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Centre de Recherche en Neuropsychologie et Cognition, Université de Montréal, Département de Psychologie, C.P. 6128, Succ. Centre-Ville, Montréal, Québec, H3C 3J7, CanadaCentre de Recherche de l’Hôpital Sainte-Justine, Hôpital Sainte-Justine, 3175 Chemin de la Côte Sainte-Catherine, Montréal, Québec, H3T 1C5, Canada
Centre de Recherche en Neuropsychologie et Cognition, Université de Montréal, Département de Psychologie, C.P. 6128, Succ. Centre-Ville, Montréal, Québec, H3C 3J7, CanadaCentre de Recherche de l’Hôpital Sainte-Justine, Hôpital Sainte-Justine, 3175 Chemin de la Côte Sainte-Catherine, Montréal, Québec, H3T 1C5, Canada
Centre de Recherche en Neuropsychologie et Cognition, Université de Montréal, Département de Psychologie, C.P. 6128, Succ. Centre-Ville, Montréal, Québec, H3C 3J7, CanadaCentre de Recherche de l’Hôpital Sainte-Justine, Hôpital Sainte-Justine, 3175 Chemin de la Côte Sainte-Catherine, Montréal, Québec, H3T 1C5, Canada
Centre de Recherche de l’Hôpital Sainte-Justine, Hôpital Sainte-Justine, 3175 Chemin de la Côte Sainte-Catherine, Montréal, Québec, H3T 1C5, CanadaService de Neurologie, Hôpital Sainte-Justine, 3175 Chemin de la Côte Sainte-Catherine, Montréal, H3T 1C5, Québec, Canada
Centre de Recherche en Neuropsychologie et Cognition, Université de Montréal, Département de Psychologie, C.P. 6128, Succ. Centre-Ville, Montréal, Québec, H3C 3J7, CanadaCentre de Recherche de l’Hôpital Sainte-Justine, Hôpital Sainte-Justine, 3175 Chemin de la Côte Sainte-Catherine, Montréal, Québec, H3T 1C5, Canada
Centre de Recherche en Neuropsychologie et Cognition, Université de Montréal, Département de Psychologie, C.P. 6128, Succ. Centre-Ville, Montréal, Québec, H3C 3J7, Canada
Near-infrared spectroscopy (NIRS) is a novel imaging technique of potential value in the pre-surgical investigation of patients with refractory epilepsy. We recorded simultaneously electrophysiology (EEG; Compumedics, USA) and near-infrared spectroscopy (NIRS; ISS, USA) to examine the localization of the ictal onset zone and assess language lateralization in a young epileptic boy (L.H., 10 years) as part of his pre-surgical evaluation. L.H. underwent a prolonged EEG–NIRS recording while electro-clinical and electrical seizures were recorded. Results were compared to those obtained with other pre-surgical techniques (SPECT, FDG–PET, EEG–fMRI and EEG–MEG) and showed good concordance for ictal onset zone localization. A second NIRS session without EEG was carried out in order to investigate language lateralization. For this purpose, the patient performed a categorical verbal-fluency task during NIRS recordings. Results showed left-hemisphere dominance for language function in this young boy. This case report illustrates that multi-channel EEG–NIRS has the potential to contribute favourably to pre-surgical investigation in young patients.
Localization of the ictal onset zone remains a challenge in patients with non-lesional neocortical epilepsy. When the magnetic resonance imaging (MRI) is normal, one must typically rely on the clinical and neuropsychological evaluations, video-electroencephalography (EEG) monitoring, inter-ictal/ictal single-photon emission computed tomography (SPECT), and positron-emission tomography (PET).
When these complementary assessments fail to adequately localize the ictal onset zone, invasive intracranial electrode studies are required but carry a risk of infection and haemorrhage.
In some cases, language lateralization investigation is also required and it is usually done using the invasive intracarotid amobarbital test (IAT) in conjunction with neuropsychological testing. In order to reduce the number of cases where invasive procedures are necessary, recent imaging techniques such as functional MRI (fMRI) and magnetoencephalography (MEG) have been used to investigate language dominance,
and have replaced the IAT in some clinical centers. Promising non-invasive techniques such as simultaneous EEG–fMRI and EEG–MEG have also been considered for localizing the ictal onset area.
However, these techniques are sometimes difficult to use with young children and patients with serious cognitive or behavioural problems. Alternatively in such cases, near-infrared spectroscopy (NIRS) can non-invasively monitor rapid changes of regional cerebral blood volume (rCBV) (as seen during seizures or functional activations). NIRS is a relatively new technique, which allows the measurement of haemodynamic changes associated with neural activity.
The different light absorption spectra of oxy-haemoglobin (HbO) and deoxy-haemoglobin (HbR) within the near-infrared spectrum allow for the measurement of concentration changes of these substances in living tissues.
Near-infrared light of two wavelengths between 680 and 1000 nm is directed through optic fibres to the head of the patient. The amount of detected light reflects the amount of absorption of the two wavelengths in targeted cerebral areas informing on concentration changes of HbO and HbR in the regions (for review see Ref.
First, it allows independent measurement of concentration changes of HbO and HbR, as well as measurement of total haemoglobin (HbT), which is the sum of HbO and HbR. Second, the equipment is portable
and less costly than fMRI or MEG. Finally, and most importantly, there are no major restrictions on movements or verbalization during recording, which renders the technique suitable for studies in mentally challenged people as well as in young children, even infants.
A potential disadvantage of this technique is the shallow penetration of the photons (between 3 and 5 cm), which renders it difficult or impossible to collect reliable data from subcortical structures. Nonetheless, a spatial resolution below 1 cm can be obtained in most patients. Furthermore, the limited penetration should not have a major impact on studies investigating cortical areas, especially in children who usually have a thinner skull than adults.
Preliminary NIRS studies with adult epileptic populations showed different haemodynamic patterns among different seizure types. More precisely, an increase of rCBV, HbT and HbO, has been reported on the side of seizure focus during complex partial seizures,
showing good concordance with other techniques (IAT and fMRI).
Case report
L.H. is a 10 year-old right-handed boy with no seizure risk factor except for an uncle who suffered from refractory seizures. Onset of seizures occurred at 4 years. Seizures were characterized by daily auras of fear with or without startle, vocalization, left ocular and head deviation. Multiple anti-epileptic drugs failed to control seizures. EEG revealed a probable right frontal origin (F4) but no clear lesion was identified on anatomical MRI. Multiple functional imaging techniques were also used. Ictal 99mTc-ECD SPECT (injection 1 s after seizure onset) showed a major activation in the right superior frontal and parasagittal areas with an extension to the right antero-lateral frontal regions. FDG–PET showed a right frontal hypometabolism. L.H. also underwent testing with novel sophisticated techniques (simultaneous EEG–fMRI and simultaneous EEG–MEG). Simultaneous EEG–fMRI acquisitions were used to measure within the whole brain haemodynamic responses that correlate with epileptic discharges detected on scalp EEG (for methodological details see Ref.
). Simultaneous EEG–MEG were used to measure directly on the scalp electric and magnetic components of signals generated by a population of pyramidal neurons synchronously active during an epileptic discharge (temporal resolution—1 ms, high density covering all the surface of the head: 275 sensors in a VSM-CTF MEG system and 64 EEG electrodes). MEG cerebral sources have been estimated using the maximun entropy on the mean (MEM) source localization technique.
Electro-clinical seizures were recorded during both procedures and marked by an expert electroencephalographer. BOLD response and MEG source localization showed a clear activation over the right frontal pole during seizure activity. Ictal EEG–fMRI and ictal EEG–MEG were made possible as the patient had several electrical seizures and clinical seizures only accompanied by a sensation of fear with little or no movement. Thus, all functional techniques pointed towards a right frontal epileptic focus. Paradoxically, neuropsychological data reported language deficits (word finding difficulties, slowing of verbal information planning and processing), and attention deficits. L.H underwent a prolonged and simultaneous EEG–NIRS recording in order to evaluate the potential of this non-invasive technique to improve the localization of the ictal onset zone. A second NIRS recording without EEG was done to assess language lateralization. The project was previously approved by the Ethics Committees of the Sainte-Justine and Notre-Dame Hospitals.
Methods
L.H. underwent a 3-h EEG–NIRS video-monitoring session during which electro-clinical and electrical seizures were recorded. During recording, the patient remained passive or slept. One hundred and twenty-eight-NIRS channels placed over right frontal, bilateral parasagittal regions and bilateral rolandic regions were recorded using a multi-channel Imagent Tissue Oxymeter (ISS, USA). Optical intensity (DC), modulation amplitude (AC) and phase data were sampled at 39.0625 Hz. DC data were filtered using a band-pass filter of 0.1–0.001 Hz, normalized by dividing each value by the mean value across time points and transformed to quantify concentration changes of HbO and HbR for each channel. Standard EEG video-monitoring was carried out simultaneously using 18 home-made carbon fibre electrodes and a NeuroScan Synamps 2™ system (Compumedics, USA). EEG data were acquired using a sampling rate of 500 Hz with a band-pass filter of 0.1–100 Hz and a central reference. Electrodes were placed based on the 10–20 system (Fz, Cz, Pz, Oz, Fp1, F3, C3, P3, O1, Fp2, F4, C4, P4, O2, VeOg, HeOg, EKG and EMG). Epileptic activity on EEG and clinical manifestations recorded on video were used to assess onset and duration of seizures. Epileptic discharges were manually marked by an expert electroencephalographer. Optical fibres and electrodes were placed on the surface of the head, using a light and comfortable, but rigid, helmet, which can be adapted to all head sizes without restricting head movements.
In order to investigate language dominance, a second NIRS recording without EEG was carried out using 128-NIRS channels that covered left Broca's and Wernicke's areas and right homologous areas. During this session, NIRS data were acquired in a block design paradigm and were then averaged by blocks (10 for each task). A categorical verbal-fluency task (language task) and a non-sense syllable repetition task (oro-motor control task) were performed by L.H. during this second NIRS recording.
For each NIRS session, an anatomically specific montage was created based on the MRI of the patient to ensure that optical fibres were placed over the regions of interest (ROI) specific to the objectives of each recording session (session 1: right frontal, bilateral parasagittal areas and bilateral rolandic regions; session 2: left Broca's and Wernicke's areas and right mirror areas). This was done using a stereotaxic system (Brainsight™ Frameless 39, Rogue Research, Canada), which enables the transfer of ROI, determined by MRI, onto the helmet. The location of each optical fibre and four fiducial points was digitized and recorded using the same stereotaxic system to allow for precise alignment of the NIRS and the anatomical data.
Results
Two electro-clinical and two electrical seizures were recorded over the prolonged EEG–NIRS session. Electro-clinical seizures lasted longer (30–36 s compared to 4–13 s) and elicited larger haemodynamic responses than electrical seizures (Fig. 1). These seizures were characterized by a sudden sensation of fear. Electrically, ictal rhythmic spikes over the right fronto-central region were noted. Ictal NIRS mapping co-registered onto the MRI revealed a high increase of rCBV over the right frontal region during both electro-clinical and electrical seizures. More specifically, an initial dip (short reduction of HbO) was measured before an increase of HbT and HbO, and small decrease of HbR, showing a cerebral activation in this region. This was in good concordance with other functional techniques that were used in this patient (Fig. 2).
Figure 1EEG–NIRS results during electro-clinical (left) and electrical seizures (right). (a) EEG data during seizures are shown on the top. Simultaneous NIRS data during seizures are shown by (b) the graphs and (c) haemoglobin map images. (b) Graphs show NIRS cerebral activation in five NIRS channels covering right fronto-polar region. Cerebral activation is characterized by an enhanced HbO (solid lines) as well as a small and late decrease of HbR (dotted lines), happening a few seconds after the beginning of the seizure. The horizontal red bar shows the seizure duration. A typical initial dip (short reduction of HbO) is also obtained before the activation. This haemodynamic signal has been measured on the right frontal area (see 3D MRI reconstruction in the middle), which is indicated by the green rectangle. It is important to note that the scale between both graphs are greatly different (5×) which is showing that cerebral activation is much more prominent during the electro-clinical seizure compared to the electrical seizure. The importance of this amplitude difference between both haemodynamic responses did not allow using the same scale. (c) Enhancement of HbT and HbO is shown by haemoglobin map images, where the rectangle is the same as the right frontal rectangle seen on the 3D MRI reconstruction. This activation corresponds temporally to the peak of HbO enhancement.
Figure 2Results from SPECT (substraction ictal SPECT co-registered on MRI, SISCOM) (top left), PET (top right), as well as BOLD response detected using EEG–fMRI (bottom left) and EEG–MEG source (bottom right) for similar EEG discharges showed a good concordance between functional neuro-imaging techniques suggesting a right frontal ictal onset zone (red arrows) in this 10 year-old epileptic boy.
Subsequent NIRS recording was carried out comparing activations elicited by the language task (verbal fluency) and the control task (non-sense syllables repetition task). Temporally, left and right cerebral activations occurred simultaneously around 30 s after baseline, at the end of the verbal-fluency task (Fig. 3c). The latter result suggests that some language reorganization may have taken place in the right hemisphere. Overall, however, a more prominent increase of HbO and decrease of HbR were measured in the left inferior–posterior frontal gyrus (Broca's area) compared to its right analogous region, indicating a left hemispheric specialization for language (Fig. 3b).
Figure 3This figure shows NIRS results during a language task used in order to investigate language lateralization. (a) A 3D MRI reconstruction of the patient with the position of the NIRS fibres is shown (sources in red, detectors in yellow). Fibre locations on left Broca's and Wernicke's areas and their right homologous regions were determined using a stereotaxic system and the patient's MRI. (b) A clear enhancement of HbO in Broca's area (left hemisphere) 28 s after the beginning of the language task suggests a left language lateralization in L.H. A decrease of HbR was also present but not shown here (see c). (c) Graph shows haemoglobin concentration changes during one-block time course including baseline (−30 to 0 s), language task (0–30 s), resting period (30–60 s), control task (60–90 s) and resting period (90–105 s). Left-hemisphere activations are illustrated in blue and right hemisphere activations in pink. HbO concentrations are indicated as solid lines whereas HbR levels correspond to the dotted lines. Left-hemisphere activation is shown by an increase in HbO concentration and a decrease in HbR concentration occurring at the end of the language task (starting 30 s after baseline, see black arrow on the graph, and peaking a few seconds later). This activation is more pronounced than the activation seen in the right hemisphere (see difference between blue and pink lines between 35 and 70 s, horizontal green line). A typical initial dip (short reduction of HbO) is also obtained before the activation (black arrow). These data are from a central channel in Broca's area (blue lines) and a central channel in the right homologous region (pink lines).
Following a case discussion and examination of different imaging results, it was decided not to proceed to a long-term invasive video-EEG study but rather limit ourselves with intraoperative electrocorticography immediately followed by surgery. Intraoperative electrocorticography revealed continuous spiking over the suspected right frontal focus and a limited right frontal corticectomy was performed. The patient has remained seizure-free since the surgery (follow-up 11 months) compared to multiple daily seizures pre-operatively. Pathological analysis of the resected tissue revealed an underlying cortical dysplasia.
Discussion
The present work is the first study using 128-NIRS channels non-invasively during spontaneous seizures to localize the ictal onset zone and investigate language lateralization in a young epileptic patient. A clear activation was obtained using simultaneous EEG–NIRS in the right frontal region during epileptic seizures in this 10 year-old boy, with refractory MRI-negative right frontal epilepsy. Adelson et al.
used NIRS to record various types of epileptic seizures in 15 children. However, these studies used only a few optodes placed over the patient's forehead and were not meant to localize the seizure focus. In another study, Watanabe et al.
applied NIRS monitoring to measure bemegride-induced ictal changes in CBV in 26 (mainly adult) patients undergoing intracranial recordings using 8–24 channels mounted on a thermoplastic splint shell so as to cover the prospective focus region as determined by a prior pre-surgical evaluation. Compared to the present study, Watanabe et al.
used much lesser NIRS channels and most of the recorded seizures were chemically induced. Surgical outcome was not mentioned.
In the present study, EEG–NIRS results were in very good concordance with the data obtained from other functional imaging techniques (SPECT, FDG–PET, EEG–fMRI and EEG–MEG) in this young patient. Furthermore, NIRS allowed to assess his language dominance non-invasively as previously reported by Watanabe et al.
This case report illustrates that continuous EEG–NIRS has the potential to contribute favourably to the localization of the ictal onset zone and assessment of language lateralization. This recent technique is non-invasive, more resistant to movement artefacts than other techniques and can easily be performed in young patients. It has a better temporal resolution than SPECT and fMRI and provides quantitative information about HbT, HbO and HbR compared to fMRI BOLD signal based principally on HbR variations.
In future studies, it would be interesting to cover even more extensively the whole scalp. For some patients, careful revision of MRI guided by NIRS could lead to the detection of subtle epileptogenic lesions previously missed by visual inspection. For others, EEG–NIRS combined with classical and other novel non-invasive techniques may reduce the need for invasive monitoring. And for those who still require an intracranial study, NIRS can potentially allow more accurate electrode positioning or reduce the extent of the studied zone. Moreover, NIRS has the potential to become a viable, non-invasive alternative to IAT, fMRI and MEG in the determination of speech lateralization in children and clinical populations who fail to remain motionless or are reluctant to submit to more invasive techniques.
Acknowledgements
We are grateful to Jean Gotman, M.D., Ph.D., Louise Tyvaert, M.D., Éliane Kobayashi, M.D., Ph.D. for providing EEG–fMRI data, helping in EEG–MEG analyses and their fruitful discussions. We are also indebted to the engineering and orthotic prosthesis team of Sainte-Justine Hospital and Marie-Enfant Hospital Center for their help in developing the optical imaging helmet.
This work was supported by funds from the Savoy Foundation held by Dr. Dang K. Nguyen, the Canada Research Chair in Developmental Neuropsychology held by Dr. Maryse Lassonde, the Canada Research Chair in Cognitive Neurosciences held by Dr. Franco Lepore, as well as scholarships by the Canadian Institutes of Health Research (CIHR), the Canadian Federation of University Women (CFUW), and the Fonds de la Recherche en Santé du Québec (FRSQ) awarded to Anne Gallagher, M.Ps. We confirm that we have read the journal's position on issues involved in ethical publication and affirm that this report is consistent with these guidelines.
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