Functional imaging in the work-up of childhood epilepsy

Functional magnetic resonance imaging

Among the range of methods available to assess neurodevelopmental disorders, functional MRI (fMRI) has been a preferred tool of choice. Indeed, fMRI can reveal functional alterations in brain networks, irrespective of their structural integrity. Yet, whether fMRI studies have provided unique added value and influenced the clinical care and assessments in children with these conditions remains controversial. This chapter aims to give an overview of the clinical use of task-based as well as resting-state fMRI in children with neurodevelopmental disorders, such as dyslexia, DLD, and epilepsy. We introduce analysis methods that appear promising (namely PPI and machine learning) and describe strengths and limitations of fMRI in the field of pediatrics. Altogether, we suggest that fMRI has provided us with a unique understanding of some developmental conditions. Indeed, findings from group studies have both informed neuroanatomical models and revealed compensation mechanisms. In addition, improvements have made fMRI an increasingly child-friendly method. Nevertheless, clinicians should be aware of limitations, including (1) lack of replication of results, (2) the limited specificity as a diagnostic tool, and (3) difficulties with interpretation of findings. The use of fMRI in the clinic currently remains restricted, with the exception of epilepsy surgery planning, where it is used routinely.

Role of presurgical functional MRI and diffusion MR tractography in pediatric low-grade brain tumor surgery: a single-center study

PURPOSE

Presurgical functional MRI (fMRI) and diffusion MRI tractography (dMRI tractography) are widely employed to delineate eloquent brain regions and their connections prior to brain tumor resection in adults. However, such studies are harder to perform in children, resulting in suboptimal neurosurgical care in pediatric brain tumor surgery as compared to adults. Thus, our objective was to assess the feasibility and the influence of presurgical advanced MR imaging on neurosurgical care in pediatric brain tumor surgery.

METHODS

Retrospective analyses of 31 presurgical fMRI/dMRI tractography studies were performed in children with low-grade tumors near eloquent brain regions at our site between 2005 and 2017.

RESULTS

In only 3/31 cases, imaging results were not interpretable (10%). All 28 successful imaging sessions were used for neurosurgical risk assessment. Based on this, surgery was canceled in 2/28 patients, and intention to treat was changed in 5/28 patients. In 4/28 cases, the surgical approach was changed and in 10/28, electrode placement for intraoperative neurophysiological monitoring was guided by imaging results. Gross total resection (GTR) was planned in 21/28 cases and could be achieved in 15/21 (71%). Despite highly eloquent tumor location, only four children suffered from a mild permanent neurological deficit after the operation.

CONCLUSIONS

We demonstrate that presurgical fMRI/dMRI tractography can have a profound impact on pediatric brain tumor management, optimizing preoperative risk-assessment and pre- as well as intraoperative decision-making. We believe that these tools should be offered to children suffering from eloquent brain tumors as part of a comprehensive operative work-up.

Language plasticity after hemispherotomy of the dominant hemisphere in 3 patients: Implication of non-linguistic networks

The neural networks involved in language recovery following hemispherotomy of the dominant hemisphere after language acquisition in children remain poorly known. Twelve hemispherotomized children (mean age at surgery: 11.3years) with comparable post-operative neuropsychological patterns underwent multi-task language functional MRI. Three of them had recovered from an initial postoperative aphasia i.e., hemispherotomy was performed on the language-dominant hemisphere. Our main results revealed (1) perisylvian activations in all patients after either left or right hemispherotomy; (2) no differences in activations between groups regarding the side of hemispherotomy; (3) additional activations in pre-frontal (3/3) and hippocampal/parahippocampal and occipito-parietal (2/3) areas, when comparing language activation in each of the three subjects with hemispherotomy of the language-dominant hemisphere to the group of 9 non-dominant hemispherotomized patients. These neural networks support the stronger engagement of learning and memory during language recovery in a hemisphere that was not initially actively subserving language.

Presurgical language mapping in children with epilepsy: clinical usefulness of functional magnetic resonance imaging for the planning of cortical stimulation

PURPOSE

Presurgical language mapping in dominant hemisphere epilepsy to evaluate the risk of postoperative deficit is particularly difficult in children. Extraoperative invasive cortical stimulation can show some areas critical to language, but not all of them, due to scarce sampling, poor cooperation, cortical immaturity, or network reorganization, whereas functional magnetic resonance imaging (fMRI) displays entire networks involved in, but not necessarily critical to, language. In a homogeneous series of children with epilepsy, we compared the contributions of language fMRI and depth electrode stimulations to optimize language mapping.

METHODS

Eight children (7.5-15.5 years) with left frontal or temporal epilepsy underwent language fMRI and language stimulation with depth electrodes as part of their comprehensive presurgical workup. fMRI data collected during sentence generation were analyzed using statistical parametric mapping (SPM2) (false discovery rate [FDR] p < 0.05). Bipolar stimulations were performed during language production tasks. By coregistering fMRI and postimplantation computed tomography (CT) images, we were able to directly compare the cortical areas identified by both investigations.

KEY FINDINGS

fMRI during sentence generation robustly showed activation in the whole perisylvian regions with little reorganization (left hemisphere dominant in 7). Of the 184 electrode contacts tested for language, only 8 were positive (language disruption) in three of the seven patients with periictal language impairment and left language dominance. All of the positive contacts colocalized with an fMRI activated cluster, that is, fMRI did not miss any region critical to language (sensitivity = 100%). However, 54 of the 176 negative contacts were within activated clusters (low specificity).

SIGNIFICANCE

In children with epilepsy, the sensitivity of fMRI during sentence generation allows for the detection of all critical regions displayed by cortical stimulation within the large perisylvian language network, but with a low specificity. It is, therefore, useful to optimize the placement of intracranial electrodes when language mapping is necessary. Systematic planning of the electrode placement according to language fMRI maps should increase the yield of extraoperative cortical stimulation, which appears rather low in children when compared to adults.

SISCOM in children with tuberous sclerosis complex-related epilepsy

Identification of a single epileptogenic focus in patients with tuberous sclerosis complex is a challenge. Noninvasive imaging modalities, including subtraction ictal single-photon emission computed tomography coregistered to magnetic resonance imaging (SISCOM), have been used to determine the dominant epileptogenic focus for surgical resection. We assessed whether complete resection of SISCOM hyperperfusion abnormality correlates with seizure-free outcome in 6 children with tuberous sclerosis complex-related epilepsy. The median seizure onset age was 4 months (range 1 day to 16 months). The age at surgery ranged from 8 months to 13 years. A dominant SISCOM hyperperfusion focus was identified in 5 patients with multiple tubers. SISCOM provided additional localizing information for epilepsy surgery in 3 patients with nonlocalizing or discordant electrophysiologic and neuroimaging findings. At a minimum of 2 years' follow-up, 3 patients were free of seizures overall. Freedom from seizures was associated with complete resection of SISCOM abnormality in 2 patients. These findings demonstrate that SISCOM can be useful in identifying the epileptogenic zone and in guiding the location and extent of epilepsy surgery in children with tuberous sclerosis complex and multifocal abnormalities. In children with tuberous sclerosis complex and intractable epilepsy, complete resection of the SISCOM hyperperfusion abnormality is associated with freedom from seizures.

Misleading functional magnetic resonance imaging mapping of the cortical hand representation in a 4-year-old boy with an arteriovenous malformation of the central region

Functional MR imaging is dependent on the hemodynamic response function of the brain. Cerebrovascular anomalies may lead to hemodynamic artifacts, contorting the true localization of neural activation. This is illustrated in the case of a 4-year-old boy with an arteriovenous malformation (AVM) of the left central region undergoing extensive functional mapping prior to surgical removal. Intraoperative electrophysiological recording confirmed presurgical results of magnetoencephalography (MEG) and transcranial magnetic stimulation (TMS) examinations, detecting the sensorimotor hand representation within the brain tissue into which the AVM extended, whereas the activation demonstrated by functional MR (fMR) imaging was proven to be falsely localized by that modality, which showed it to be posterior to the affected central region. Thus, this case demonstrates that functional mapping can be performed even in very young patients and that combining fMR imaging with TMS and MEG is especially important in patients with vascular lesions, in whom fMR imaging can be misleading due to changes in blood flow.

Intraoperative electrocorticography and cortical stimulation in children

Intraoperative electrocorticography has been used in the surgical management of children with medically refractory epilepsy to localize anatomic areas of focal seizure onset, guide the extent, and completeness of resective epilepsy surgery, aid in functional mapping of cortical anatomy, and predict epilepsy surgical outcome. Evidence to support its utility for these purposes is somewhat controversial, particularly in children where the literature is substantially lacking. Usefulness is often dependent on the underlying pathology, and type of resective surgery. It seems to be valuable in the following circumstances: (1) tailoring the extent of hippocampal resection during temporal lobectomies, (2) guiding resection of cortical brain malformations, low-grade tumors, and other neocortical lesions, especially those involving eloquent cortex, and (3) monitoring for afterdischarges during functional cortical mapping. Most literature on this topic is not purely pediatric, and in most circumstances, is the result of combination of both children and adults cases. Cortical stimulation has been shown to be a useful, reliable and safe technique to assess motor, sensory, and speech function in children. As compared with adults, children manifest the following: (1) they need higher Amperage thresholds to elicit responses. In children younger than 10 years, cortical stimulation identifies language cortex at a lower rate than in children older than 10 years or in adults. (2) They have variability within the same individual in the stimulation threshold depending on the cortical site. This often results in the need to maximize stimulation of currents at each cortical site regardless of adjacent afterdischarge threshold. (3) They demonstrate more difficultly to evoke motor responses particularly with certain pathologies such as retrorolandic low-grade tumors. Often also the effective current intensity decreases after lesion removal. As a consequence of the above, the pulse width used for cortical stimulation in children often varies between 0.14 and 200 ms, the frequency ranges between 20 and 50 Hz, the current intensity between 0.5 and 20 mA, and the train between 3 and 25 seconds. Cortical stimulation can identify cortex with reorganized function secondary to congenital lesions and cerebral plasticity. These lesions include brain tumors, cortical dysplasia resulting in intractable epilepsy, and cavernous angioma causing epilepsy. When compared with other presurgical tests, cortical stimulation was shown to have results consistent with those of functional magnetic resonance imaging and Wada testing. It has the disadvantage of being invasive but the advantage of being highly accurate allowing for surgical tailored resections. Although the evidence for the utility of electrocorticography and cortical stimulation is exclusively level IV evidence, it is unlikely that randomized studies are going to be performed due to practical, ethical, and other reasons. The weight of the uncontrolled data at this stage is such that in children electrocorticography remains to be a useful test in some cases of cortical resection and that cortical stimulation is usually indicated when resection in or near eloquent cortex is needed.

INFLUENCE OF MOTOR FUNCTIONAL MAGNETIC RESONANCE IMAGING ON THE SURGICAL MANAGEMENT OF CHILDREN AND ADOLESCENTS WITH SYMPTOMATIC FOCAL EPILEPSY

OBJECTIVE

To determine the clinical value of motor functional magnetic resonance imaging (fMRI) in the presurgical evaluation of a large group of children and adolescents with epilepsy caused by lesions close to the central sulcus.

METHODS

Forty-three patients (19 males; mean age, 13 years) with lesional focal epilepsy underwent motor fMRI as part of a multidisciplinary standardized presurgical evaluation between 2000 and 2006. fMRI data were analyzed using statistical parametric mapping (SPM2) and screened for the presence of movement-related artifacts. The ways in which the results of motor fMRI influenced the decision-making process were reviewed.

RESULTS

The success rate of motor fMRI was 93% and data were of high quality in 67.5% of the patients. Together with other clinical considerations, motor fMRI results contributed to the surgical management of 32 patients (74%). They helped 1) to determine the type of surgery in 23 patients (72%; 12 cases with and 11 cases without invasive functional mapping), 2) to indicate a reduced benefit-risk ratio with the consequence that surgery was not further considered in 5 patients (16%), and 3) to indicate that surgery was not an appropriate option because of the high risk of motor function deficit in 4 patients (12%).

CONCLUSION

Motor fMRI can be performed with a high degree of success and good data quality in this population of patients. It has an important additive role in the discussion of the feasibility of resective surgery contributing to the decision-making process for children with epilepsy caused by brain lesions close to the central sulcus.

Role of fMRI in the decision-making process: epilepsy surgery for children

Functional MRI (fMRI) is increasingly being used to evaluate children and adolescents who are candidates for surgical treatment of intractable epilepsy. It has the advantage of being noninvasive and well tolerated by young people. By identifying important functional regions within the brain, including unpredictable patterns of functional reorganization, it can aid in surgical decision-making. Here we illustrate this using a number of case studies from the pediatric epilepsy surgery program at our institution. We describe how fMRI, used in conjunction with conventional investigative methods such as neuropsychological assessment, MRI, and electrophysiology, can 1) help to improve functional outcome by enabling resective surgery that spares functional cortex, 2) guide surgical intervention by revealing when reorganization of function has occurred, and 3) show when abnormal cortex is also functionally active, and hence that surgery may not be the best option. Altogether, these roles have reduced the need for invasive procedures that can be both risky and distressing for young people with epilepsy. In our experience, fMRI has significantly contributed to the decision-making process, and improved the counseling and management of young people with intractable epilepsy.

The influence of cortical maturation on the BOLD response: an fMRI study of visual cortex in children

We performed blood oxygenation level-dependent (BOLD) functional MR imaging in 11 children younger than 5 y of age and 10 children older than 5 y of age. All but five of the children in the older age group were tested under light anesthesia. We examined the cerebral oxidative metabolism (CMRO(2)) associated with the processing of a flashed and a reversing checkerboard stimulus. These stimuli had been shown in a previous study to induce identical vascular responses. The reversing checkerboard activated twice the neuronal population of the flashed checkerboard, doubling the CMRO(2) associated with it. We compared the extent of activation for the positive BOLD response and found that it did not differ between the different age groups. We estimated the oxidative metabolism by examining the change in the local deoxyhemoglobin (HbR) concentration using Delta R2*. Because both stimuli induced the same vascular response, any increase in oxygen requirement would have to be met by the identical blood volume. Increasing CMRO(2) will therefore result in an increase in the oxygen extraction fraction (OEF), which raises the local HbR concentration. In the younger children, both checkerboard stimuli produced identical, high HbR concentrations. In the older children, the HbR concentration to the flashed stimulus was significantly lower than to the reversing stimulus. We conclude that, for identical stimuli, the oxidative energy requirement associated with the cortical processing is higher in young children than in older children because the presence of superfluous synaptic connections in the immature visual system activates a larger neuronal population.

Functional MRI and Wada Determination of Language Lateralization: A Case of Crossed Dominance

The Wada test has historically been the conventional procedure for determining language lateralization before neurosurgery. However, functional magnetic resonance imaging (fMRI) offers a less invasive alternative to the Wada procedure. Research indicates that the two techniques used together may provide comparable, and sometimes complementary, information that results in improved prediction of postsurgical language ability. We present a case in which use of fMRI in conjunction with Wada testing provided complementary information about language lateralization before neurosurgical resection of a mesial temporal subependymoma for seizure control in a patient with schizencephaly.

Language lateralization in patients with temporal lobe epilepsy: a comparison of functional transcranial Doppler sonography and the Wada test

This study prospectively investigates whether noninvasive functional transcranial Doppler sonography (fTCD) is a useful tool to determine hemispheric language lateralization in the presurgical evaluation of patients with medically intractable temporal lobe epilepsy (TLE). fTCD results were compared with the Wada test as the gold standard. Wada test and fTCD were performed in 13 patients suffering from TLE. fTCD continuously measured blood flow velocities in both middle cerebral arteries, while the patient was performing a cued word generation task. During the Wada test, spontaneous speech, comprehension, reading, naming, and repetition were investigated. A laterality index (LI) was obtained by both procedures. Due to a lack of an acoustic temporal bone window, fTCD could not be performed in two patients (15%). In 9 of the remaining 11 patients hemispheric language dominance was found on the left side, in 1 patient on the right side, and 1 patient showed bihemispheric language representation. In all patients fTCD and the Wada test were in good agreement regarding hemispheric language lateralization, and the LI of both techniques were highly correlated (r = 0.776, P = 0.005). fTCD gives predictions of hemispheric language dominance consistent with the Wada test results even in children, patients with low IQ, and nonnative speakers. It is an alternative to the Wada test in determining language lateralization in patients with temporal lobe epilepsy.

Late plasticity for language in a child's non-dominant hemisphere: a pre- and post-surgery fMRI study

The ability of the right hemisphere to sustain the acquisition or the recovery of language after extensive damage to the left hemisphere has been essentially related to the age at the time of injury. Better language abilities are acquired when the insult occurs in early childhood (perinatal insults) compared with later occurrence. However, while previous studies have described the neuropsychological pattern of language development in typical cases, the neural bases of such plasticity remain unexplored. Non-invasive functional MRI (fMRI) is a unique tool to assess the neural correlates of brain plasticity through repeated studies, but the technique has not been widely used in children because of methodological limitations. Plasticity of language was studied in a boy who developed intractable epilepsy related to Rasmussen's syndrome of the left hemisphere at age 5 years 6 months, after normal language acquisition. The first fMRI study at age 6 years 10 months showed left lateralization of language networks during a word fluency task. After left hemispherotomy at age 9 years, the child experienced profound aphasia and alexia, with rapid recovery of receptive language but slower and incomplete recovery of expressive language and reading. Postoperative fMRI at age 10 years 6 months showed a shift of language-related networks to the right during expressive and receptive tasks. Right activation was seen mainly in regions that could not be detected preoperatively, but mirrored those previously found in the left hemisphere (inferior frontal, temporal and parietal cortex), suggesting reorganization in a pre-existing bilateral network. In addition, neuropsychological data of this case support the hypothesis of innately more bilateral distribution of receptive than expressive language. This first serial fMRI study illustrates the great plasticity of the child's brain and the ability of the right hemisphere to take over some expressive language functions, even at a relatively late age. It also suggests a limit for removal of the dominant hemisphere beyond the age of 6 years, a classical limit for the critical period of language acquisition.