The role of the corpus callosum in seizure spread: MRI lesion mapping in oligodendrogliomas

Brain lesion locations associated with secondary seizure generalization in tumors and strokes

Structural brain lesions are the most common cause of adult-onset epilepsy. The lesion location may contribute to the risk for epileptogenesis, but whether specific lesion locations are associated with a risk for secondary seizure generalization from focal to bilateral tonic-clonic seizures, is unknown. We identified patients with a diagnosis of adult-onset epilepsy caused by an ischemic stroke or a tumor diagnosed at the Turku University Hospital in 2004-2017. Lesion locations were segmented on patient-specific MR imaging and transformed to a common brain atlas (MNI space). Both region-of-interest analyses (intersection with the cortex, hemisphere, and lobes) and voxel-wise analyses were conducted to identify the lesion locations associated with focal to bilateral tonic-clonic compared to focal seizures. We included 170 patients with lesion-induced epilepsy (94 tumors, 76 strokes). Lesions predominantly localized in the cerebral cortex (OR 2.50, 95% C.I. 1.21-5.15, p = .01) and right hemisphere (OR 2.22, 95% C.I. 1.17-4.20, p = .01) were independently associated with focal to bilateral tonic-clonic seizures. At the lobar-level, focal to bilateral tonic-clonic seizures were associated with lesions in the right frontal cortex (OR 4.41, 95% C.I. 1.44-13.5, p = .009). No single voxels were significantly associated with seizure type. These effects were independent of lesion etiology. Our results demonstrate that lesion location is associated with the risk for secondary generalization of epileptic seizures. These findings may contribute to identifying patients at risk for focal to bilateral tonic-clonic seizures.

Corpus callosum thickness: A predictive factor for the first drug efficiency of self-limited epilepsy with centrotemporal spikes (selects)?

OBJECTIVE

To investigate the existence of a possible linkage between the thickness of corpus callosum (CC) regions and the first antiepileptic drug response in patients with Selects.

MATERIALS AND METHODS

CC thickness of 68 patients with Selects and 42 healthy controls between 4 and 12 years of age were measured using brain magnetic resonance imaging (MRI). Clinical and EEG features of newly diagnosed Selects patients were recorded. Patients were divided into two groups: good-response (patients without seizures within 24 weeks) and poor-response (patients with ≥ 1 seizure within 24 weeks). Thickness of CC was compared between patients (good-response and poor-response groups).and healthy controls.

RESULTS

The thicknesses of genu and isthmus were significantly reduced in the Selects group than healthy controls. Isthmus and splenium were significantly thinner in poor responders than those in the good-response group (p = 0.005 and p < 0.001, respectively). The total number of seizures was negatively correlated with the thickness of the body, isthmus, and splenium (p < 0.001). There was no significant difference in CC thickness of the children with and without electrical status epilepticus in sleep (ESES). The thickness of the isthmus and splenium were significantly thinner in patients receiving ≥ 2 antiepileptic drugs (p = 0.002 and p = 0.001, respectively).

CONCLUSIONS

Our study highlights the notable differences in areas of CC in Selects patients. These changes may help uncover the underlying cause of seizure recurrence and antiepileptic drug (AED) response. Different thinner parts of CC may be a protective mechanism to prevent seizure spread to other brain regions. CC thickness can be used as a new radiologic biomarker for predicting first AED response and seizure recurrence in Selects patients.

Altered integrity of corpus callosum in generalized epilepsy in relation to seizure lateralization after corpus callosotomy

OBJECTIVE

Generalized-onset seizures are usually conceptualized as engaging bilaterally distributed networks with no clear focus. However, the authors previously reported a case series demonstrating that in some patients with generalized-onset seizures, focal seizure onset could be discovered after corpus callosotomy. The corpus callosum is considered to be a major pathway for seizure generalization in this group of patients. The authors hypothesized that, in patients with generalized-onset seizures, the structure of the corpus callosum could be different between patients who have lateralized seizures and those who have nonlateralized seizures after corpus callosotomy. The authors aimed to evaluate the structural difference through statistical analysis of diffusion tensor imaging (DTI) scalars between these two groups of patients.

METHODS

Thirty-two patients diagnosed with generalized-onset motor seizures and without an MRI lesion were included in this study. Among them, 16 patients developed lateralized epileptic activities after corpus callosotomy, and the remaining 16 patients continued to have nonlateralized seizures after corpus callosotomy. Presurgical DTI studies were acquired to quantify the structural integrity of the corpus callosum.

RESULTS

The DTI analysis showed significant reduction of fractional anisotropy (FA) and increase in radial diffusivity (RD) in the body of the corpus callosum in the lateralized group compared with the nonlateralized group.

CONCLUSIONS

The authors' findings indicate the existence of different configurations of bilateral epileptic networks in generalized epilepsy. Generalized seizures with focal onset relying on rapid spread through the corpus callosum might cause more structural damage related to demyelination in the corpus callosum, showing reduced FA and increased RD. This study suggests that presurgical DTI analysis of the corpus callosum might predict the seizure lateralization after corpus callosotomy.

Patterns of seizure spread in temporal lobe epilepsy are associated with distinct white matter tracts

OBJECTIVE

It is commonly hypothesized that seizure spread patterns in patients with focal epilepsy are associated with structural brain pathways. However, this relationship is poorly understood and has not been fully demonstrated in patients with temporal lobe epilepsy. Here, we sought to determine whether directionality of seizure spread (DSS) is associated with specific cerebral white matter tracts in patients with temporal lobe epilepsy.

METHODS

Thirty-three adult patients with temporal lobe epilepsy who underwent stereoelectroencephalography (sEEG) and magnetic resonance diffusion tensor imaging (MR-DTI) as part of their standard-of-care clinical evaluation were included in the study. DSS was defined as anterior-posterior (AP) or medial-lateral (ML) spread based upon sEEG evaluation by two independent specialists who demonstrated excellent inter-rater agreement (Cohen's kappa = .92). DTI connectometry was used to assess differences between seizure spread pattern groups along major fiber pathways regarding fractional anisotropy (FA).

RESULTS

Twenty-four participants showed seizures with AP spread and nine participants showed seizures with ML spread. There were no significant differences between the groups on their demographic and clinical profile. Patients with ML seizures had higher FA along the corpus callosum and, to a lesser degree, some portions of the bilateral cingulate tracts. In contrast, patients with AP seizures had higher FA along several anterior-posterior white matter projections bundles, including the cingulate fasciculus and the inferior longitudinal, with significantly less involvement of the corpus callosum compared with ML seizures.

SIGNIFICANCE

This study confirms the hypothesis that the anatomical pattern of electrophysiological ictal propagation is associated with the structural reinforcement of supporting pathways in temporal lobe epilepsy. This observation can help elucidate mechanisms of ictal propagation and may guide future translational approaches to curtail seizure spread.

Focal to bilateral tonic-clonic seizures are associated with widespread network abnormality in temporal lobe epilepsy

OBJECTIVE

Our objective was to identify whether the whole-brain structural network alterations in patients with temporal lobe epilepsy (TLE) and focal to bilateral tonic-clonic seizures (FBTCS) differ from alterations in patients without FBTCS.

METHODS

We dichotomized a cohort of 83 drug-resistant patients with TLE into those with and without FBTCS and compared each group to 29 healthy controls. For each subject, we used diffusion-weighted magnetic resonance imaging to construct whole-brain structural networks. First, we measured the extent of alterations by performing FBTCS-negative (FBTCS-) versus control and FBTCS-positive (FBTCS+) versus control comparisons, thereby delineating altered subnetworks of the whole-brain structural network. Second, by standardizing each patient's networks using control networks, we measured the subject-specific abnormality at every brain region in the network, thereby quantifying the spatial localization and the amount of abnormality in every patient.

RESULTS

Both FBTCS+ and FBTCS- patient groups had altered subnetworks with reduced fractional anisotropy and increased mean diffusivity compared to controls. The altered subnetwork in FBTCS+ patients was more widespread than in FBTCS- patients (441 connections altered at t > 3, p < .001 in FBTCS+ compared to 21 connections altered at t > 3, p = .01 in FBTCS-). Significantly greater abnormalities-aggregated over the entire brain network as well as assessed at the resolution of individual brain areas-were present in FBTCS+ patients (p < .001, d = .82, 95% confidence interval = .32-1.3). In contrast, the fewer abnormalities present in FBTCS- patients were mainly localized to the temporal and frontal areas.

SIGNIFICANCE

The whole-brain structural network is altered to a greater and more widespread extent in patients with TLE and FBTCS. We suggest that these abnormal networks may serve as an underlying structural basis or consequence of the greater seizure spread observed in FBTCS.

White matter microstructural differences between right and left mesial temporal lobe epilepsy

PURPOSE

Mesial temporal lobe epilepsy (mTLE) is a chronic focal epileptic disorder characterized by recalcitrant seizures often necessitating surgical intervention. Identifying the laterality of seizure focus is crucial for pre-surgical planning. We implemented diffusion MRI (DMRI) connectometry to identify differences in white matter connectivity in patients with left and right mTLE relative to healthy control subjects.

METHOD

We enrolled 12 patients with right mTLE, 12 patients with left mTLE, and 12 age/sex matched healthy controls (HCs). We used DMRI connectometry to identify local connectivity patterns of white matter tracts, based on quantitative anisotropy (QA). We compared QA of white matter to reconstruct tracts with significant difference in connectivity between patients and HCs and then between patients with left and right mTLE.

RESULTS

Right mTLE patients show higher anisotropy in left inferior longitudinal fasciculus (ILF) and forceps minor and lower QA in genu of corpus callosum (CC), bilateral corticospinal tracts (CSTs), and bilateral middle cerebellar peduncles (MCPs) compared to HCs. Left mTLE patients show higher anisotropy in genu of CC, bilateral CSTs, and right MCP and decreased anisotropy in forceps minor compared to HCs. Compared to patients with right mTLE, left mTLE patients showed increased and decreased connectivity in some major tracts.

CONCLUSIONS

Our study showed the pattern of microstructural disintegrity in mTLE patients relative to HCs. We demonstrated that left and right mTLE patients have discrepant alternations in their white matter microstructure. These results may indicate that left and right mTLE have different underlying pathologic mechanisms.

Concurrence of multiple sclerosis, oligodendroglioma, and autosomal recessive cerebellar ataxia with spasticity in the same patient: A challenging diagnosis

Multiple sclerosis (MS) has been described in several case reports to coexist with brain tumors. This unusual concurrence has been the subject of research projects with a common question of whether these pathological entities share common roots. However, no clear association has proved that either of them could provoke the other, and mere chance is the only acceptable explanation. Along all reported cases, oligodendroglioma has been rarely reported to coexist with MS. In this paper, we report a unique case with a triad of MS, oligodendroglioma, and autosomal recessive cerebellar ataxia with spasticity and discuss possible theories that might have attributed to these three conditions. To our knowledge, this is the first case ever to have these three conditions present in one patient. The most likely explanation is believed to be that this patient was unfortunate to have three unrelated diseases.

Circuits generating secondarily generalized seizures

Mapping the circuits underlying the generation and propagation of seizures is critically important for understanding their pathophysiology. We review evidence to suggest that circuits engaged in secondarily generalized seizures are likely to be more complex than those currently proposed. Focal seizures have been proposed to engage canonical thalamocortical circuits that mediate primarily generalized absence seizures, leading to secondarily generalized tonic-clonic seizures. In addition to traveling through the canonical thalamocortical circuits, secondarily generalized seizures could also travel through the striatum, globus pallidus, substantia nigra reticulata, and corpus callosum to the contralateral hemisphere. Recruitment of principal neurons in superficial layers 2/3 of the cortex can play a critical role in corticocortical seizure spread. Understanding the neuronal structures engaged in generating secondarily generalized seizures could provide novel targets for neuromodulation for the treatment of seizures. Furthermore, these sites may be loci of neuronal plasticity facilitating epileptogenesis. This article is part of the Special Issue "Proceedings of the 7th London-Innsbruck Colloquium on Status Epilepticus and Acute Seizures".

The Impacts of Tumor and Tumor Associated Epilepsy on Subcortical Brain Structures and Long Distance Connectivity in Patients With Low Grade Glioma

Low grade gliomas in cerebral cortex often cause symptoms related to higher cerebral functions such as attention, memory and executive function before treatment is initiated. Interestingly, focal tumors residing in one cortical region can lead to a diverse range of symptoms, indicating that the impact of a tumor is extended to multiple brain regions. We hypothesize that the presence of focal glioma in the cerebral cortex leads to alterations of distant subcortical areas and essential white matter tracts. In this study, we analyzed diffusion tensor imaging scans in glioma patients to study the effect of glioma on subcortical gray matter nuclei and long-distance connectivity. We found that the caudate nucleus, putamen and thalamus were affected by cortical glioma, displaying both volumetric and diffusion alterations. The cerebellar cortex contralateral to the tumor side also showed significant volume decrease. Additionally, tractography of the cortico-striatal and cortico-thalamic projections shows similar diffusion alterations. Tumor associated epilepsy might be an important contributing factor to the found alterations. Our findings indeed confirm concurrent structural and connectivity abrasions of brain areas distant from brain tumor, and provide insights into the pathogenesis of diverse neurological symptoms in glioma patients.

Dynamical properties of LFPs from mice with unilateral injection of TeNT

Local field potential (LFP) recordings were performed from the visual cortex (V1) of a focal epilepsy mouse model. Epilepsy was induced by a unilateral injection of the synaptic blocker tetanus neurotoxin (TeNT). LFP signals were simultaneously recorded from V1 of both hemispheres of each animal under acute and chronic conditions (i.e. during and after the period of TeNT action). All data were analysed by using nonlinear time series methods. Suitable values of the lag time and embedding dimension for phase space reconstruction were estimated by employing well-known methods. The results showed that lag times are sensitive to the presence of TeNT. Interestingly, TeNT promoted an increase in the level of linear and nonlinear correlation of LFP signals. The values of the embedding dimension failed to show any dependence on the presence of the neurotoxin. However, a local nonlinear prediction method showed that the presence of TeNT increases the predictability, quantified by the normalized prediction error, of the neural recordings. From a neurophysiological point of view, the above results suggest that TeNT injected in one hemisphere strongly impacts the local electrical activity of the neural populations in the opposite hemisphere. We hypothesize that this could arise from a qualitative and quantitative alteration of the transmission properties of the callosal fibers.

Functional Connectivity of the Corpus Callosum in Epilepsy Patients with Secondarily Generalized Seizures

The corpus callosum (CC) plays an important role in generalization of seizure activity. We used resting-state function magnetic resonance imaging (rs-fMRI) to investigate the regional and interregional functional connectivity of CC in patients with magnetic resonance imaging (MRI)-negative and secondarily generalized seizures. We measured the multi-regional coherences of blood oxygen level-dependent (BOLD) signals via rs-fMRI, cortical thickness via high-resolution T1-weighted MRI, and white matter (WM) integrity via diffusion-tensor imaging in 16 epilepsy patients as well as in 16 age- and gender-matched healthy subjects. All patients had non-lesional MRI, medically well-controlled focal epilepsy and history of secondarily generalized convulsions. Individuals with epilepsy had significant differences in regional and interregional hypersynchronization of BOLD signals intrahemispherically and interhemispherically, but no difference in cortical thickness and WM integrity. The only area with increased regional hypersynchrony in WM was over the anterior CC, which also exhibited lower activation of neighboring resting-state networks. The present study revealed abnormal local and distant synchronization of spontaneous neural activities in epileptic patients with secondarily generalized seizures.

Novel Neuromodulation Techniques to Assess Interhemispheric Communication in Neural Injury and Neurodegenerative Diseases

Interhemispheric interaction has a major role in various neurobehavioral functions. Its disruption is a major contributor to the pathological changes in the setting of brain injury such as traumatic brain injury, peripheral nerve injury, and stroke, as well as neurodegenerative diseases. Because interhemispheric interaction has a crucial role in functional consequence in these neuropathological states, a review of noninvasive and state-of-the-art molecular based neuromodulation methods that focus on or have the potential to elucidate interhemispheric interaction have been performed. This yielded approximately 170 relevant articles on human subjects or animal models. There has been a recent surge of reports on noninvasive methods such as transcranial magnetic stimulation and transcranial direct current stimulation. Since these are noninvasive techniques with little to no side effects, their widespread use in clinical studies can be easily justified. The overview of novel neuromodulation methods and how they can be applied to study the role of interhemispheric communication in neural injury and neurodegenerative disease is provided. Additionally, the potential of each method in therapeutic use as well as investigating the pathophysiology of interhemispheric interaction in neurodegenerative diseases and brain injury is discussed. New technologies such as transcranial magnetic stimulation or transcranial direct current stimulation could have a great impact in understanding interhemispheric pathophysiology associated with acquired injury and neurodegenerative diseases, as well as designing improved rehabilitation therapies. Also, advances in molecular based neuromodulation techniques such as optogenetics and other chemical, thermal, and magnetic based methods provide new capabilities to stimulate or inhibit a specific brain location and a specific neuronal population.

Clinical manifestations in children and adolescents with corpus callosum abnormalities

Corpus callosum abnormality (CCA) outcomes are quite unpredictable and variable, from asymptomatic forms to mild or severe neurodevelopment disorders. The aim of this study was to examine clinical outcomes in CCA patients. The study included 61 children and adolescents in whom brain magnetic resonance imaging (MRI) scans showed CCA, isolated or associated to other central nervous system lesions. All patients underwent anamnesis, physical and neurological examination, routine laboratory tests, electroencephalogram (EEG), and MRI scans. In all participants, the intelligence quotient (IQ) was determined. We divided the participants into two subgroups: the first subgroup included patients with an isolated CCA, and the second subgroup included patients with CCA associated with extra-callosal brain lesions (complex CCA). We found that CCA were associated with elevated frequency to intellectual disability (ID), other neurodevelopment disorders, epilepsy, and isolated EEG anomalies. Mild ID (p = 0.003) was more frequent in the isolated subgroup, while epilepsy (p = 0.036) and pre-perinatal risk factors (p = 0.023) were more frequent in the complex CCA subgroup. Although the role of the CC in the interhemispheric communication is known, neurological and neurodevelopment outcomes of CCA are extremely variable and unpredictable. The presence of extra-callosal brain anomalies is one of the major prognostic factor, and probably, they have an important impact on the clinical outcome.

Oligodendroglioma: pathology, molecular mechanisms and markers

For nearly a century, the diagnosis and grading of oligodendrogliomas and oligoastrocytomas has been based on histopathology alone. Roughly 20 years ago, the first glioma-associated molecular signature was found with complete chromosome 1p and 19q codeletion being particularly common in histologically classic oligodendrogliomas. Subsequently, this codeletion appeared to not only carry diagnostic, but also prognostic and predictive information, the latter aspect only recently resolved after carefully constructed clinical trials with very long follow-up times. More recently described biomarkers, including the non-balanced translocation leading to 1p/19q codeletion, promoter hypermethylation of the MGMT gene, mutations of the IDH1 or IDH2 gene, and mutations of FUBP1 (on 1p) or CIC (on 19q), have greatly enhanced our understanding of oligodendroglioma biology, although their diagnostic, prognostic, and predictive roles are less clear. It has therefore been suggested that complete 1p/19q codeletion be required for the diagnosis of 'canonical oligodendroglioma'. This transition to an integrated morphological and molecular diagnosis may result in the disappearance of oligoastrocytoma as an entity, but brings new challenges as well. For instance it needs to be sorted out how (histopathological) criteria for grading of 'canonical oligodendrogliomas' should be adapted, how pediatric oligodendrogliomas (known to lack codeletions) should be defined, which platforms and cut-off levels should ideally be used for demonstration of particular molecular aberrations, and how the diagnosis of oligodendroglioma should be made in centers/countries where molecular diagnostics is not available. Meanwhile, smart integration of morphological and molecular information will lead to recognition of biologically much more uniform groups within the spectrum of diffuse gliomas and thereby facilitate tailored treatments for individual patients.