How to establish causality in epilepsy surgery
Eishi Asano, Erik C Brown, Csaba Juhász, Eishi Asano, Erik C Brown, Csaba Juhász
Abstract
Focality in electro-clinical or neuroimaging data often motivates epileptologists to consider epilepsy surgery in patients with medically-uncontrolled seizures, while not all focal findings are causally associated with the generation of epileptic seizures. With the help of Hill's criteria, we have discussed how to establish causality in the context of the presurgical evaluation of epilepsy. The strengths of EEG include the ability to determine the temporal relationship between cerebral activities and clinical events; thus, scalp video-EEG is necessary in the evaluation of the majority of surgical candidates. The presence of associated ictal discharges can confirm the epileptic nature of a particular spell and whether an observed neuroimaging abnormality is causally associated with the epileptic seizure. Conversely, one should be aware that scalp EEG has a limited spatial resolution and sometimes exhibits propagated epileptiform discharges more predominantly than in situ discharges generated at the seizure-onset zone. Intraoperative or extraoperative electrocorticography (ECoG) is utilized when noninvasive presurgical evaluation, including anatomical and functional neuroimaging, fails to determine the margin between the presumed epileptogenic zone and eloquent cortex. Retrospective as well as prospective studies have reported that complete resection of the seizure-onset zone on ECoG was associated with a better seizure outcome, but not all patients became seizure-free following such resective surgery. Some retrospective studies suggested that resection of sites showing high-frequency oscillations (HFOs) at >80Hz on interictal or ictal ECoG was associated with a better seizure outcome. Others reported that functionally-important areas may generate HFOs of a physiological nature during rest as well as sensorimotor and cognitive tasks. Resection of sites showing task-related augmentation of HFOs has been reported to indeed result in functional loss following surgery. Thus, some but not all sites showing interictal HFOs are causally associated with seizure generation. Furthermore, evidence suggests that some task-related HFOs can be transiently suppressed by the prior occurrence of interictal spikes. The significance of interictal HFOs should be assessed by taking into account the eloquent cortex, seizure-onset zone, and cortical lesions. Video-EEG and ECoG generally provide useful but still limited information to establish causality in presurgical evaluation. A comprehensive assessment of data derived from multiple modalities is ultimately required for successful management.
Keywords: Concept of the epileptogenic zone; Eloquent cortex; Functional brain mapping; High-frequency oscillations (HFOs); Hill’s criteria for causality; In-vivo animation of event-related gamma activity; Infantile spasms; Language; Pediatric epilepsy surgery.
Copyright © 2013 The Japanese Society of Child Neurology. Published by Elsevier B.V. All rights reserved.
Figures
(A) The illustration shows a case in which the presumed epileptogenic zone (red) is localized to a region distant from eloquent cortex (blue). The primary intent of a surgeon would be to maximize removal of the presumed epileptogenic zone while preserving eloquent cortex. The symptomatogenic zone is dependent on the propagation pattern of ictal discharges. The sensorimotor cortex here may be considered as the symptomatogenic zone if the propagation of ictal discharges to it resulted in seizure-related symptoms characterized by jerking of the contralateral body. (B) The illustration shows a case in which the presumed epileptogenic zone involves a large portion of eloquent cortex also serving as the symptomatogenic zone. The surgeon's intent would be to remove the symptomatogenic zone together with the presumed epileptogenic zone; for example, hemispherectomy including the sensorimotor area is a likely approach. The resulting functional losses due to hemispherectomy would include visual field loss, if the function was intact preoperatively, and a variable degree of enhancement in sensorimotor deficits. (C) The illustration shows a case in which the presumed epileptogenic zone involves, to a lesser degree, eloquent cortex. This scenario is the case, for example, when the seizure onset zone is localized outside of eloquent cortex but very frequent interictal spike-wave discharges involve the eloquent cortex. One approach would be partial removal of the presumed epileptogenic zone along with maximal preservation of the eloquent cortex. An alternative approach would be maximal removal of the presumed epileptogenic zone together with eloquent cortex (serving as the symptomatogenic zone). With the later approach, a greater chance of seizure-freedom is plausible since resection includes the symptomatogenic zone responsible for generating the ictal symptom; the drawback of this more aggressive approach is a greater chance of development of a new or further enhancement of an existing functional deficit.
A 9-year-old, right-handed girl with focal epilepsy and normal MRI underwent extraoperative ECoG. Her habitual seizures were characterized by sudden-onset and brief hypermotor automatism followed by minimal postictal confusion. Both seizure onset and irritative zones were localized to the ventral surface of the left frontal lobe on ECoG [3]. While MRI failed to show a cortical lesion, FDG-PET showed severe glucose hypometabolism near the seizure onset zone in the ventral surface of the left frontal lobe [18]. Glucose metabolism in the red-shaded area was >15% lower than that of the contralateral homotopic region [3]. Electrical stimulation of the seizure onset zone elicited her habitual seizures. The intent of cortical resection was to completely remove the presumed epileptogenic zone while preserving the sensorimotor and language cortices, as suggested by electrical stimulation and anatomical landmarks. After surgery, pathological examination revealed focal cortical dysplasia; she has been seizure-free without a new cognitive deficit for 6 years. The epileptogenic zone is included within the resection cavity, the margin of which is denoted by broken lines, but it remains unknown if this extent of resection was necessary to achieve long-term seizure-freedom. As demonstrated in Table 3 below, it was reasonably straightforward to establish the causal relationship between imaging abnormalities of the ventral surface of the left frontal lobe and generation of epileptic seizures in this patient.
A 7-year-old, right-handed girl with a history of intractable epilepsy associated with Sturge-Weber syndrome underwent epilepsy surgery. A port wine stain was noted on her right face shortly after birth; seizures developed by 11 months of age. Before surgery, she exhibited mild hemiparesis as well as a left-sided visual field deficit. Her habitual seizures were characterized by behavioral arrest, transient worsening of hemiparesis on the left side, as well as occasional generalized tonic-clonic convulsions. (A) MRI showed severe atrophy of the right hemisphere involving the right Rolandic cortex (arrow). (B) Ictal EEG (arrowhead) showed sustained rhythmic slow wave discharges mixed with low-amplitude fast waves maximally involving the left posterior head region; seizure-onset is marked by the black arrowhead. High-pass filter cutoff: 1.6 Hz. Low-pass filter cutoff: 30 Hz. The intraoperative photograph showed diffuse angiomatosis involving the right hemisphere. Prior to surgery, we concluded the probable causal relationship between the broad right-hemispheric lesion and epilepsy (Table 4). The intent was to completely remove the presumed epileptogenic zone including the right Rolandic cortex by anatomical hemispherectomy [19]. Following surgery, our presumed causal relationship was confirmed by her ongoing seizure freedom, but she has suffered a worsening of her left hemiparesis, which required physical therapy.
A 16-month-old girl with a history of intractable epileptic spasms associated with a brain tumor and Down syndrome underwent epilepsy surgery. She developed epileptic spasms at 6 months of age. (A) MRI showed a left temporal lobe tumor with gadolinium enhancement (arrow). (B) Interictal scalp video-EEG showed independent spike-wave discharges arising from the left and right temporal regions as well as posterior head regions, independently. Generalized spike-wave discharges were also noted, and some of the sleep EEG segments showed hypsarrhythmia (not shown here). A high-pass filter cutoff of 1.6 Hz and a low-pass filter cutoff of 70 Hz were applied. (C) Ictal EEG associated with each spasm event showed diffuse, low-amplitude, fast wave bursts without a consistent leading wave in the left hemisphere. The aforementioned data failed to suggest a causal relationship between the cortical lesion in the left temporal lobe and generation of epileptic spasms (Table 5). Our surgical intention was complete removal of the brain tumor and the surrounding tissues. Intraoperative electrocorticography (ECoG) showed multifocal interictal epileptiform discharges in the left temporal and extratemporal regions. Postoperative ECoG following a left temporal lobectomy that included the brain tumor continued to show interictal epileptiform discharges in the extratemporal region. (D) Postoperative MRI confirmed successful resection of the brain tumor. However, postoperative scalp EEG showed multifocal and independent interictal spike-wave discharges bilaterally and the patient continued to have epileptic spasms. Pathology revealed a high-grade glioma.
A 16-month-old girl with a history of intractable epileptic spasms associated with a brain tumor and Down syndrome underwent epilepsy surgery. She developed epileptic spasms at 6 months of age. (A) MRI showed a left temporal lobe tumor with gadolinium enhancement (arrow). (B) Interictal scalp video-EEG showed independent spike-wave discharges arising from the left and right temporal regions as well as posterior head regions, independently. Generalized spike-wave discharges were also noted, and some of the sleep EEG segments showed hypsarrhythmia (not shown here). A high-pass filter cutoff of 1.6 Hz and a low-pass filter cutoff of 70 Hz were applied. (C) Ictal EEG associated with each spasm event showed diffuse, low-amplitude, fast wave bursts without a consistent leading wave in the left hemisphere. The aforementioned data failed to suggest a causal relationship between the cortical lesion in the left temporal lobe and generation of epileptic spasms (Table 5). Our surgical intention was complete removal of the brain tumor and the surrounding tissues. Intraoperative electrocorticography (ECoG) showed multifocal interictal epileptiform discharges in the left temporal and extratemporal regions. Postoperative ECoG following a left temporal lobectomy that included the brain tumor continued to show interictal epileptiform discharges in the extratemporal region. (D) Postoperative MRI confirmed successful resection of the brain tumor. However, postoperative scalp EEG showed multifocal and independent interictal spike-wave discharges bilaterally and the patient continued to have epileptic spasms. Pathology revealed a high-grade glioma.
A 16-month-old girl with a history of intractable epileptic spasms associated with a brain tumor and Down syndrome underwent epilepsy surgery. She developed epileptic spasms at 6 months of age. (A) MRI showed a left temporal lobe tumor with gadolinium enhancement (arrow). (B) Interictal scalp video-EEG showed independent spike-wave discharges arising from the left and right temporal regions as well as posterior head regions, independently. Generalized spike-wave discharges were also noted, and some of the sleep EEG segments showed hypsarrhythmia (not shown here). A high-pass filter cutoff of 1.6 Hz and a low-pass filter cutoff of 70 Hz were applied. (C) Ictal EEG associated with each spasm event showed diffuse, low-amplitude, fast wave bursts without a consistent leading wave in the left hemisphere. The aforementioned data failed to suggest a causal relationship between the cortical lesion in the left temporal lobe and generation of epileptic spasms (Table 5). Our surgical intention was complete removal of the brain tumor and the surrounding tissues. Intraoperative electrocorticography (ECoG) showed multifocal interictal epileptiform discharges in the left temporal and extratemporal regions. Postoperative ECoG following a left temporal lobectomy that included the brain tumor continued to show interictal epileptiform discharges in the extratemporal region. (D) Postoperative MRI confirmed successful resection of the brain tumor. However, postoperative scalp EEG showed multifocal and independent interictal spike-wave discharges bilaterally and the patient continued to have epileptic spasms. Pathology revealed a high-grade glioma.
(A) A 30-month-old boy with a history of intractable epileptic spasms underwent implantation of subdural electrodes over the left hemisphere. (B) Ictal HFOs were initially generated by the left frontal region and propagated to other sites. High-pass filer cutoff: 0.08 Hz. Low-pass filter cutoff: 300 Hz. (C) Time-frequency analysis showed that augmentation of HFOs at 80–200 Hz preceded those at beta range.
A 3-year-old, ambidextrous girl with a history of daily epileptic spasms underwent epilepsy surgery. She developed seizures at 6 months of age and had no focal neurological deficits prior to surgery. (A) Preoperative MRI showed no discrete cortical lesion except for a smaller white matter volume in the left hemisphere, especially in the frontal lobe. (B) Interictal scalp EEG is presented with a high-pass filter cuttoff of 1.0 Hz and low-pass filter cutoff of 70 Hz. The posterior background rhythm was 8 Hz alpha activity, and interictal epileptiform discharges involved the left hemisphere slightly more frequently than the right hemisphere. The left temporal lobe showed the most frequent focal spike-wave discharges. (C) Ictal EEG showed diffuse fast wave bursts superimposed on delta waves with anterior dominance; an arrow indicates the onset of a spasm. (D) Subdural grid and strip electrodes were placed on the left hemisphere. The right frontal-parietal regions were also sampled by four strip electrodes (not shown). (E) Interictal ECoG is presented with a high-pass filter cuttoff of 160 Hz and low-pass filter cutoff of 300 Hz. Interictal epileptiform discharges independently involved multiple locations in the left hemisphere such as the left frontal, temporal, parietal and occipital regions as well as the right hemisphere. The rate of interictal spike discharges in the left primary motor area for the hand (Channel 3) was much smaller than those of the remaining regions. Spontaneous high-frequency oscillations (HFOs) at >160 Hz were generated at Channel 3 during non-REM sleep (arrow). (F) Ictal ECoG during a spasm event (arrow) showed sustained and widespread HFOs involving the left premotor and supplementary motor regions earlier than the onset of epileptic spasms denoted by an arrow. Ictal HFOs involved the left hemisphere often more intensely and earlier than the right hemisphere. Following considerable discussion with the family, we decided to perform multilobar resection involving the left frontal, parietal, occipital and temporal regions, but not to remove the left Rolandic or insular cortices, in order to preserve motor function. Pathology showed gliosis without definitive dysplasia. She has been seizure-free following this surgery for four months with development of mild right hemiparesis but with maintained grasping function on the hemiparetic side. Achievement of post-operative seizure-freedom is strong evidence that the left hemispheric cortex was causally associated with the generation of epileptic spasms in this patient, and it is likely that a substantial proportion of the epileptogenic zone has been removed. Short-term follow-up data failed to support the causal relationship between HFOs spontaneously generated by the left Rolandic cortex (Channel 3) and generation of epileptic spasms, in this case. Long-term follow-up is needed to determine whether the epileptogenic zone still exists in the remaining cortical regions and whether anatomical hemispherectomy was the better option to completely eliminate seizures. It will remain unknown whether a less extensive resection would have achieved similar seizure-freedom.
A 3-year-old, ambidextrous girl with a history of daily epileptic spasms underwent epilepsy surgery. She developed seizures at 6 months of age and had no focal neurological deficits prior to surgery. (A) Preoperative MRI showed no discrete cortical lesion except for a smaller white matter volume in the left hemisphere, especially in the frontal lobe. (B) Interictal scalp EEG is presented with a high-pass filter cuttoff of 1.0 Hz and low-pass filter cutoff of 70 Hz. The posterior background rhythm was 8 Hz alpha activity, and interictal epileptiform discharges involved the left hemisphere slightly more frequently than the right hemisphere. The left temporal lobe showed the most frequent focal spike-wave discharges. (C) Ictal EEG showed diffuse fast wave bursts superimposed on delta waves with anterior dominance; an arrow indicates the onset of a spasm. (D) Subdural grid and strip electrodes were placed on the left hemisphere. The right frontal-parietal regions were also sampled by four strip electrodes (not shown). (E) Interictal ECoG is presented with a high-pass filter cuttoff of 160 Hz and low-pass filter cutoff of 300 Hz. Interictal epileptiform discharges independently involved multiple locations in the left hemisphere such as the left frontal, temporal, parietal and occipital regions as well as the right hemisphere. The rate of interictal spike discharges in the left primary motor area for the hand (Channel 3) was much smaller than those of the remaining regions. Spontaneous high-frequency oscillations (HFOs) at >160 Hz were generated at Channel 3 during non-REM sleep (arrow). (F) Ictal ECoG during a spasm event (arrow) showed sustained and widespread HFOs involving the left premotor and supplementary motor regions earlier than the onset of epileptic spasms denoted by an arrow. Ictal HFOs involved the left hemisphere often more intensely and earlier than the right hemisphere. Following considerable discussion with the family, we decided to perform multilobar resection involving the left frontal, parietal, occipital and temporal regions, but not to remove the left Rolandic or insular cortices, in order to preserve motor function. Pathology showed gliosis without definitive dysplasia. She has been seizure-free following this surgery for four months with development of mild right hemiparesis but with maintained grasping function on the hemiparetic side. Achievement of post-operative seizure-freedom is strong evidence that the left hemispheric cortex was causally associated with the generation of epileptic spasms in this patient, and it is likely that a substantial proportion of the epileptogenic zone has been removed. Short-term follow-up data failed to support the causal relationship between HFOs spontaneously generated by the left Rolandic cortex (Channel 3) and generation of epileptic spasms, in this case. Long-term follow-up is needed to determine whether the epileptogenic zone still exists in the remaining cortical regions and whether anatomical hemispherectomy was the better option to completely eliminate seizures. It will remain unknown whether a less extensive resection would have achieved similar seizure-freedom.
A 3-year-old, ambidextrous girl with a history of daily epileptic spasms underwent epilepsy surgery. She developed seizures at 6 months of age and had no focal neurological deficits prior to surgery. (A) Preoperative MRI showed no discrete cortical lesion except for a smaller white matter volume in the left hemisphere, especially in the frontal lobe. (B) Interictal scalp EEG is presented with a high-pass filter cuttoff of 1.0 Hz and low-pass filter cutoff of 70 Hz. The posterior background rhythm was 8 Hz alpha activity, and interictal epileptiform discharges involved the left hemisphere slightly more frequently than the right hemisphere. The left temporal lobe showed the most frequent focal spike-wave discharges. (C) Ictal EEG showed diffuse fast wave bursts superimposed on delta waves with anterior dominance; an arrow indicates the onset of a spasm. (D) Subdural grid and strip electrodes were placed on the left hemisphere. The right frontal-parietal regions were also sampled by four strip electrodes (not shown). (E) Interictal ECoG is presented with a high-pass filter cuttoff of 160 Hz and low-pass filter cutoff of 300 Hz. Interictal epileptiform discharges independently involved multiple locations in the left hemisphere such as the left frontal, temporal, parietal and occipital regions as well as the right hemisphere. The rate of interictal spike discharges in the left primary motor area for the hand (Channel 3) was much smaller than those of the remaining regions. Spontaneous high-frequency oscillations (HFOs) at >160 Hz were generated at Channel 3 during non-REM sleep (arrow). (F) Ictal ECoG during a spasm event (arrow) showed sustained and widespread HFOs involving the left premotor and supplementary motor regions earlier than the onset of epileptic spasms denoted by an arrow. Ictal HFOs involved the left hemisphere often more intensely and earlier than the right hemisphere. Following considerable discussion with the family, we decided to perform multilobar resection involving the left frontal, parietal, occipital and temporal regions, but not to remove the left Rolandic or insular cortices, in order to preserve motor function. Pathology showed gliosis without definitive dysplasia. She has been seizure-free following this surgery for four months with development of mild right hemiparesis but with maintained grasping function on the hemiparetic side. Achievement of post-operative seizure-freedom is strong evidence that the left hemispheric cortex was causally associated with the generation of epileptic spasms in this patient, and it is likely that a substantial proportion of the epileptogenic zone has been removed. Short-term follow-up data failed to support the causal relationship between HFOs spontaneously generated by the left Rolandic cortex (Channel 3) and generation of epileptic spasms, in this case. Long-term follow-up is needed to determine whether the epileptogenic zone still exists in the remaining cortical regions and whether anatomical hemispherectomy was the better option to completely eliminate seizures. It will remain unknown whether a less extensive resection would have achieved similar seizure-freedom.
A 3-year-old, ambidextrous girl with a history of daily epileptic spasms underwent epilepsy surgery. She developed seizures at 6 months of age and had no focal neurological deficits prior to surgery. (A) Preoperative MRI showed no discrete cortical lesion except for a smaller white matter volume in the left hemisphere, especially in the frontal lobe. (B) Interictal scalp EEG is presented with a high-pass filter cuttoff of 1.0 Hz and low-pass filter cutoff of 70 Hz. The posterior background rhythm was 8 Hz alpha activity, and interictal epileptiform discharges involved the left hemisphere slightly more frequently than the right hemisphere. The left temporal lobe showed the most frequent focal spike-wave discharges. (C) Ictal EEG showed diffuse fast wave bursts superimposed on delta waves with anterior dominance; an arrow indicates the onset of a spasm. (D) Subdural grid and strip electrodes were placed on the left hemisphere. The right frontal-parietal regions were also sampled by four strip electrodes (not shown). (E) Interictal ECoG is presented with a high-pass filter cuttoff of 160 Hz and low-pass filter cutoff of 300 Hz. Interictal epileptiform discharges independently involved multiple locations in the left hemisphere such as the left frontal, temporal, parietal and occipital regions as well as the right hemisphere. The rate of interictal spike discharges in the left primary motor area for the hand (Channel 3) was much smaller than those of the remaining regions. Spontaneous high-frequency oscillations (HFOs) at >160 Hz were generated at Channel 3 during non-REM sleep (arrow). (F) Ictal ECoG during a spasm event (arrow) showed sustained and widespread HFOs involving the left premotor and supplementary motor regions earlier than the onset of epileptic spasms denoted by an arrow. Ictal HFOs involved the left hemisphere often more intensely and earlier than the right hemisphere. Following considerable discussion with the family, we decided to perform multilobar resection involving the left frontal, parietal, occipital and temporal regions, but not to remove the left Rolandic or insular cortices, in order to preserve motor function. Pathology showed gliosis without definitive dysplasia. She has been seizure-free following this surgery for four months with development of mild right hemiparesis but with maintained grasping function on the hemiparetic side. Achievement of post-operative seizure-freedom is strong evidence that the left hemispheric cortex was causally associated with the generation of epileptic spasms in this patient, and it is likely that a substantial proportion of the epileptogenic zone has been removed. Short-term follow-up data failed to support the causal relationship between HFOs spontaneously generated by the left Rolandic cortex (Channel 3) and generation of epileptic spasms, in this case. Long-term follow-up is needed to determine whether the epileptogenic zone still exists in the remaining cortical regions and whether anatomical hemispherectomy was the better option to completely eliminate seizures. It will remain unknown whether a less extensive resection would have achieved similar seizure-freedom.
A 3-year-old, ambidextrous girl with a history of daily epileptic spasms underwent epilepsy surgery. She developed seizures at 6 months of age and had no focal neurological deficits prior to surgery. (A) Preoperative MRI showed no discrete cortical lesion except for a smaller white matter volume in the left hemisphere, especially in the frontal lobe. (B) Interictal scalp EEG is presented with a high-pass filter cuttoff of 1.0 Hz and low-pass filter cutoff of 70 Hz. The posterior background rhythm was 8 Hz alpha activity, and interictal epileptiform discharges involved the left hemisphere slightly more frequently than the right hemisphere. The left temporal lobe showed the most frequent focal spike-wave discharges. (C) Ictal EEG showed diffuse fast wave bursts superimposed on delta waves with anterior dominance; an arrow indicates the onset of a spasm. (D) Subdural grid and strip electrodes were placed on the left hemisphere. The right frontal-parietal regions were also sampled by four strip electrodes (not shown). (E) Interictal ECoG is presented with a high-pass filter cuttoff of 160 Hz and low-pass filter cutoff of 300 Hz. Interictal epileptiform discharges independently involved multiple locations in the left hemisphere such as the left frontal, temporal, parietal and occipital regions as well as the right hemisphere. The rate of interictal spike discharges in the left primary motor area for the hand (Channel 3) was much smaller than those of the remaining regions. Spontaneous high-frequency oscillations (HFOs) at >160 Hz were generated at Channel 3 during non-REM sleep (arrow). (F) Ictal ECoG during a spasm event (arrow) showed sustained and widespread HFOs involving the left premotor and supplementary motor regions earlier than the onset of epileptic spasms denoted by an arrow. Ictal HFOs involved the left hemisphere often more intensely and earlier than the right hemisphere. Following considerable discussion with the family, we decided to perform multilobar resection involving the left frontal, parietal, occipital and temporal regions, but not to remove the left Rolandic or insular cortices, in order to preserve motor function. Pathology showed gliosis without definitive dysplasia. She has been seizure-free following this surgery for four months with development of mild right hemiparesis but with maintained grasping function on the hemiparetic side. Achievement of post-operative seizure-freedom is strong evidence that the left hemispheric cortex was causally associated with the generation of epileptic spasms in this patient, and it is likely that a substantial proportion of the epileptogenic zone has been removed. Short-term follow-up data failed to support the causal relationship between HFOs spontaneously generated by the left Rolandic cortex (Channel 3) and generation of epileptic spasms, in this case. Long-term follow-up is needed to determine whether the epileptogenic zone still exists in the remaining cortical regions and whether anatomical hemispherectomy was the better option to completely eliminate seizures. It will remain unknown whether a less extensive resection would have achieved similar seizure-freedom.
A 3-year-old, ambidextrous girl with a history of daily epileptic spasms underwent epilepsy surgery. She developed seizures at 6 months of age and had no focal neurological deficits prior to surgery. (A) Preoperative MRI showed no discrete cortical lesion except for a smaller white matter volume in the left hemisphere, especially in the frontal lobe. (B) Interictal scalp EEG is presented with a high-pass filter cuttoff of 1.0 Hz and low-pass filter cutoff of 70 Hz. The posterior background rhythm was 8 Hz alpha activity, and interictal epileptiform discharges involved the left hemisphere slightly more frequently than the right hemisphere. The left temporal lobe showed the most frequent focal spike-wave discharges. (C) Ictal EEG showed diffuse fast wave bursts superimposed on delta waves with anterior dominance; an arrow indicates the onset of a spasm. (D) Subdural grid and strip electrodes were placed on the left hemisphere. The right frontal-parietal regions were also sampled by four strip electrodes (not shown). (E) Interictal ECoG is presented with a high-pass filter cuttoff of 160 Hz and low-pass filter cutoff of 300 Hz. Interictal epileptiform discharges independently involved multiple locations in the left hemisphere such as the left frontal, temporal, parietal and occipital regions as well as the right hemisphere. The rate of interictal spike discharges in the left primary motor area for the hand (Channel 3) was much smaller than those of the remaining regions. Spontaneous high-frequency oscillations (HFOs) at >160 Hz were generated at Channel 3 during non-REM sleep (arrow). (F) Ictal ECoG during a spasm event (arrow) showed sustained and widespread HFOs involving the left premotor and supplementary motor regions earlier than the onset of epileptic spasms denoted by an arrow. Ictal HFOs involved the left hemisphere often more intensely and earlier than the right hemisphere. Following considerable discussion with the family, we decided to perform multilobar resection involving the left frontal, parietal, occipital and temporal regions, but not to remove the left Rolandic or insular cortices, in order to preserve motor function. Pathology showed gliosis without definitive dysplasia. She has been seizure-free following this surgery for four months with development of mild right hemiparesis but with maintained grasping function on the hemiparetic side. Achievement of post-operative seizure-freedom is strong evidence that the left hemispheric cortex was causally associated with the generation of epileptic spasms in this patient, and it is likely that a substantial proportion of the epileptogenic zone has been removed. Short-term follow-up data failed to support the causal relationship between HFOs spontaneously generated by the left Rolandic cortex (Channel 3) and generation of epileptic spasms, in this case. Long-term follow-up is needed to determine whether the epileptogenic zone still exists in the remaining cortical regions and whether anatomical hemispherectomy was the better option to completely eliminate seizures. It will remain unknown whether a less extensive resection would have achieved similar seizure-freedom.
Source: PubMed