing. In others, functional imaging (SPECT and or PET) scans, three-dimensional spike source localization, and functional MRI for language and motor regions are obtained. The data are reviewed at a multidisciplinary case conference where the surgical strategy is defined. Guided by the pragmatic considerations discussed previously, IEM is generally recommended for the following indications: inconclusive preoperative data (e.g., normal or nonspecific computed tomography [CT]/MRI scans, structural lesions), divergent preoperative data, or encroachment on eloquent cortex.

Inconclusive Preoperative Data: Each Test Has Limitations

Normal or Nonspecific CT/MRI Scans

Despite advances in MRI imaging, many children with localization-related intractable epilepsy have normal scans; approximately one in every four children in the ILAE series did not show a definite lesion on MRI.2 IEM continues to play a significant role in this patient subgroup, especially when functional imaging data are inconclusive; removal of the entire region of significant abnormalities identified on IEM is generally required to achieve seizure freedom.3 In our recent series,4 80 of the 102 patients underwent a two-stage evaluation, the rest could be adequately localized based on scalp EEG, functional imaging, and ECoG.

Structural Lesion

In general, a discrete structural abnormality on CT/MRI scans is regarded as a very reliable marker of the ER and biases the presurgical evaluation against IEM. However, there are many documented failures after lesionectomy in children,5,6 partly because lesional epilepsy does not represent a homogeneous substrate. Whereas developmental tumors, hippocampal sclerosis, low-flow vascular lesions, or Sturge-Weber syndrome can often be successfully treated after noninvasive evaluation alone, children with ill-defined cortical dysplasia or multiple lesions such as tuberous sclerosis often reveal complex and rapid seizure propagation that make interpretation of the scalp EEG and functional imaging data difficult. IEM often helps clarify ambiguities of seizure origin and propagation, thus facilitating surgery in these difficult cases.6–8 In some cases albeit rare, when the MRI shows widespread lesions but other noninvasive data suggest that seizures arise from only a restricted region, IEM may allow successful focal resections averting a hemispherectomy in a functional child.9

Divergent Preoperative Data

The criteria of what exactly constitutes divergence are variably defined by different centers; we regard divergence when the clinical semiology, EEG, and functional/structural imaging data implicate separate regions. In this scenario, IEM serves as the last resort to help define the ER and may permit successful focal resection. Since the divergence may arise from complex and rapid interaction between noncontiguous cortical sites, all suspected sites must be adequately sampled. When divergence occurs in the context of large or deepseated lesions, a combination of subdural and strategically placed depth electrodes is recommended.

Encroachment on Eloquent Cortex

In our series of discrete perirolandic foci, aggressive resections tailored to the ER and eloquent motor cortex led to seizure freedom in more than half the cases, even in the absence of a structural lesion.10 In some cases, a calculated decision to remove part of the motor cortex revealing face or proximal limb function enhances the chances of success without incurring risks of significant clinical deficits. Likewise, in patients with occipital foci who have intact visual fields, accurate demarcation of the ER over the occipital convexity or base may allow a restricted corticectomy preserving most of the calcarine cortex and visual pathways, making IEM a worthwhile endeavor.11

The issues surrounding the need to identify and preserve language cortex are more complex. Because language cortex is plastic under age 5 years, many centers opt for more aggressive large resections in the hope of forcing language transfer. Our presurgical evaluation strategy is driven by the intent to preserve predestined language sites unless they are involved at ictal onset. As illustrated in Fig. 5.1 , we have used IEM to map and tailor resections, even in the very young, with the hope of maximizing language outcomes in those who are rendered seizure free.

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