Requirement of longitudinal synchrony of epileptiform discharges in the hippocampus for seizure generation: a pilot study

Ultrasound-Navigated Multiple Hippocampal Transections: An Anatomical Study

BACKGROUND

 Multiple hippocampal transection (MHT) is a surgical technique used for the treatment of drug-resistant mesial temporal lobe epilepsy in situations where standard procedures would pose a high risk for memory deterioration. During MHT, the longitudinal fibers of the hippocampus, implicated in epilepsy spreading, are interrupted, while the transverse memory circuits are spared. The extent of MHT is governed by intraoperative electrocorticography; abolition of epileptic discharges serves as an end point to terminate the transection. In other words, the aim of MHT is not the anatomical completeness of hippocampal transection. In contrast, we hypothesize that only the complete transection of hippocampal cross-section is needed to durably terminate epilepsy, avoiding possible postoperative reorganization of longitudinal pathways. Here, we report an anatomical study designed to evaluate the feasibility of complete transection of hippocampus with the aid of ultrasound neuronavigation and we propose new instruments to reach this goal.

METHODS

 Five cadaveric brains were analyzed in this study. MHT was performed on both sides of each brain either with or without ultrasound neuronavigation. The percentage of transected cross-section of the hippocampus was measured using magnetic resonance imaging (MRI) and both sides were compared.

RESULTS

 The ultrasound-guided MHTs were more likely to achieve complete hippocampal transection compared with the nonnavigated MHT transection (73 vs 58%; p < 0.01). Our study also allowed us to propose specialized transectors to minimize invasivity of this procedure.

CONCLUSION

 Completeness of MHT can be better reached with the aid of an ultrasound neuronavigation system; modified instruments for this procedure were also designed.

Identifying sources of human interictal discharges with travelling wave and white matter propagation

Interictal epileptiform discharges have been shown to propagate from focal epileptogenic sources as travelling waves or through more rapid white matter conduction. We hypothesize that both modes of propagation are necessary to explain interictal discharge timing delays. We propose a method that, for the first time, incorporates both propagation modes to identify unique potential sources of interictal activity. We retrospectively analysed 38 focal epilepsy patients who underwent intracranial EEG recordings and diffusion-weighted imaging for epilepsy surgery evaluation. Interictal discharges were detected and localized to the most likely source based on relative delays in time of arrival across electrodes, incorporating travelling waves and white matter propagation. We assessed the influence of white matter propagation on distance of spread, timing and clinical interpretation of interictal activity. To evaluate accuracy, we compared our source localization results to earliest spiking regions to predict seizure outcomes. White matter propagation helps to explain the timing delays observed in interictal discharge sequences, underlying rapid and distant propagation. Sources identified based on differences in time of receipt of interictal discharges are often distinct from the leading electrode location. Receipt of activity propagating rapidly via white matter can occur earlier than more local activity propagating via slower cortical travelling waves. In our cohort, our source localization approach was more accurate in predicting seizure outcomes than the leading electrode location. Inclusion of white matter in addition to travelling wave propagation in our model of discharge spread did not improve overall accuracy but allowed for identification of unique and at times distant potential sources of activity, particularly in patients with persistent postoperative seizures. Since distant white matter propagation can occur more rapidly than local travelling wave propagation, combined modes of propagation within an interictal discharge sequence can decouple the commonly assumed relationship between spike timing and distance from the source. Our findings thus highlight the clinical importance of recognizing the presence of dual modes of propagation during interictal discharges, as this may be a cause of clinical mislocalization.

Hippocampal interlamellar cell-cell connectome that counts

The hippocampus is regarded as a cognition hub, particularly for learning and memory. Previously, neuronal mechanisms underlying various cognitive functions are delineated with the lamellar hippocampal circuitry, dentate gyrus-CA3 or CA2-CA1, within the transverse plane. More recently, interlamellar (often referred to as longitudinal) projections have received intensive attention to help understand signal convergence and divergence in cognition and behavior. Signal propagation along the longitudinal axis is evidenced by axonal arborization patterns and synaptic responses to electro- and photo-stimulation, further demonstrating that information flow is more enriched in the longitudinal plane than the transverse plane. Here, we review the significance of longitudinal connections for cognition, discuss a putative circuit mechanism of place coding, and suggest the reconceptualization of the hippocampal circuitry.

Multiple hippocampal transection for mesial temporal lobe epilepsy: A systematic review

PURPOSE

Multiple hippocampal transection (MHT) is a surgical technique that offers adequate seizure control with minimal perioperative morbidity. However, there is little evidence available to guide neurosurgeons in selecting this technique for use in appropriate patients. This systematic review analyzes patient-level data associated with MHT for intractable epilepsy, focusing on postoperative seizure control and memory outcomes.

METHODS

The systematic review was conducted in accordance with Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. Relevant articles were identified from 3 databases (PubMed, Medline, Embase) up to August 1, 2021. Inclusion criteria were that the majority of patients had received a diagnosis of intractable epilepsy, the article was written in English, MHT was the primary procedure, and patient-level metadata were included.

RESULTS

Fifty-nine unique patients who underwent MHT were identified across 11 studies. Ten (17%) of 59 patients underwent MHT alone. Forty-three (75%) of 57 patients who had a follow-up 12 months or longer were seizure free at last follow-up. With respect to postoperative verbal memory retention, 9 of 38 (24%) patient test scores did not change, 14 (37%) decreased, and 16 (42%) increased. With respect to postoperative nonverbal memory retention, 12 of 38 (34%) patient test scores did not change, 13 (34%) decreased, and 13 (33%) increased.

CONCLUSION

There are few reported patients analyzed after MHT. Although the neurocognitive benefits of MHT are unproven, this relatively novel technique has shown promise in the management of seizures in patients with intractable epilepsy. However, structured trials assessing MHT in isolation are warranted.

Multiple hippocampal transections for mesial temporal lobe epilepsy

Background:

The purpose of this study was to evaluate the effectiveness of multiple hippocampal transections (MHT) in the treatment of drug-resistant mesial temporal lobe epilepsy.

Methods:

Six patients underwent MHT at Burdenko Neurosurgery Center in 2018. The age of the patients varied from 18 to 43 years. All patients suffered from refractory epilepsy caused by focal lesions of the mesial temporal complex or temporal pole in dominant side. Postoperative pathology revealed neuronal-glial tumors in two patients, focal cortical dysplasia (FCD) of the temporal pole – in two patients, cavernous angioma – in one patient, and encephalocele of the preuncal area – in one patient.

Results:

All patients underwent surgery satisfactorily. There were no postoperative complications except for homonymous superior quadrantanopia. This kind of visual field loss was noted in four cases out of six. During the follow-up period five patients out of six had Engel Class I outcome (83.3%). In one case, seizures developed after 1 month in a patient with FCD in the uncus (Engel IVA). After surgery, three out of six patients developed significant nominative aphasia. Two patients relative to the preoperative level demonstrated improvement in delayed verbal memory after MHT. Two patients showed a decrease level in delayed verbal memory. In preoperative period, visual memory was below the normal in one patient. Delayed visual memory in two cases impaired compared to the preoperative level.

Conclusion:

MHT can be considered as an effective method of drug-resistant mesial temporal lobe epilepsy caused by tumors of the medial temporal complex. At the same time, MHT makes it possible to preserve memory in patients with structurally preserved hippocampus. However, MHT do not guarantee the preservation of memory after surgery.

Disrupted functional connectivity in white matter resting-state networks in unilateral temporal lobe epilepsy

Unilateral temporal lobe epilepsy (TLE) is the most common type of focal epilepsy characterized by foci in the unilateral temporal lobe grey matters of regions such as the hippocampus. However, it remains unclear how the functional features of white matter are altered in TLE. In the current study, resting-state functional magnetic resonance imaging (fMRI) was performed on 71 left TLE (LTLE) patients, 79 right TLE (RTLE) patients and 47 healthy controls (HC). Clustering analysis was used to identify fourteen white matter networks (WMN). The functional connectivity (FC) was calculated among WMNs and between WMNs and grey matter. Furthermore, the FC laterality of hemispheric WMNs was assessed. First, both patient groups showed decreased FCs among WMNs. Specifically, cerebellar white matter illustrated decreased FCs with the cerebral superficial WMNs, implying a dysfunctional interaction between the cerebellum and the cerebral cortex in TLE. Second, the FCs between WMNs and the ipsilateral hippocampus (grey matter foci) were also reduced in patient groups, which may suggest insufficient functional integration in unilateral TLE. Interestingly, RTLE showed more severe abnormalities of white matter FCs, including links to the bilateral hippocampi and temporal white matter, than LTLE. Taken together, these findings provide functional evidence of white matter abnormalities, extending the understanding of the pathological mechanism of white matter impairments in unilateral TLE.

Neural recruitment by ephaptic coupling in epilepsy

OBJECTIVE

One of the challenges in treating patients with drug-resistant epilepsy is that the mechanisms of seizures are unknown. Most current interventions are based on the assumption that epileptic activity recruits neurons and progresses by synaptic transmission. However, several experimental studies have shown that neural activity in rodent hippocampi can propagate independently of synaptic transmission. Recent studies suggest these waves are self-propagating by electric field (ephaptic) coupling. In this study, we tested the hypothesis that neural recruitment during seizures can occur by electric field coupling.

METHODS

4-Aminopyridine was used in both in vivo and in vitro preparation to trigger seizures or epileptiform activity. A transection was made in the in vivo hippocampus and in vitro hippocampal and cortical slices to study whether the induced seizure activity can recruit neurons across the gap. A computational model was built to test whether ephaptic coupling alone can account for neural recruitment across the transection. The model prediction was further validated by in vitro experiments.

RESULTS

Experimental results show that electric fields generated by seizure-like activity in the hippocampus both in vitro and in vivo can recruit neurons locally and through a transection of the tissue. The computational model suggests that the neural recruitment across the transection is mediated by electric field coupling. With in vitro experiments, we show that a dielectric material can block the recruitment of epileptiform activity across a transection, and that the electric fields measured within the gap are similar to those predicted by model simulations. Furthermore, this nonsynaptic neural recruitment is also observed in cortical slices, suggesting that this effect is robust in brain tissue.

SIGNIFICANCE

These results indicate that ephaptic coupling, a nonsynaptic mechanism, can underlie neural recruitment by a small electric field generated by seizure activity and could explain the low success rate of surgical transections in epilepsy patients.

Multiple hippocampal transections for refractory pediatric mesial temporal lobe epilepsy: seizure and neuropsychological outcomes

OBJECTIVE

Temporal lobe epilepsy (TLE) is the most common focal epilepsy across adult and pediatric age groups. It is also the most amenable to surgery, with excellent long-term seizure outcome. Most TLE cases have an epileptogenic zone in the mesial temporal structures, namely the hippocampus. Resecting the dominant hippocampus has been shown to be associated with significant verbal memory deficits, especially in patients with intact verbal memory scores presurgically. Multiple hippocampal transection (MHT) is a relatively new surgical technique designed to interrupt the longitudinal hippocampal circuitry involved in seizure propagation yet preserve the circular fibers involved in memory function. This technique has been used to treat mesial TLE in both dominant- and nondominant-hemisphere cases, almost exclusively in adults. It has been applied to normal and sclerotic hippocampi.

METHODS

In this study, information on 3 pediatric patients who underwent MHT for mesial TLE at Children's Wisconsin between 2017 and 2018 is included. Clinical, electroencephalographic, and neuropsychological features and outcomes are described in detail.

RESULTS

MRI revealed a tumor in the amygdala with a normal hippocampus in 1 patient and hippocampal sclerosis in 2 patients. All patients underwent stereoelectroencephalography confirming the involvement of the hippocampus in seizure onset. MHTs were completed under intraoperative monitoring, with amygdala and temporal tip resection in all patients due to early spread to these regions. All patients had excellent seizure outcomes at 1 year, and 2 of the 3 patients remain seizure free at last follow-up (range 20-36 months), all with stable or improved neuropsychological profiles, including verbal memory.

CONCLUSIONS

MHT is a relatively new surgical procedure designed to preserve essential memory circuitry while disrupting seizure propagation pathways in the hippocampus. A growing body of literature shows good seizure and neuropsychological results, but mainly in adults. This is the first series of MHTs used exclusively in children at one medical center, showcasing excellent seizure control and preservation of neuropsychological functioning. One of the patients is also the first described to have MHT in the setting of an amygdalar tumor abutting the hippocampus, further expanding the pathological setting in which MHT can be used effectively.

Multiple hippocampal transections: Post-operative Memory Outcomes and Seizure Control

OBJECT

Temporal lobectomy with amygdalohippocampectomy is the standard surgical treatment for appropriate candidates with medically-intractable temporal lobe epilepsy. More recently, because of the risk of postoperative language/memory decline in a subset of patients with intact memory, a multiple hippocampal transection (MHT) approach has been proposed to preserve function.

METHODS

Studies of MHT reporting both Engel and verbal memory outcome measures were included in accordance with Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines for reporting of systematic reviews. Data were extracted on verbal memory function pre- and postoperatively, seizure outcome, and demographic factors. A random effects model was used to determine overall verbal memory function after MHT, and a meta-regression model was applied to identify factors associated with outcome.

RESULTS

A total of 114 patients across five studies were included. Engel class I seizure outcome across all studies ranged from 64.7% to 94.7%, with 84 of the 114 patients achieving this outcome. Preoperative verbal memory score was most strongly associated with postoperative verbal memory preservation (p = 0.003). Of 59 patients with full verbal memory outcome scores, 86.8% (95% CI [confidence interval]: 77.6%-96%) had complete preservation of verbal memory relative to preoperative functional baseline.

CONCLUSION

Multiple hippocampal transection is an evolving surgical technique. Although the present data are limited, the current systematic review suggests that this approach is effective at preserving verbal memory in patients with good baseline function. Although reasonable seizure outcomes have been reported with MHT, comparison to a well-established procedure such as temporal lobectomy and amydalohippocampectomy must be guided by further evidence.

Hippocampal transection for stereo-electroencephalography-proven dominant mesial temporal lobe epilepsy in a child: a detailed case report and critical review

Resection of the hippocampus ipsilateral to the verbal memory-dominant hemisphere frequently results in severe memory deficits. In adults with epilepsy, multiple hippocampal transections (MHTs) have resulted in excellent seizure outcome with preservation of verbal memory. The authors report the first detailed case of a child undergoing MHTs for mesial temporal lobe epilepsy. A 13-year-old right-handed boy had intractable seizures characterized by epigastric discomfort evolving to unresponsiveness and chewing automatisms, lasting 1 minute and occurring 2-3 times weekly, sometimes ending in a generalized tonic-clonic seizure. He had no seizure risk factors and nonfocal examination results. Interictal electroencephalography (EEG) showed frequent left temporal epileptiform discharges (maximum FT9) and intermittent slowing. Video EEG, FDG-PET, and 1.5-T MRI were nonlocalizing. Neuropsychological evaluation suggested left temporal lobe dysfunction. A stereo-EEG investigation using 8 electrodes localized the seizure onset zone to the anterior mesial temporal region, immediately involving the hippocampus. The temporal pole and amygdala were resected en bloc with 3 MHTs. Comparison of neuropsychological tests 4 months before and 6 months after the surgery showed a significant decline only in confrontational naming and no significant change in verbal memory. Six and a half years later, the patient remains seizure free with no antiepileptic drugs. In children with established hemispheric dominance suffering from mesial temporal lobe epilepsy, MHTs may be an option.

Long term potentiation, but not depression, in interlamellar hippocampus CA1

Synaptic plasticity in the lamellar CA3 to CA1 circuitry has been extensively studied while interlamellar CA1 to CA1 connections have not yet received much attention. One of our earlier studies demonstrated that axons of CA1 pyramidal neurons project to neighboring CA1 neurons, implicating information transfer along a longitudinal interlamellar network. Still, it remains unclear whether long-term synaptic plasticity is present within this longitudinal CA1 network. Here, we investigate long-term synaptic plasticity between CA1 pyramidal cells, using in vitro and in vivo extracellular recordings and 3D holography glutamate uncaging. We found that the CA1-CA1 network exhibits NMDA receptor-dependent long-term potentiation (LTP) without direction or layer selectivity. By contrast, we find no significant long-term depression (LTD) under various LTD induction protocols. These results implicate unique synaptic properties in the longitudinal projection suggesting that the interlamellar CA1 network could be a promising structure for hippocampus-related information processing and brain diseases.

Dynamic Expression of CX36 Protein in Kainic Acid Kindling induced Epilepsy

Connexin (Cx) 36 is known to be a component of gap junctions, and has been suggested to play an important role in epilepsy. In order to determine dynamic changes of Cx36 protein expression in epilepsy and investigate the role of Cx36 in electroencephalographic activity and pathogenesis, we utilized kainic acid (KA) to induce epileptogenesis. We found that epileptic discharges began 71.8 ± 23.7 s after KA administration. Spike frequency and amplitude of epileptiform activity reached maximal levels at 30 ± 5.2 min. The maximum level of spike frequency and amplitude of epileptiform activity was 13.9 ± 0.3 Hz and 198 ± 14.3mV respectively. Employing Western blotting and immunohistochemistry, we demonstrated that hippocampal Cx36 protein expression was significantly increased 6 h after KA kindling compared to control or sham groups, but decreased in 3 d and 7d groups. Our results suggested that the dynamic change of Cx36 expression may play an important role inepilepsy, and the specific manipulation of Cx36 expression may be a potential target for the treatment of epilepsy.

Resection of Temporal Neocortex During Multiple Hippocampal Transections for Mesial Temporal Lobe Epilepsy Does not Affect Seizure or Memory Outcome

BACKGROUND

Multiple hippocampal transection (MHT) is a surgical treatment for mesial temporal lobe epilepsy associated with improved postoperative neuropsychological outcomes compared with lobectomy.

OBJECTIVE

To determine whether resection of the amygdala and anterior temporal neocortex during MHT affects postoperative seizure/memory outcome.

METHODS

Seventeen patients with normal magnetic resonance imaging and stereo-electroencephalogram-proven drug-resistant dominant mesial temporal lobe epilepsy were treated with MHT. Nine patients underwent MHT alone (MHT–) and 8 patients underwent MHT plus removal of the amygdala and anterior 4.5 cm of temporal neocortex lateral to the fusiform gyrus (MHT+). Verbal and visual-spatial memory were assessed in all patients preoperatively and in 14 patients postoperatively using the Wechsler Memory Scale. Postoperative seizure control was assessed at 12 months for all patients.

RESULTS

Overall, 11 of 17 patients (64.7%) were Engel class 1 at 1 year (6/9 MHT–, 5/8 MHT+, P = .38), and 10 of 14 patients (71.4%) had no significant postoperative decline in either verbal or visual memory (6/8 MHT–, 4/6 MHT+, P = .42). Verbal memory declined in 2 of 8 MHT– and 1 of 6 MHT+ patients, and visual memory declined in 1 of 8 MHT– and 2 of 6 MHT+ patients. Two patients had improved visual memory postoperatively, both in the MHT+ group.

CONCLUSION

MHT on the dominant side is associated with high rates of seizure freedom and favorable memory preservation outcomes regardless of the extent of neocortical resection. Preservation of the temporal neocortex and amygdala during MHT does not appear to decrease the risk of postoperative memory decline, nor does it alter seizure outcome.

Multiple hippocampal transections for intractable hippocampal epilepsy: Seizure outcome

PURPOSE

The purpose of this study was to evaluate the seizure outcomes after transverse multiple hippocampal transections (MHTs) in 13 patients with intractable TLE.

METHODS

Thirteen patients with normal memory scores, including 8 with nonlesional hippocampi on MRI, had temporal lobe epilepsy (TLE) necessitating depth electrode implantation. After confirming hippocampal seizure onset, they underwent MHT. Intraoperative monitoring was done with 5-6 hippocampal electrodes spaced at approximately 1-cm intervals and spike counting for 5-8min before each cut. The number of transections ranged between 4 and 7. Neuropsychological assessment was completed preoperatively and postoperatively for all patients and will be reported separately.

RESULTS

Duration of epilepsy ranged between 5 and 55years. There were no complications. Intraoperatively, MHT resulted in marked spike reduction (p=0.003, paired t-test). Ten patients (77%) are seizure-free (average follow-up was 33months, range 20-65months) without medication changes. One of the 3 patients with persistent seizures had an MRI revealing incomplete transections, another had an additional neocortical seizure focus (as suggested by pure aphasic seizures), and the third had only 2 seizures in 4years, one of which occurred during antiseizure medication withdrawal. Verbal and visual memory outcomes will be reported separately. Right and left hippocampal volumes were not different preoperatively (n=12, p=0.64, Wilcoxon signed-rank test), but the transected hippocampal volume decreased postoperatively (p=0.0173).

CONCLUSIONS

Multiple hippocampal transections provide an effective intervention and a safe alternative to temporal lobectomy in patients with hippocampal epilepsy.

Long-term outcome and neuroradiologic changes after multiple hippocampal transection combined with multiple subpial transection or lesionectomy for temporal lobe epilepsy

OBJECTIVE

Multiple hippocampal transection (MHT) is a surgical procedure developed to avoid postoperative memory decline. Its efficacy has been documented in only a few small series with relatively short observation periods. We prospectively evaluated the long-term seizure and cognitive outcomes of MHT combined with multiple subpial transection or lesionectomy (MHT + MST/L). Moreover, we quantitatively evaluated the structural and metabolic neuroradiologic changes after the procedure to elucidate the anatomofunctional correlates of memory preservation.

METHODS

Twenty-four patients underwent MHT + MST/L for treatment of drug-resistant mesial temporal lobe epilepsy (mTLE) and were followed for more than 5 years. Indications for the procedure were the following: (1) verbally dominant-sided surgery in patients with a radiologically normal hippocampus or normal/near normal memory, and (2) surgery for patients with concomitant epileptic activity on the contralateral side, that is, when the surgery was considered a high risk for severe postoperative memory decline. Seizure outcome was evaluated using Engel's classification 1, 2, and 3 years after surgery, and at the last visit (LV). Three subgroups were evaluated as well: magnetic resonance imaging (MRI) negative (MN), hippocampal sclerosis (HS), and normal hippocampus with extrahippocampal lesion (NHEL). The long-term cognitive outcome was followed through to LV in patients who underwent verbally dominant-sided surgery. Hippocampal volume (HV), diffusion tensor parameters (DTP), and glucose utilization (GU) were determined from MRI and fluorodeoxyglucose-positron emission tomography (FDG-PET) studies performed before and >6 months after surgery.

RESULTS

Whereas the rate of Engel class I as a whole was 71% at 1 year and 67% at LV, the rates in the MN, HS, and NHEL groups were 60%, 67%, and 100% at 1 year, respectively, and 70%, 56%, and 80% at LV, respectively. Memory indices after verbally dominant-sided surgery transiently declined at 1 month but recovered to and remained at the preoperative level through LV. The HV, DTP of the fornix, and GU of the temporal lobe on the treated side showed pathologic changes even when the transiently declined memory indices had recovered to the preoperative level.

SIGNIFICANCE

The long-term outcome for complex partial seizures after MHT + MST/L was comparable to that seen after anterior temporal lobectomy. The long-term cognitive outcome was favorable, even for patients with a high risk of severe postoperative memory decline. MHT + MST/L may be a treatment option for mTLE in which resective surgery carries a risk of postoperative memory decline, particularly in patients without MRI lesion. A discrepancy between the preserved memory and the pathologic neuroradiologic changes indicates the necessity for further studies including functional MRI.

Hippocampal Transections for Epilepsy

Multiple hippocampal transection (MHT) is a novel surgical procedure that serves to disrupt seizure propagation fibers within the hippocampus without impairing verbal memory or the loss of stem cells. Given the paucity of literature regarding the utility and long-term outcome of MHT, a review is presented of the current literature to support the utility of this procedure in the treatment of intractable temporal lobe epilepsy. Long-term outcome analysis of this technique has been reported by 2 independent groups. Both groups used intraoperative electrocorticography. All patients underwent multiple subpial transection on the neocortex and MHT on the hippocampus.

The piriform, perirhinal, and entorhinal cortex in seizure generation

Understanding neural network behavior is essential to shed light on epileptogenesis and seizure propagation. The interconnectivity and plasticity of mammalian limbic and neocortical brain regions provide the substrate for the hypersynchrony and hyperexcitability associated with seizure activity. Recurrent unprovoked seizures are the hallmark of epilepsy, and limbic epilepsy is the most common type of medically-intractable focal epilepsy in adolescents and adults that necessitates surgical evaluation. In this review, we describe the role and relationships among the piriform (PIRC), perirhinal (PRC), and entorhinal cortex (ERC) in seizure-generation and epilepsy. The inherent function, anatomy, and histological composition of these cortical regions are discussed. In addition, the neurotransmitters, intrinsic and extrinsic connections, and the interaction of these regions are described. Furthermore, we provide evidence based on clinical research and animal models that suggest that these cortical regions may act as key seizure-trigger zones and, even, epileptogenesis.

Epilepsy surgery: current status and ongoing challenges

This article reviews the current status of surgical treatment of epilepsy and introduces the ongoing challenges. Seizure outcome of resective surgery for focal seizures associated with focal lesions is satisfactory. Particularly for mesial temporal lobe epilepsy, surgical treatment should be considered from the earlier stage of the disease. Meanwhile, surgical outcome in nonlesional extratemporal lobe epilepsy is still to be improved using various approaches. Disconnective surgeries reduce surgical complications of extensive resections while achieving equivalent or better seizure outcomes. Multiple subpial transection is still being modified expecting a better outcome by transection to the vertical cortices along the sulci- and multi-directional transection from a single entry point. Hippocampal transection is expected to preserve memory function while interrupting the abnormal epileptic synchronization. Proper selection or combination of subdural and depth electrodes and a wide-band analysis of electroencephalography may improve the accurate localization of epileptogenic region. Patients for whom curative resective surgery is not indicated because of generalized or bilateral multiple nature of their epilepsies, neuromodulation therapies are options of treatment which palliate their seizures.

Status epilepticus results in region-specific alterations in seizure susceptibility along the hippocampal longitudinal axis

Temporal lobe epilepsy (TLE) is the most common epilepsy syndrome in adults. In particular, the hippocampus is highly susceptible to abnormal synchronization. Recent advances in the surgical treatment of patients with refractory TLE have shown that multiple hippocampal transections can effectively control seizures. It has been suggested that in TLE the synchrony in the longitudinal connections is required for seizure generation; however the physiological background for the increase in hippocampal synchronization along the longitudinal axis is not fully understood. The hippocampus varies in seizure susceptibility along its longitudinal axis with the ventral hippocampus (VH) region being more seizure-prone and susceptible to neuronal damage than the dorsal hippocampus (DH). In the present study we studied seizure susceptibility along the longitudinal axis of the hippocampus following pilocarpine-induced status epilepticus (SE). In control conditions the VH generates epileptiform activity (EA) more frequently than the DH when exposed to a low Mg(2+)/1Ca(2+)/5K(+) solution. Following SE the probability of inducing epileptiform activity (EA) is similar in the VH and DH slices. This SE-induced change is due to an increase in the proportion of DH slices responding to the low Mg(2+)/1Ca(2+)/5K(+) solution with EA. Moreover, both the VH and DH show similar responses to a low Mg(2+)/1Ca(2+)/5K(+) solution. These findings indicate that the hippocampus undergoes significant functional changes following SE, which may provide the necessary increase of synchrony along the longitudinal axis to generate seizures in TLE.

Transsylvian hippocampal transection for mesial temporal lobe epilepsy: surgical indications, procedure, and postoperative seizure and memory outcomes

Object

Amygdalohippocampectomy is a well-established, standard surgery for medically intractable mesial temporal lobe epilepsy (MTLE). However, in the case of MTLE without hippocampal atrophy or sclerosis, amygdalohippocampectomy is associated with decreased postoperative memory function. Hippocampal transection (HT) has been developed to overcome this problem. In HT the hippocampus is not removed; rather, the longitudinal hippocampal circuits of epileptic activities are disrupted by transection of the pyramidal layer of the hippocampus. The present study describes a less invasive modification of HT (transsylvian HT) and presents the seizure and memory outcomes for this procedure.

Methods

Thirty-seven patients with MTLE (18 men and 19 women; age range 9–63 years; 19 with surgery on the right side and 18 with surgery on the left side; seizure onset from 3 to 34 years) who were treated with transsylvian HT were retrospectively analyzed. All patients had left-side language dominance, and follow-up periods ranged from 12 to 94 months (median 49 months). Seizure outcomes were evaluated for all patients by using the Engel classification. Memory function was evaluated for 22 patients based on 3 indices (verbal memory, nonverbal memory, and delayed recall), with those scores obtained using the Wechsler Memory Scale–Revised. Patients underwent evaluation of the memory function before and after surgery (6 months–1 year).

Results

Engel Class I (completely seizure free) was achieved in 25 patients (67.6%). Class II and Class III designation was achieved in 10 (27%) and 2 patients (5.4%), respectively. There were differences in memory outcome between the sides of operation. On the right side, verbal memory significantly increased postoperatively (p = 0.003) but nonverbal memory and delayed recall showed no significant change after the operation (p = 0.718 and p = 0.210, respectively). On the left side, all 3 indices (verbal memory, nonverbal memory, and delayed recall) showed no significant change (p = 0.331, p = 0.458, and p = 0.366, respectively).

Conclusions

Favorable seizure outcome and preservation of verbal memory were achieved with transsylvian HT for the treatment of MTLE without hippocampal atrophy or sclerosis.