The medial pulvinar as a subcortical relay in temporal lobe status epilepticus

Magnetic resonance imaging fingerprints of status epilepticus: A case-control study

OBJECTIVE

Status epilepticus (SE) is frequently associated with peri-ictal magnetic resonance imaging (MRI) abnormalities (PMA). However, the anatomical distribution of these alterations has not been systematically studied. The aim of this study was to assess the localization patterns of PMA in patients with SE.

METHODS

In this prospective case-control study, we compared the distribution and combinations of diffusion-restricted PMA to diffusion-restricted lesions caused by other neurological conditions. All patients of the SE group and the control group underwent MRI including a diffusion-weighted imaging sequence. Patients with SE were imaged within 48 h after its onset.

RESULTS

We enrolled 201 patients (51 with SE and 150 controls). The most frequent locations of PMA in SE were cortex (25/51, 49%), followed by hippocampus (20/51, 39%) and pulvinar of thalamus (10/51, 20%). In the control group, the cortex was involved in 80 of 150 (53%), white matter in 53 of 150 (35%), and basal ganglia in 33 of 150 (22%). In the control group, the pulvinar of thalamus was never affected and hippocampal structures were rarely involved (7/150, 5%). Involvement of the pulvinar of thalamus and the hippocampus had high specificity for SE at 100% (95% confidence interval [CI] = 98-100) and 95% (95% CI = 91-98), respectively. The sensitivity, however, was low for both locations (pulvinar of thalamus: 20%, 95% CI = 10-33; hippocampus: 39%, 95% CI = 26-54).

SIGNIFICANCE

Diffusion-restricted MRI lesions observed in the pulvinar of thalamus and hippocampus are strongly associated with SE. These changes may help physicians in diagnosing SE-related changes on MRI in an acute setting, especially in cases of equivocal clinical and electroencephalographic manifestations of SE.

Stimulation to probe, excite, and inhibit the epileptic brain

Direct cortical stimulation has been applied in epilepsy for nearly a century and has experienced a renaissance, given unprecedented opportunities to probe, excite, and inhibit the human brain. Evidence suggests stimulation can increase diagnostic and therapeutic utility in patients with drug-resistant epilepsies. However, choosing appropriate stimulation parameters is not a trivial issue, and is further complicated by epilepsy being characterized by complex brain state dynamics. In this article derived from discussions at the ICTALS 2022 Conference (International Conference on Technology and Analysis for Seizures), we succinctly review the literature on cortical stimulation applied acutely and chronically to the epileptic brain for localization, monitoring, and therapeutic purposes. In particular, we discuss how stimulation is used to probe brain excitability, discuss evidence on the usefulness of stimulation to trigger and stop seizures, review therapeutic applications of stimulation, and finally discuss how stimulation parameters are impacted by brain dynamics. Although research has advanced considerably over the past decade, there are still significant hurdles to optimizing use of this technique. For example, it remains unclear to what extent short timescale diagnostic biomarkers can predict long-term outcomes and to what extent these biomarkers add information to already existing biomarkers from passive electroencephalographic recordings. Further questions include the extent to which closed loop stimulation offers advantages over open loop stimulation, what the optimal closed loop timescales may be, and whether biomarker-informed stimulation can lead to seizure freedom. The ultimate goal of bioelectronic medicine remains not just to stop seizures but rather to cure epilepsy and its comorbidities.

Advancing the frontiers of thalamic neuromodulation: A review of emerging targets and paradigms

The thalamus is a key structure that plays a crucial role in initiating and propagating seizures. Recent advancements in neuroimaging and neurophysiology have identified the thalamus as a promising target for neuromodulation in drug-resistant epilepsies. This review article presents the latest innovations in thalamic targets and neuromodulation paradigms being explored in pilot or pivotal clinical trials. Multifocal temporal plus or posterior quadrant epilepsies are evaluated with pulvinar thalamus neuromodulation, while centromedian thalamus is explored in generalized epilepsies and Lennox Gastaut syndrome. Multinodal thalamocortical neuromodulation with novel stimulation paradigms such as long bursting or low-frequency stimulation is being investigated to quench the epileptic network excitability. Beyond seizure control, thalamic neuromodulation to restore consciousness is being studied. This review highlights the promising potential of thalamic neuromodulation in epilepsy treatment, offering hope to patients who have not responded to conventional medical therapies. However, it also emphasizes the need for larger randomized controlled trials and personalized stimulation paradigms to improve patient outcomes further.

The anterior and pulvinar thalamic nuclei interactions in mesial temporal lobe seizure networks

OBJECTIVE

To evaluate the respective roles of the anterior thalamic nucleus (ANT) and the medial pulvinar (PuM) during mesial temporal lobe seizures recorded by stereoelectroencephalography (SEEG).

METHODS

We assessed functional connectivity (FC) in 15 SEEG recorded seizures from 6 patients using a non-linear correlation method. Functional interactions were explored between the mesial temporal region, the temporal neocortex, ANT and PuM. The node total-strength (the summed connectivity of the node with all other nodes) as well as the directionality of the links (IN and OUT strengths) were calculated to estimate drivers and receivers during the cortico-thalamic interactions.

RESULTS

Significant increased thalamo-cortical FC during seizures was observed, with the node total-strength reaching a maximum at seizure end. There was no significant difference in global connectivity values between ANT and PuM. Regarding directionality, significantly higher thalamic IN strength values were observed. However, compared to ANT, PuM appeared to be the driver at the end of seizures with synchronous termination.

CONCLUSIONS

This work demonstrates that during temporal seizures, both thalamic nuclei are highly connected with the mesial temporal region and that PuM could play a role in seizure termination.

SIGNIFICANCE

Understanding functional connectivity between the mesial temporal and thalamic nuclei could contribute to the development of target-specific deep brain stimulation strategies for drug-resistant epilepsy.

Medial Pulvinar Stimulation in Temporal Lobe Epilepsy: A Literature Review and a Hypothesis Based on Neuroanatomical Findings

While bilateral stimulation of the anterior thalamic nuclei remains the only approved deep brain stimulation (DBS) option for focal epilepsy, two additional thalamic targets have been proposed. Earlier work indicated the potential of centromedian thalamic nucleus stimulation with recent findings highlighting the medial pulvinar nucleus. The latter has been shown to exhibit electrophysiological and imaging alterations in patients with partial status epilepticus and temporal lobe epilepsy. On this basis, recent studies have begun assessing the feasibility and efficacy of pulvinar stimulation, with encouraging results on the reduction of seizure frequency and severity. Building on existing neuroanatomical knowledge, indicating that the medial pulvinar is connected to the temporal lobe via the temporopulvinar bundle of Arnold, we hypothesize that this is one of the routes through which medial pulvinar stimulation affects temporal lobe structures. We suggest that further anatomic, imaging, and electrophysiologic studies are warranted to deepen our understanding of the subject and guide future clinical applications.

Thalamic neuromodulation in epilepsy: A primer for emerging circuit-based therapies

INTRODUCTION

Epilepsy is a common, often debilitating disease of hyperexcitable neural networks. While medically intractable cases may benefit from surgery, there may be no single, well-localized focus for resection or ablation. In such cases, approaching the disease from a network-based perspective may be beneficial.

AREAS COVERED

Herein, the authors provide a narrative review of normal thalamic anatomy and physiology and propose general strategies for preventing and/or aborting seizures by modulating this structure. Additionally, they make specific recommendations for targeting the thalamus within different contexts, motivated by a more detailed discussion of its distinct nuclei and their respective connectivity. By describing important principles governing thalamic function and its involvement in seizure networks, the authors aim to provide a primer for those now entering this fast-growing field of thalamic neuromodulation for epilepsy.

EXPERT OPINION

The thalamus is critically involved with the function of many cortical and subcortical areas, suggesting it may serve as a compelling node for preventing or aborting seizures, and so it has increasingly been targeted for the surgical treatment of epilepsy. As various thalamic neuromodulation strategies for seizure control are developed, there is a need to ground such interventions in a mechanistic, circuit-based framework.

Deep brain stimulation of the anterior nuclei of the thalamus in focal epilepsy

OBJECTIVE

To review the therapeutic effects of deep brain stimulation of the anterior nuclei of the thalamus (ANT-DBS) and the predictors of its effectiveness, safety, and adverse effects.

METHODS

A comprehensive search of the medical literature (PubMed) was conducted to identify relevant articles investigating ANT-DBS therapy for epilepsy. Out of 332 references, 77 focused on focal epilepsies were reviewed.

RESULTS

The DBS effect is probably due to decreased synchronization of epileptic activity in the cortex. The potential mechanisms from cellular to brain network levels are presented. The ANT might participate actively in the network elaborating focal seizures. The effects of ANT-DBS differed in various studies; ANT-DBS was linked with a 41% seizure frequency reduction at 1 year, 69% at 5 years, and 75% at 7 years. The most frequently reported adverse effects, depression and memory impairment, were considered non-serious in the long-term follow-up view. ANT-DBS also has been used in a few cases to treat status epilepticus.

CONCLUSIONS

We reviewed the clinical literature and identified several factors that may predict seizure outcome following DBS therapy. More large-scale trials are required since there is a need to explore stimulation settings, apply patient-tailored therapy, and identify the presurgical predictors of patient response.

SIGNIFICANCE

A critical review of the published literature on ANT-DBS in focal epilepsy is presented. ANT-DBS mechanisms are not fully understood; possible explanations are provided. Biomarkers of ANT-DBS effectiveness may lead to patient-tailored therapy.

Brain Imaging in Epilepsy-Focus on Diffusion-Weighted Imaging

Epilepsy is a common neurological disorder; 1% of people worldwide have epilepsy. Differentiating epileptic seizures from other acute neurological disorders in a clinical setting can be challenging. Approximately one-third of patients have drug-resistant epilepsy that is not well controlled by current antiepileptic drug therapy. Surgical treatment is potentially curative if the epileptogenic focus is accurately localized. Diffusion-weighted imaging (DWI) is an advanced magnetic resonance imaging technique that is sensitive to the diffusion of water molecules and provides additional information on the microstructure of tissue. Qualitative and quantitative analysis of peri-ictal, postictal, and interictal diffusion images can aid the differential diagnosis of seizures and seizure foci localization. This review focused on the fundamentals of DWI and its associated techniques, such as apparent diffusion coefficient, diffusion tensor imaging, and tractography, as well as their impact on epilepsy in terms of differential diagnosis, epileptic foci determination, and prognosis prediction.

Identifying the neural network for neuromodulation in epilepsy through connectomics and graphs

Deep brain stimulation is a treatment option for patients with drug-resistant epilepsy. The precise mechanism of neuromodulation in epilepsy is unknown, and biomarkers are needed for optimizing treatment. The aim of this study was to describe the neural network associated with deep brain stimulation targets for epilepsy and to explore its potential application as a novel biomarker for neuromodulation. Using seed-to-voxel functional connectivity maps, weighted by seizure outcomes, brain areas associated with stimulation were identified in normative resting state functional scans of 1000 individuals. To pinpoint specific regions in the normative epilepsy deep brain stimulation network, we examined overlapping areas of functional connectivity between the anterior thalamic nucleus, centromedian thalamic nucleus, hippocampus and less studied epilepsy deep brain stimulation targets. Graph network analysis was used to describe the relationship between regions in the identified network. Furthermore, we examined the associations of the epilepsy deep brain stimulation network with disease pathophysiology, canonical resting state networks and findings from a systematic review of resting state functional MRI studies in epilepsy deep brain stimulation patients. Cortical nodes identified in the normative epilepsy deep brain stimulation network were in the anterior and posterior cingulate, medial frontal and sensorimotor cortices, frontal operculum and bilateral insulae. Subcortical nodes of the network were in the basal ganglia, mesencephalon, basal forebrain and cerebellum. Anterior thalamic nucleus was identified as a central hub in the network with the highest betweenness and closeness values, while centromedian thalamic nucleus and hippocampus showed average centrality values. The caudate nucleus and mammillothalamic tract also displayed high centrality values. The anterior cingulate cortex was identified as an important cortical hub associated with the effect of deep brain stimulation in epilepsy. The neural network of deep brain stimulation targets shared hubs with known epileptic networks and brain regions involved in seizure propagation and generalization. Two cortical clusters identified in the epilepsy deep brain stimulation network included regions corresponding to resting state networks, mainly the default mode and salience networks. Our results were concordant with findings from a systematic review of resting state functional MRI studies in patients with deep brain stimulation for epilepsy. Our findings suggest that the various epilepsy deep brain stimulation targets share a common cortico-subcortical network, which might in part underpin the antiseizure effects of stimulation. Interindividual differences in this network functional connectivity could potentially be used as biomarkers in selection of patients, stimulation parameters and neuromodulation targets.

Management of Status Epilepticus, Refractory Status Epilepticus, and Super-refractory Status Epilepticus

PURPOSE OF REVIEW

Status epilepticus is a serious condition caused by disorders and diseases that affect the central nervous system. In status epilepticus, hypersynchronous epileptic activity lasts longer than the usual duration of isolated self-limited seizures (time t1), which causes neuronal damage or alteration of neuronal networks at a certain time point (time t2), depending on the type of and duration of status epilepticus. The successful management of status epilepticus includes both the early termination of seizure activity and the earliest possible identification of a causative etiology, which may require independent acute treatment. In nonconvulsive status epilepticus, patients present only with subtle clinical signs or even without any visible clinical manifestations. In these cases, EEG allows for the assessment of cerebral function and identification of patterns in need of urgent treatment.

RECENT FINDINGS

In 2015, the International League Against Epilepsy proposed a new definition and classification of status epilepticus, encompassing four axes: symptomatology, etiology, EEG, and age. Various validation studies determined the practical usefulness of EEG criteria to identify nonconvulsive status epilepticus. The American Clinical Neurophysiology Society has incorporated these criteria into their most recent critical care EEG terminology in 2021. Etiology, age, symptomatology, and the metabolic demand associated with an increasing duration of status epilepticus are the most important determinants of prognosis. The consequences of status epilepticus can be visualized in vivo by MRI studies.

SUMMARY

The current knowledge about status epilepticus allows for a more reliable diagnosis, earlier treatment, and improved cerebral imaging of its consequences. Outcome prediction is a soft tool for estimating the need for intensive care resources.

Emerging Trends in Neuromodulation for Treatment of Drug-Resistant Epilepsy

Epilepsy is a neurological disorder that affects more than 70 million people globally. A considerable proportion of epilepsy is resistant to anti-epileptic drugs (AED). For patients with drug-resistant epilepsy (DRE), who are not eligible for resective or ablative surgery, neuromodulation has been a palliative option. Since the approval of vagus nerve stimulation (VNS) in 1997, expansion to include other modalities, such as deep brain stimulation (DBS) and responsive neurostimulation (RNS), has led to improved seizure control in this population. In this article, we discuss the current updates and emerging trends on neuromodulation for epilepsy.

Current perspectives on neuroimaging techniques used to identify stroke mimics in clinical practice

INTRODUCTION

Urgent clinical assessment and brain imaging are essential for differentiating stroke mimics from stroke and to avoid unnecessary initiation of reperfusion and other therapies in stroke mimic patients.

AREAS COVERED

In this article, the authors will review acute stroke imaging and then the imaging patterns of the most common stroke mimics. The authors have focused our review on brain CT scan, and more specifically CT perfusion, as this is the most commonly available and emerging tool in emergency settings. The authors also provide information on acute brain MRI and MR perfusion.

EXPERT OPINION

Imaging can contribute to the detection and diagnosis of acute stroke mimics. Knowledge of imaging findings in different stroke mimics can help distinguish these from patients with stroke who require timely reperfusion therapy. CT and MRI perfusion and diffusion-weighted imaging (DWI) MRI are useful imaging modalities for the assessment of acute stroke patients as they provide more accurate information than plain CT scan. Some of these modalities should be available in the emergency setting. The authors recommended CT perfusion as a useful tool for stroke management and differentiation with stroke mimics.