Etiologic factors for unitemporal vs bitemporal epileptiform discharges

Seizure freedom after laser amygdalohippocampotomy guided by bilateral responsive neurostimulation in pediatric epilepsy: illustrative case

BACKGROUND

For patients with difficult-to-lateralize temporal lobe epilepsy, the use of chronic recordings as a diagnostic tool to inform subsequent surgical therapy is an emerging paradigm that has been reported in adults but not in children.

OBSERVATIONS

The authors reported the case of a 15-year-old girl with pharmacoresistant temporal lobe epilepsy who was found to have bitemporal epilepsy during a stereoelectroencephalography (sEEG) admission. She underwent placement of a responsive neurostimulator system with bilateral hippocampal depth electrodes. However, over many months, her responsive neurostimulation (RNS) recordings revealed that her typical, chronic seizures were right-sided only. This finding led to a subsequent right-sided laser amygdalohippocampotomy, resulting in seizure freedom.

LESSONS

In this case, RNS chronic recording provided real-world data that enabled more precise seizure localization than inpatient sEEG data, informing surgical decision-making that led to seizure freedom. The use of RNS chronic recordings as a diagnostic adjunct to seizure localization procedures and laser ablation therapies in children is an area with potential for future study.

Semiology, EEG, and neuroimaging findings in temporal lobe epilepsies

Temporal lobe epilepsy (TLE) is the most common type of focal epilepsy. First descriptions of TLE date back in time and detailed portraits of epileptic seizures of temporal origin can be found in early medical reports as well as in the works of various artists and dramatists. Depending on the seizure onset zone, several subtypes of TLE have been identified, each one associated with peculiar ictal semiology. TLE can result from multiple etiological causes, ranging from genetic to lesional ones. While the diagnosis of TLE relies on detailed analysis of clinical as well as electroencephalographic (EEG) features, the lesions responsible for seizure generation can be highlighted by multiple brain imaging modalities or, in selected cases, by genetic investigations. TLE is the most common cause of refractory epilepsy and despite the great advances in diagnostic tools, no lesion is found in around one-third of patients. Surgical treatment is a safe and effective option, requiring presurgical investigations to accurately identify the seizure onset zone (SOZ). In selected cases, presurgical investigations need intracerebral investigations (such as stereoelectroencephalography) or dedicated metabolic imaging techniques (interictal PET and ictal SPECT) to correctly identify the brain structures to be removed.

Clinical significance of bilateral epileptiform discharges in temporal lobe epilepsy

OBJECTIVE

The aim of the current study was to investigate the rate and clinical significance of bitemporal interictal epileptiform discharges (IEDs) in a large cohort of patients with temporal lobe epilepsy (TLE).

METHODS

The data used in this study were collected at the Epilepsy Care Unit, Namazi Hospital, Shiraz University of Medical sciences, Shiraz, Iran, from 2008 to 2020. Inclusion criteria were a confirmed diagnosis of TLE based on the clinical grounds (history and the described seizure semiology) and a 2-hour interictal video-electroencephalography (EEG) monitoring. The EEG recording of each patient included both sleep (about 90 minutes) and wakefulness (about 30 minutes).

RESULTS

532 patients were included in this study [420 patients (79%) had unilateral IEDs, and 112 patients (21%) had bilateral IEDs]. Patients with bilateral IEDs less often had auras with their seizures and had higher frequencies of seizures (as a trend for focal to bilateral tonic-clonic seizures and significantly in focal seizures with impaired awareness) compared with those who had unilateral IEDs. Patients with bilateral epileptiform discharges showed a trend to experiencing ictal injury more frequently. Brain MRI findings were different between these two groups (p = 0.0001).

CONCLUSION

It is important to recognize that a patient with TLE has unilateral vs. bilateral IEDs. Bilateral IEDs in a patient with TLE may suggest a more severe disease (with a higher risk for ictal injuries and other significant consequences of frequent seizures). It may also suggest a somewhat different etiology.

Focal epilepsies and focal disorders

Electroencephalographic (EEG) investigations are crucial in the diagnosis and management of patients with focal epilepsies. EEG may reveal different interictal epileptiform discharges (IEDs: abnormal spikes, sharp waves). The EEG visibility of a spike depends on the surface area of cortex involved (>10cm²) and the brain localization of cortical generators. Regions generating IEDs (defining the "irritative zone") are not necessarily equivalent to the seizure onset zone. Focal seizures are dynamic processes originating from one or several brain regions (that generate fast oscillations and are called the epileptogenic zone) before spreading to other structures (that generate lower frequency oscillations and are called the propagation zone). Several factors limit the expression of seizures on scalp EEG, such as the area involved, degree of synchronization, and depth of the cortical generators. Different scalp EEG seizure onset patterns may be observed: fast discharge, background flattening, rhythmic spikes, sinusoidal discharge, or sharp activity. However, to a large extent EEG changes are linked to seizure propagation. Finally, in the context of presurgical evaluation, the combination of interictal and ictal EEG features is crucial to provide an optimal hypothesis concerning the epileptogenic zone.

A Mesiotemporal Lobe Epilepsy Mouse Model

Among the different forms of epilepsies, mesiotemporal lobe epilepsy (MTLE) is one of the most common and represents the main pharmaco-resistant form of epilepsy. There is therefore an urgent need to better understand this form of epilepsy to develop better anti-epileptic drugs. Many rodent models are mimicking some aspects of the human temporal lobe epilepsy but only few are addressing most of the human mesiotemporal lobe epilepsy. In this article, we describe the main characteristics of a mouse of model of mesial temporal lobe epilepsy. This model is generated by a single injection of kainic acid into the dorsal hippocampus which reproduces most of the morphological and electrophysiological features of human MTLE in a mouse. This model may help to better understand mesial temporal lobe epilepsy and the development of new therapeutic drugs.

The role of mirror focus in the surgical outcome of patients with indolent temporal lobe tumors

PURPOSE

To review the clinical and neurophysiological data of 21 patients with epilepsy due to temporal lobe tumors and who had undergone evaluation and surgery at the Hospital das Clínicas da Universidade de São Paulo. The aim of this study was to investigate whether the occurrence of a mirror focus was influenced either by certain clinical factors or if the surgical outcome was influenced by the presence of a mirror focus.

METHOD

We included these 21 patients who had undergone at least one interictal electroencephalogram in the pre- and post-surgical periods. They had had a minimum follow-up of one year.

RESULTS

Eight patients had mirror focus (Group 1) and 13 did not (Group 2). The mean age at seizure onset, duration of epilepsy disorder and total number of seizures did not vary statistically between the two groups of patients. Generalized tonic-clonic seizures occurred more frequently in the mirror focus group. All, but one patient, with a mirror focus were seizure free at follow- up. The mirror focus disappeared in all eight patients in the post-surgical electroencephalogram. In this group, the patient who was not seizure - free had a seizure recorded in his post-surgical electroencephalogram with seizure onset ipsilateral to the resected tumor. The patients who were not seizure-free had either been submitted to an incomplete resection of the tumor or showed evidence of associated cortical dysplasia.

CONCLUSION

The occurrence of mirror focus is not a contraindication to surgery even when interictal epileptiform activity predominates contralaterally to the tumor and neither when seizures appear to arise from the mirror focus on scalp EEG. Good surgical outcome is expected despite EEG findings that may conflict with tumor location.

Clinical differences in patients with unilateral hippocampal sclerosis and unitemporal or bitemporal epileptiform discharges

PURPOSE

To investigate factors determining the presence of bilateral interictal epileptiform discharges (IEDs) in temporal lobe epilepsy (TLE) with unilateral hippocampal sclerosis (HS).

METHODS

We analysed data of 243 TLE patients with unilateral HS who had long-term video-EEG. Eighty-one patients (33%) had bitemporal IEDs.

RESULTS

We categorised patients into a unilateral group (UG), a bilateral group (BG) according to presence of bitemporal IEDs. We found no difference between UG and BG regarding epilepsy duration, secondarily generalised seizures, and history of febrile seizures. Mean seizure frequency was significantly higher in the BG (UG: 7.7+/-14.7 seizures/month; BG: 13+/-35 seizures/month, P=0.01). We found a significant correlation between late epilepsy onset and the presence of bitemporal IEDs. The mean age at epilepsy onset in UG was 10.1+/-7.9 years, while in BG it was 13+/-9.2 years (P=0.02).

CONCLUSIONS

The traditional concept of the evolution of mirror focus cannot apply for humans because the duration of epilepsy does not influence the evolution of bitemporal IEDs. Other factors, i.e. age at onset and seizure frequency may play a role in this process. The association between the higher seizure frequency and mirror foci indicates that the development of mirror focus depends on seizures and not on a progressive 'interictal' epileptogenesis.

Refractory epilepsy: a progressive, intractable but preventable condition?

Intractable seizures are just one manifestation of 'refractory epilepsy', which can be recognized as a distinct condition with multifaceted dimensions, including neurobiochemical plastic changes, cognitive decline and psychosocial dysfunction, leading to dependent behaviour and a restricted lifestyle. The biological basis of 'refractoriness' is likely to be multifactorial, and may include the severity of the syndrome and/or underlying neuropathology, abnormal reorganization of neuronal circuitry, alteration in neurotransmitter receptors, ion channelopathies, reactive autoimmunity, and impaired antiepileptic drug (AED) penetration to the seizure focus. Some of these deleterious changes may be a consequence of recurrent seizures. We hypothesize that 'refractory epilepsy' may be prevented by interrupting this self-perpetuating progression. There is increasing evidence that these patients can be identified early in the clinical course and, thus, be targeted early for effective therapeutic intervention. Failure of two first-line AEDs due to lack of efficacy or poor tolerability should prompt consideration of epilepsy surgery in a patient with a resectable brain abnormality. For the majority not suitable for 'curative' surgery, AEDs should be combined with the aim of achieving 'synergism'. This strategy has the potential to improve outcome by preventing the insidious progression to intractable 'refractoriness' and a downward spiraling quality of life.

Spatiotemporal relationship between seizure activity and interictal spikes in temporal lobe epilepsy

PURPOSE

To analyze the spatio-temporal relationship between seizure propagation and interictal epileptiform discharges (IEDs) in patients with bitemporal epilepsy.

METHODS

We investigated 18 adult patients with intractable temporal lobe epilepsy (TLE) who had undergone continuous video-EEG monitoring during presurgical evaluation. Only those patients were selected who had independent IEDs over both temporal lobes. Two authors evaluated the ictal and interictal EEG data independently.

RESULTS

We analyzed 52 lateralized seizures of 18 patients. Thirty-one seizures showed ipsilateral seizure spread exclusively, whereas in 21 seizures the contralateral hemisphere was also involved. In lateralized seizures without contralateral propagation, we found that spikes ipsilateral to the seizure onset occurred postictally in a greater ratio than preictally (P<0.001). In lateralized seizures with contralateral propagation, we found no significant changes in the postictal spike distribution.

CONCLUSIONS

Our findings showed that the lateralization of IEDs may depend on the brain areas involved by the preceding seizures, suggesting that spikes can be influenced by the seizure activity, and are not independent signs of epileptogenicity.

Secondary epileptogenesis in humans

Secondary epileptogenesis as it applies to humans remains a controversial topic despite 40 years of investigation. Part of the controversy stems from disagreement about the definition of secondary epileptogenesis, and part of the controversy stems from the imperfect fit of animal models to the human epileptic syndromes. It may be that models of secondary epileptogenesis can be useful to describe specific epileptic syndromes such as bitemporal epilepsy and secondary bilateral synchrony, but other models may be required for remitting syndromes such as the Landau-Kleffner syndrome. The concept of secondary epileptogenesis may also provide a useful construct for evaluating patients with partial epileptic syndromes, especially those under consideration for epilepsy surgery, and for the evaluation of preventive strategies in epilepsy.

The development of psychosis in epilepsy: a re-examination of the kindling hypothesis

It is generally acknowledged that psychosis occurs with increased frequency within epileptic populations. There are several possible explanations for this over-representation: (1) psychosis may develop as a result of anti-epileptic drug or surgical treatment, or as a result of the psychosocial effects of living with epilepsy; (2) epilepsy and psychosis may, in some cases, have a common cause: and (3) chronic seizure activity may sometimes cause psychosis. The objective of this review is to evaluate the hypothesis that focal seizure activity may lead to the development of psychosis through a kindling process. There is some evidence to suggest that secondary epileptogenesis may develop following the spread of seizure activity from a primary focus, possible via a kindling mechanism. Although it has been suggested that long-term potentiation (LTP) may result in the development of secondary epileptic foci; LTP is not necessarily implicated. The kindling hypothesis of the development of psychosis in epilepsy must address the neural mechanism by which the spread of seizures might result in psychosis. At present, the neurochemical mechanisms by which psychosis could result from epilepsy are unclear.

Propagation of epileptiform activity during development of amygdala kindling in rats: linear and non-linear association between ipsi- and contralateral sites

The relationship between ipsi- and contralateral epileptiform electroencephalographic (EEG) activity was investigated in rats that were kindled daily in the amygdala. Two types of relationships--linear and non-linear associations--were studied and used to estimate time delays of EEG activity between homotopic amygdalar sites during consecutive tetanizations. The progressive development of epileptiform EEG and convulsive behaviour was accompanied by an increase in association. Maximal association values of the non-linear function were significantly higher than linear association values. The gradual development of motor seizure severity was correlated with increased non-linearity. Time delays between the two amygdalae were estimated comparably with the linear and non-linear function: 30.0 +/- 3.3 and 24.6 +/- 1.7 ms (ipsilateral leading contralateral), respectively. However, in rats displaying exclusively bilaterally generalized motor convulsions, maximal values of both functions decreased but were still significantly higher than control values of phase-randomized EEG. Corresponding positive as well as negative interhemispheric time delays were recorded during the afterdischarge. These results demonstrated a strengthened association between the ipsi- and contralateral amygdala during primary epileptogenesis induced by amygdala kindling. In contrast, development of a secondary focus in the contralateral homotopic region resulted in a weakened interhemispheric association. Secondary bilateral synchrony between the ipsi- and contralateral amygdala occurred during the evoked epileptiform EEG activity.