Intracranial EEG Substrates of Scalp EEG Interictal Spikes

University of Chicago, Chicago, Illinois, United States
Epilepsia (Impact Factor: 4.57). 06/2005; 46(5):669-76. DOI: 10.1111/j.1528-1167.2005.11404.x
Source: PubMed


To determine the area of cortical generators of scalp EEG interictal spikes, such as those in the temporal lobe epilepsy.
We recorded simultaneously 26 channels of scalp EEG with subtemporal supplementary electrodes and 46 to 98 channels of intracranial EEG in 16 surgery candidates with temporal lobe epilepsy. Cerebral discharges with and without scalp EEG correlates were identified, and the area of cortical sources was estimated from the number of electrode contacts demonstrating concurrent depolarization.
We reviewed approximately 600 interictal spikes recorded with intracranial EEG. Only a very few of these cortical spikes were associated with scalp recognizable potentials; 90% of cortical spikes with a source area of >10 cm(2) produced scalp EEG spikes, whereas only 10% of cortical spikes having <10 cm(2) of source area produced scalp potentials. Intracranial spikes with <6 cm(2) of area were never associated with scalp EEG spikes.
Cerebral sources of scalp EEG spikes are larger than commonly thought. Synchronous or at least temporally overlapping activation of 10-20 cm(2) of gyral cortex is common. The attenuating property of the skull may actually serve a useful role in filtering out all but the most significant interictal discharges that can recruit substantial surrounding cortex.

    • "For neurologists and psychiatrists alike, these patients with a strong clinical case for paroxysmal seizures, but a normal EEG represent a challenging diagnostic dilemma [15]. However, a negative result can in many cases be explained by the low sensitivity of surface EEG to detect a majority of the epileptiform features that can be seen when simultaneously recording with intracranial electrodes [16]. "
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    ABSTRACT: Episodic psychiatric symptoms are not uncommon and range from panic attacks to repeated violent acts. Some evidence has accumulated over the years that at least in a subset of patients exhibiting these symptoms there may be evidence for the presence of focal cortical/subcortical hyperexcitability. In these cases the condition could be conceptualized as an epilepsy spectrum disorder (ESD) with significant treatment implications. There is currently no clear demarcation of this category of symptoms, their prevalence, an understanding of how these symptoms occur, what is appropriate work up and possible treatments. In this article, we propose that milder degrees of increased neural excitability (i.e., a subthreshold excitation insufficient to cause seizures) may nonetheless be capable of causing observable phenotypic changes. The observable phenotypic changes depend on the degree of hyperexcitability and the location of the hyperexcitable neural tissue. The location of the abnormal neural tissue may dictate the initial manifestation of an attack resulting from activation of the hyperexcitable tissue, but the anatomical connectivity of the abnormal region will dictate the breadth of manifestations. We provide some evidence, derived mainly from either electroencephalography studies of these populations or clinical reports of response to anti-epilepsy treatment, for the assumption and propose methods to test the advanced hypothesis. Copyright © 2015 Elsevier Ltd. All rights reserved.
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    • "i z i o n i I n t e r n a z i o n a l i activity recorded by intracranial EEG (Tao et al., 2005). Answers to these questions might be found thanks to the recent possibility of recording intracranial EEG and fMRI simultaneously (icEEG-fMRI). "
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    ABSTRACT: Functional magnetic resonance imaging (fMRI), which has high spatial resolution, is increasingly used to evaluate cerebral functions in neurological and psychiatric diseases. The main limitation of fMRI is that it detects neural activity indirectly, through the associated slow hemodynamic variations. Because neurovascular coupling can be regionally altered by pathological conditions or drugs, fMRI responses may not truly reflect neural activity. Electroencephalography (EEG) recordings, which directly detect neural activity with optimal temporal resolution, can now be obtained during fMRI data acquisition. Therefore, there is a growing interest in combining the techniques to obtain simultaneous EEG-fMRI recordings. The EEG-fMRI approach has several promising clinical applications. The first is the detection of cortical areas involved in interictal and ictal epileptic activity. Second, combining evoked potentials with fMRI could be an accurate way to study eloquent cortical areas for the planning of neurosurgery or rehabilitation, circumventing the above-mentioned limitation of fMRI. Finally, the use of this approach to evaluate the functional connectivity of resting-state networks would extend the applications of EEG-fMRI to uncooperative or unconscious patients. Integration of multimodal neuroimaging methods: a rationale for clinical applications of simultaneous EEG-fMRI.
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    • "Therefore, these epileptiform waves most likely represent activation of the parahippocampal tissue {Ltiders 2008). Along these lines, true hippocampal epileptiform activity is not seen on scalp EEG due to its small area of activation as intracranial EEG has indicated that approximately 10 em of cortex is required to generate scalp interictal discharges (Tao et al. 2005). Placement of basal temporal electrodes (i.e., sphenoidal or FT9/FT10 electrodes) can provide additional localizing information that is useful in both scalp EEG (Figure 2) as well as more advanced modalities such as electrical source imaging (ESI) (Cherian et al. 2012). "
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    ABSTRACT: Mesial temporal lobe epilepsy is a common subtype of temporal lobe epilepsy. Its most common cause is hippocampal sclerosis, which contributes to its distinct electroclinical phenotype that is seen commonly in the epilepsy monitoring unit setting. The common electrophysiological data show anterior temporal interictal sharp waves as well as rhythmic theta activity in the same localization. While the electrophysiological data can at times be misleading, its stereotyped and characteristic semiology can often allow for accurate diagnosis on its own. As patients with mesial temporal lobe epilepsy often fail medical therapy, surgical therapy can be considered. Early accurate diagnosis in these patients is essential for optimal care.
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